High Power GaN Electronic Devices

A.P. Zhang^1*, F. Ren¹, T.J. Anderson¹, C.R. Abernathy², R.K. Singh², P.H. Holloway²,

S.J. Pearton², D. Palmer³ and G.E.McGuire^3**

¹ Department of Chemical Engineering

University of Florida, Gainesville, FL 32611

² Department of Materials Science and Engineering

University of Florida, Gainesville, FL 32611

³ Microelectronics Center of North Carolina

Research Triangle Park, NC 27709

Abstract

Gallium Nitride (GaN) and related materials (especially AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures and high frequencies. The AlGaInN system offers numerous advantages. These include wide bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light-emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices.

Al_xGa_1-xN (x=0 ~.25) Schottky rectifiers were fabricated in a lateral geometry employing p⁺-implanted guard rings and rectifying contact overlap onto an SiO₂ passivation layer. The reverse breakdown voltage (V_B) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al_0.25Ga_0.75N and 6350 V for GaN, respectively, for 100 mm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of £9.67x10⁵ Vcm^-2, which is roughly a factor of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (V_B)²/R_ON , where R_ON is the on-state resistance, was in the range 94-268 MWcm^-2 for all the devices. Edge-terminated Schottky rectifiers were also fabricated on quasi-bulk GaN substrates grown by HVPE. For small diameter (75 mm) Schottky contacts, Vs measured in the vertical geometry was ~700 V, with an on-state resistance (R_ON) of 3 mWcm², producing a figure-of-merit V_B²/R_ON of 162.8 MW-cm^-2.

GaN p-i-n diodes were also fabricated. A direct comparison of GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher reverse breakdown voltage for the former (490 V versus 347 V for the Schottky diodes), but lower forward turn-on voltages for the latter (~3.5 V versus ~5 V for the p-i-n diodes). The forward I-V characteristics of the p-i-n rectifiers show behavior consistent with a multiple recombination center model. The reverse current in both types of rectifiers was dominated by surface perimeter leakage at moderate bias. Finally, all of the devices we fabricated showed negative temperature coefficients for reverse breakdown voltage, which is a clear disadvantage for elevated temperature operation.

Bipolar devices are particularly interesting for high current applications such as microwave power amplifiers for radar, satellite and communication in the l~5 GHz range, powers >l00 W and operating temperatures >425°C. pnp Bipolar Junction Transistors and pnp Heterojunction Bipolar Transistors were demonstrated for the first time. For power microwave applications, small area self-aligned npn GaN/AlGaN HBTs were attempted. The devices showed very promising direct current characteristics.

1. Introduction

The nitride material growth technology developed for optical devices (lasers, light-emitting diodes) has also proven to be compatible with the development of electronic devices. In the past several years, the electronic device development has emphasized field effect transistor (FET) structures, because this important class of devices places smaller demands on the growth and fabrication technique compared to bipolar transistors. The rapid progress that has been made, especially in modulation-doped FETs (MODFETs), has been sufficient to show that GaN and related alloys will play a significant role in the future development of high temperature, high power and high frequency electronic devices [1-14].

GaN-based transistors have a unique combination of high current density, high breakdown electric field, and good thermal conductivity, that enable previously unrealizable microwave power performance for solid state transistors. For microwave transistor performance, two figures of merit (FOMs) have been developed for comparing the inherent semiconductor material capabilities. These FOMs are Johnson’s FOM (n_satE_C)² and the Baliga’s high frequency FOM (mE_C²), where E_C is the critical breakdown field, n_sat is the electron saturation velocity , and m is the low field electron mobility. Figure 1 shows these figures of merit normalized to silicon for all the potential microwave semiconductor materials. The FOM comparison clearly shows the advantage of the GaN material system [14].

Figure 2 presents a schematic representation of a GaN/AlGaN heterostructure. Due to the large conduction band discontinuity, the electrons diffusing from the large bandgap AlGaN into the smaller bandgap GaN form a two-dimensional electron gas (2DEG) in the triangle quantum well at the interface, which is the hallmark of MODFET. The sheet carrier density of the 2DEG was found to be further enhanced by the strong piezoelectric effect in GaN. Piezoelectric coefficients in nitrides were measured to be about an order of magnitude higher than in traditional Group III-V semiconductors [5]. Theoretical simulations have predicted a high peak electron velocity of ~3´10⁷ cm/s and an electron mobility of ~2000 cm²/V×s in the GaN channel at room temperature at a carrier concentration of 10¹⁷cm^-3 [6]. Gaska et al. [15] found the highest measured Hall mobility at room temperature was 2019 cm²/V×s, and increased approximately fivefold to 10,250 cm²/V×s below 10 K for growth on 6H SiC substrate.

In 1993, Khan et al. [1] demonstrated the first AlGaN/GaN MODFET, with a g_m of 23 mS/mm and 2DEG mobility of 563 cm²/V×s at 300 K. They also reported the first microwave results with f_t of 11 GHz and f_max of 14 GHz [2]. In the early stages, the MODFETs exhibited very low transconductances and relatively poor frequency response. This is consistent with the defect-laden nature of the early GaN and AlGaN layers. With improvements in the materials quality, the transconductance, current capacity, and drain breakdown voltage are all increased to the point that GaN-based MODFETs are now strong contenders in the arena of high power devices and amplifiers. To date, the highest power density achieved for a 0.45´125 mm GaN MODFET is 6.8 W/mm at 10 GHz and associated gain of 10.65 dB. The operation temperature has been pushed to 750ºC by employing a thermally stable Pt/Au gate contact [4].

The published performances of epitaxial GaN-based MESFETs demonstrate that all the required components for a MESFET based technology are in place [7,8], i.e., an appropriate high resistivity buffer and substrate combination has been developed for doped layer epitaxial growth, FET channels can be grown with thin n⁺ contact layers on which Ohmic contact with adequate contact resistances have been achieved, gate metallizations which can pinchoff the channel and support a high drain bias have been demonstrated, and it has shown that both mesa etch and implant isolation can be used to define the active device area. Recently, an all implanted GaN junction FET, a Si₃N₄ gated GaN MISFET [10], and a Ga₂O₃ (Gd₂O₄) gated GaN MOSFET with reasonable performance were also reported. These types of devices potentially have advantage over MESFET, especially at high temperatures due to low reverse leakage currents.

So far, there are only few reports of development of GaN based bipolar transistors [12,13]. Basically the device performance is limited by the difficulty in growth and processing related to the buried p-type layer and the small minority carrier lifetime. It is still far from commercialization of these devices, but their developments will follow the material improvements in the new decade, and much impetus comes from defense applications where ultrawide bandwidth and linearity are desired.

2. Effect of N₂ Inductively Coupled Plasma Discharge Treatment on n-AlGaN/GaN Ohmic Contacts

Both the dc and rf performance of AlGaN/GaN High Electron Mobility Transistors (HEMTs) are strongly dependent on the specific contact resistance of the source/drain contacts [16-25]. There have been four basic classes of metallization employed for n-type ohmic contacts to GaN-based materials, namely Al [26-28], Ti or TiN [29-36], W or other refractory metals [35-38] or multilayers such as Ti/Al/Ni/Au [39-41] which appear to give wider process windows by reducing oxidation of the Ti [39-41]. Modifications to the GaN by high temperature annealing [42] or reactive ion etching [39,43] to produce preferential loss of nitrogen can improve n-type ohmic contact resistance by increasing electron concentration in the near-surface region. In all cases, the best specific contact resistivity has been achieved after annealing the metallization at 900-950 °C [39,44,45].

We have previously found that exposure of n- or p- type GaN to high density Inductively Coupled Plasmas (ICP) degrades the rectifying properties of subsequently deposited Schottky contacts [46]. The degradation mechanism is loss of nitrogen, as described above. To this point, there have been no investigation of the effect of ICP exposure on the properties of n-type ohmic contacts, especially on HEMT structures where the contact resistivity can be high due to presence of AlGaN donor and contact layers. In this section we report the results of a systematic study to understand the effect of ion energy, ion flux and exposure time of N₂ ICP discharges on the contact resistance of Ti/Al/Pt/Au metallization on AlGaN/GaN HEMTs.

The AlGaN/GaN structures were grown by rf plasma activated Molecular Beam Epitaxy on (0001) sapphire [47]. After nitridation of the surface, at low temperature, 300Å thick AlN buffer was grown, followed by a 1mm undoped GaN layer grown at 750 °C under Ga-rich growth conditions. This was followed with a 30Å undoped Al_0.15Ga_0.85N spacer layer, 100Å Al_0.15Ga_0.85N donor layer (Si-doped, n=10¹⁹ cm^-3) and a 100Å undoped Al_0.15Ga_0.85N cap layer. A schematic of the structure is shown in Figure 3. Typical room temperature sheet electron densities were ~3.5´10¹² cm^-2, with Hall mobilities of ~400 cm^-2V^-1·sec^-1 [44].

The N₂ plasma exposures were carried out in a Plasma Therm 790 reactor, in which the ion flux is controlled by a 1500W ICP source operating at 2MHz, and the ion energy is controlled by rf power (13.56MHz) applied to the sample chuck. The N₂ gas was injected into the source at a total flow rate of 15 standard cubic centimeter per minute and process pressure was held constant at 2mTorr. After plasma exposure, e-beam deposited Ti(200Å)/Al(800Å)/Pt(400Å)/Au(1500Å) was patterned by lift-off and annealed under N₂ in an AG associates Heatpulse 610T system. The specific contact resistance was obtained from Transmission Line Method (TLM) measurement using gap spacings of 2,4,8,16 and 32mm. In some cases the plasma exposed AlGaN/GaN structures were examined by Atomic Force Microscopy (AFM) and Auger Electron Spectroscopy (AES)for measurement of surface morphology and composition, respectively.

Figure 4 shows the measured contact resistances for the Ti/Al/Pt/Au metallization on unexposed (control) samples, as a function of post-deposition annealing temperature. We will use this data for comparison with the plasma-exposed samples. Note that a value of 7´10^-3 W×cm² was obtained for 950°C annealing.

The effect of rf chuck power on contact resistance of the N₂ plasma exposed samples is shown in Figure 5. In this case the ICP source power was held constant at 300W (equivalent to an ion flux of ~4´10¹⁶ cm^-2×sec^-1). The lowest contact resistances were obtained for samples exposed at 40W chuck power and subsequent annealed for 30sec at 950°C, producing a value of 2´10^-4 W×cm². This is approximately a factor of three improvement over contacts annealed at the same temperature on control samples. The ion energy at this condition is roughly -125 eV, the sum of the dc self-bias (lower part of Figure 5) and plasma potential (about -25eV under these conditions).

We fixed the rf chuck power at 40W and examined the effect of varying the ICP source power during the plasma exposure (Figure 6). For annealing at 800 or 950°C, there is a broad minimum in contact resistance centered at 300W source power. We believe that at lower powers the ion flux is too low to produce efficient preferential loss of the nitrogen, while at higher fluxes there are large concentrations of defects created that degrade current transport in the AlGaN. Note that while flux increases with source power, the ion energy decreases slightly due to the higher plasma conductivity.

The improvement in contact resistance saturated with exposure time, as shown in Figure 7. This result is not unexpected, since part of the surface is removed by sputtering during plasma exposure and the creation of an N₂-deficient surface region will come to an equilibrium condition. The plasma exposure did not roughen the AlGaN surface, as shown by the AFM scans of Figure 8. The root-mean-square (RMS) roughness of the controlled sample was 1.3nm, compared to 1.0nm for the sample exposed to a 300W source power, 40W rf chuck power N₂ discharge for 30 secs. It is likely that at high chuck power, corresponding to high ion energies, surface roughening should be more prevalent.

To confirm that the mechanism for the contact resistance improvement was loss of nitrogen, we performed AES measurements. Surface scans before and after N₂ plasma exposure (300W source power, 40W rf chuck power, 30sec) showed that the average composition of N in the top 100Å of the surface dropped from 10% in the control sample to 7.1% in the plasma exposed sample (Figure 9). Scanning electron microscopy of the contact metallization showed good morphology and edge definition for both the control and plasma exposed samples.

In summary, ICP N₂ discharges were used to improve contact resistances on AlGaN/GaN HEMT structures by inducing preferential loss of nitrogen from the near- surface 100Å) region. The N₂ plasma chemistry is a good choice for this application, since it produces light ions (N₂⁺, N⁺) for bombardment of the AlGaN surface that do not create heavy lattice disorder and associated trapping states that could degrade current transport in the semiconductor. It also avoids the chemical effects of H₂ or O₂ discharges on the AlGaN surface. Under optimized conditions, the contact resistance of Ti/Al/Pt/Au metallization deposited on the plasma exposed samples and subsequently annealed at 950°C was lowered by a factor of 3 relative to unexposed contact samples annealed in the same fashion. This is a simple and effective method for reducing ohmic contact resistance on AlGaN/GaN HEMTs.

3. GaN and AlGaN HIGH VOLTAGE POWER RECTIFIERS

There is a strong interest in developing wide bandgap power devices for use in the electric power utility industry [48-52]. With the onset of deregulation in the industry, there will be increasing numbers of transactions on the power grid in the US, with different companies buying and selling power. The main applications are in the primary distribution system (100~2000 kVA) and in subsidiary transmission systems (1~50 MVA). A major problem in the current grid is momentary voltage sags, which affect motor drives, computers and digital controls. Therefore, a system for eliminating power sags and switching transients would dramatically improve power quality. For example it is estimated that a 2-second outage at a large computer center can cost US$ 600,000 or more, and an outage of less than one cycle, or a voltage sag of 25% for two cycles, can cause a microprocessor to malfunction. In particular, computerized technologies have led to strong consumer demands for less expensive electricity, premium quality power and uninterruptible power.

The basic power electronics hierarchy would include the use of widegap devices such as Gate Turn-Off Thyristors (GTOs), MOS-Controlled Thyristors (MCT) or Insulated Gate Bipolar Transistors (IGBTs) combined with appropriate packaging and thermal management techniques to make subsystems (such as switches, rectifiers or adjustable speed devices) which then comprise a system such as Flexible AC Transmissions (FACTS). Common power electronics systems, which are inserted between the incoming power and the electrical load include uninterruptible power supplies, advanced motors, adjustable speed drives and motor controls, switching power supplies, solid-state circuit breakers and power conditioning equipment. About 50% of the electricity in the US is consumed by motors. Motor repairs cost ~US$ 5 billion each year and could be dramatically reduced by high power electronic devices that permit smoother switching and control. Moreover, control electronics could dramatically improve motor efficiency. Other end uses include lighting, computers, heating and air-conditioning.

Some desirable attributes of next generation, widegap power electronics include the ability to withstand currents in excess of 5 kA and voltages in excess of 50 kV, provide rapid switching, maintain good thermal stability while operating at temperatures above 250°C, have small size and light-weight, and be able to function without bulky heat-dissipating systems.

The primary limits of Si-based power electronics are:

1. Maximum voltage ratings < 7 kV
• Multiple devices must be placed in series for high-voltage systems.
2. Insufficient current-carrying capacity
• Multiple devices must be placed in parallel for typical power grid applications.
3. Conductivity in one direction only
• Identical pairs of devices must be installed in anti-parallel for switchable circuits.
4. Inadequate thermal management
• Heat damage is a primary cause of failure and expense.
5. High initial cost
• Applications are limited to the highest-value settings.
6. Large and heavy components
• Costs are high for installation and servicing, and equipment is unsuitable for many customers.

For these reasons, there is a strong development effort on widegap power devices, predominantly SiC, with lesser efforts in GaN and diamond, which should have benefits that Si-based or electromechanical power electronics cannot attain. The higher standoff voltages should eliminate the need for series stacking of devices and the associated packaging difficulties. In addition these widegap devices should have higher switching frequency in pulse-width-modulated rectifiers and inverters.

The absence of Si devices capable of application to 13.8 kV distribution lines (a common primary distribution mode) opens a major opportunity for widegap electronics. However, cost will be an issue, with values of US$ 200~2000 per kVA necessary to have an impact. It is virtually certain that SiC switches will become commercially available within 3~5 years, and begin to be applied to the 13.8 kV lines. MOS Turn-Off-Thyristors involving a SiC GTO and SiC MOSFET are a promising approach [103]. An inverter module can be constructed from an MOS turn-off thyristor (MTO) and a SiC power diode.

Packaging and thermal management will be a key part of future power devices. For current Si IGBTs, there are two basic package types - the first is a standard attached die, wire bond package utilizing soft-solder and wire-bonds as contacts, while the second is the presspack, which employs dry-pressed contacts for both electrical and thermal paths [53,54]. In the classical package the IGBTs and control diodes are soldered onto ceramic substrates, such as AlN, which provide electrical insulation, and this in turn is mounted to a heat sink (typically Cu). Thick Al wires (500 mm) are used for electrical connections, while silicone gel fills the package [53]. In the newer presspack style, the IGBT and diode are clamped between Cu electrodes, buffered by materials such as molybdenum or composites [54], whose purpose is to account for the thermal expansion coefficient differences between Si and Cu. The package is again filled with gel for electrical insulation and corrosion resistance.

3.1 GaN Schottky Rectifiers with 3.1 kV Reverse Breakdown Voltage

The GaN materials system is attractive from the viewpoint of fabricating unipolar power devices because of its large bandgap and relatively high electron mobility [55-58]. An example is the use of Schottky diodes as high-voltage rectifiers in power switching applications. These diodes will have lower blocking voltages than p-i-n rectifiers, but have advantages in terms of switching speed and lower forward voltage drop. Edge termination techniques such as, field rings on filed plates, bevels or surface ion implantation are relatively well-developed for Si and SiC and maximize the high voltage blocking capability by avoiding sharp field distributions within the device. However, in the few GaN Schottky diode rectifiers reported to date [55,56], there has been little effort made on developing edge termination techniques. Proper design of the edge termination is critical both for obtaining a high breakdown voltage and reducing the on-state voltage drop and switching time.

Based on the punch-through model, Figure 10 shows a plot of avalanche and punch through breakdown of GaN Schottky diodes calculated as a function of doping concentration and standoff layer thickness. It can be seen that 20 kV device may be obtained with ~100mm thick GaN layer with doping concentration <10¹⁵ cm^-3.

In this section we report on the effect of various edge termination techniques on the reverse breakdown voltage, V_B, of planar GaN Schottky diodes which deplete in the lateral direction. A maximum V_B of 3.1 kV at 25ºC was achieved with optimized edge termination, which is a record for GaN devices. We also examined the temperature dependence of V_B in mesa diodes and found a negative temperature coefficient of this parameter in these structures.

The GaN was grown on c-plane Al₂O₃ substrates by MOCVD using trimethylgallium and ammonia as the precursors. To create a Schottky rectifier with high breakdown voltage, one needs a thick, very pure GaN depletion layer. Figure 11 shows SIMS profile of H and other background impurities in a 2 mm thick, high resistivity (10⁷W×cm) GaN layer grown by MOCVD. The reverse breakdown voltage of simple Schottky rectifiers fabricated on this material was > 2 kV, a record for GaN. Notice that in this material the hydrogen concentration is at the detection sensitivity of the SIMS apparatus. The amount of hydrogen present in GaN after cooldown from the growth temperature will depend on the number of sites to which it can bond, including dopants and point and line defects. In the absence of p-type doping, it is clear that the number of these sites is £ 8´10¹⁷ cm^-3 under our growth conditions.

For vertically-depleting devices, the structure consisted of a 1 mm n⁺ (3×10¹⁸ cm^-3, Si-doped) contact layer, followed by undoped (n~2.5×10¹⁶ cm^-3) blocking layers which ranged from 3 to 11 mm thick. These samples were formed into mesa diodes using ICP etching with Cl₂/Ar discharges (300 W source power, 40 W rf chuck power). The dc self-bias during etching was -85 V. To remove residual dry etch damage, the samples were annealed under N₂ at 800ºC for 30 s. Ohmic contacts were formed by lift-off of e-beam evaporated Ti/Al, annealed at 700ºC for 30 s under N₂ to minimize the contact resistance. Finally, the rectifying contacts were formed by lift-off of e-beam evaporated Pt/Au. Contact diameters of 60-1100 mm were examined.

For laterally depleting devices, the structure consisted of ~3 mm of resistive (10⁷ W/ٱ) GaN. To form Ohmic contacts, Si⁺ was implanted at 5 ´10¹⁴ cm^-2, 50 keV into the contact region and activated by annealing at 150°C for 10 s under N₂. The Ohmic and rectifying contact metallization was the same as described above.

Three different edge termination techniques were investigated for the planar diode:

1. Use of a p-guard ring formed by Mg⁺ implantation at the edge of the Schottky barrier metal. In these diodes the rectifying contact diameter was held constant at 124 mm, while the distance of the edge of this contact from the edge of the Ohmic contact was 30 mm in all cases.
2. Use of p-floating field rings of width 5 mm to extend the depletion boundary along the surface of the SiO₂ dielectric, which reduces the electric field crowding at the edge of this boundary. In these structures a 10 µm wide p-guard ring was used, and one to three floating field rings employed.
3. Use of junction barrier controlled Schottky (JBS) rectifiers, i.e., a Schottky rectifier structure with a p-n junction grid integrated into its drift region.

In all of the edge-terminated devices the Schottky barrier metal was extended over an oxide layer at the edge to further minimize field crowding, and the guard and field rings formed by Mg⁺ implantation and 1100ºC annealing.

Figure 12 shows a schematic of the planar diodes fabricated with the p-guard rings, while the lower portion of the figure shows the influence of guard ring width on V_B at 25ºC. Without any edge termination, V_B is ~2300 V for these diodes. The forward turn-on voltage was in the range 15~50 V, with a best on-resistance of 0.8 W cm². The figure-of-merit (V_B)²/R_ON was 6.8 MW/cm². As the guard-ring width was increased, we observed a monotonic increase in V_B, reaching a value of ~3100 V for 30 µm wide rings (Figure 13). The figure-of-merit was 15.5 MW/cm² under these conditions. The reverse leakage current of the diodes was still in the nA range at voltages up to 90% of the breakdown value.

Figure 14 shows a schematic of the floating field ring structures, while Figure 15 shows the effect of different edge termination combinations on the resulting V_B at 25ºC. Note that the addition of the floating field rings to a guard ring structure further improves V_B, with the improvement saturating for a three-floating field ring geometry.

Figure 16 shows the effect of the junction barrier control on V_B, together with a schematic of the p-n junction grid in Figure 17. In our particular structure we found that junction barrier control slightly degraded V_B relative to devices with guard rings and various numbers of floating field rings. We believe that with optimum design of the grid structure we should achieve higher V_B values and that the current design allows Schottky barrier lowering since the depletion regions around each section of the grid do not completely overlap. This is consistent with the fact that we did not observe the decrease in forward turn-on voltage expected for JBS rectifiers relative to conventional Schottky rectifiers.

The results of Figures 14-17 are convincing evidence that proper design and implementation of edge termination methods can significantly increase reverse breakdown voltage in GaN diode rectifiers and will play an important role in applications at the very highest power levels. For example, the target goals for devices, intended to be used for transmission and distribution of electric power or in single-pulse switching in the subsystem of hybrid-electric contact vehicles are 25 kV standoff voltage, 2 kA conducting current and a forward voltage drop <2% of the standoff voltage. At these power levels, it is expected that edge termination techniques will be essential for reproducible operation.

The devices designed for vertical depletion had lower on-state voltages than the lateral diodes, due to the fact that a highly-doped n⁺ contact layer can be included in the epitaxial structure, obviating the need for implantation. However, we have not yet perfected the ability to grow resistive GaN on top of conducting GaN and, therefore, the depletion layers in the vertical devices typically had lightly n-type conductivity (2×10¹⁶ to 5×10 ¹⁶ cm^-3). The typical on-state resistances were 6~10 m W cm², with reverse breakdown voltages at 25ºC of 200~550 V (depending on doping level and layer thickness). The maximum figure-of-merit in these devices was higher than for the planar diodes, reaching values as high as 48 MW/cm².

In summary, GaN Schottky diodes with vertical and lateral geometries were fabricated. A reverse breakdown voltage of 3.1 kV was achieved on a lateral device incorporating p-type guard rings. Several types of edge termination were examined, with floating field rings and guard rings found to increase V_B. The best on-state resistance obtained in these lateral devices was 0.8 W cm². In mesa diodes incorporating n⁺ contact layers, the best on-state resistance was 6 m W cm², while V_B values were in the range 200~550 V. These GaN rectifiers show promise for high power electronics applications.

3.2 AlGaN Schottky Rectifiers with 4.1 kV Reverse Breakdown Voltage

There is a strong interest in developing high current, high voltage switches in the AlGaN materials system for applications in the transmission and distribution of electric power and in the electrical subsystems of emerging vehicle, ship, and aircraft technology [57,59,60]. It is expected that packaged switches made from AlGaN may operate at temperatures in excess of 250 °C without liquid cooling, therefore reducing system complexity, weight, and cost. In terms of voltage requirements, there is a strong need for power quality enhancement in the 13.8 kV class, while it is estimated that availability of 20–25 kV switches in a single unit would cause a sharp drop in the cost of power flow control circuits. Schottky and p-i-n rectifiers are an attractive vehicle for demonstrating the high-voltage performance of different materials systems, and blocking voltages from 3–5.9 kV have been reported in SiC devices [61-63]. The reverse leakage current in Schottky rectifiers is generally far higher than expected from thermionic emission, most likely due to defect states around the contact periphery [61]. To reduce this leakage current and prevent breakdown by surface flashover, edge termination techniques such as guard rings, field plates, beveling or surface ion implantation are necessary [64,65]. However, in the GaN rectifiers reported so far, there has been little effort in employing edge termination methods and no investigation of the effect of increasing the band gap by use of AlGaN.

We report on the reverse breakdown voltage (V_RB) of AlGaN Schottky rectifiers for different Al compositions (0–0.25) and on the effect of various edge termination techniques in suppressing premature edge breakdown. A maximum V_RB of 4.3 kV was achieved for Al_0.25Ga_0.75N diodes, with very low reverse current densities. At low reverse biases the rectifiers typically show currents which are proportional to the contact perimeter, whereas at higher biases the current is proportional to contact area. The forward current characteristics show ideality factors of 2 at low bias (Shockley–Read–Hall recombination) and 1.5 at higher voltage (diffusion current).

The undoped Al_XGa_1–XN layers were grown by atmospheric pressure metalorganic chemical vapor deposition at 1040 °C (pure GaN) or 1100 °C (AlGaN) on (0001) oriented sapphire substrates. The precursors were trimethylgallium, trimethylaluminum, and ammonia, with H₂ used as a carrier gas. The growth was performed on either GaN (in the case of GaN active layers) or AlN (in the case of AlGaN active layers) low temperature buffers with nominal thicknesses of 200 Å. The active layer thickness was ~2.5 µm in all cases and the resistivity of these films was of order 10⁷ W×cm [66]. To form ohmic contacts in some cases, Si ⁺ was implanted at 5×10¹⁴ cm^–2, 50 keV into the contact region and activated by annealing at 1150 °C for 10 s under N₂. The contacts were then formed by lift off of e-beam evaporated Ti/Al/Pt/Au annealed at 700 °C for 30 s under N₂. The rectifying contacts were formed by lift off of e-beam evaporated Pt/Ti/Au (diameter 60–1100µm). A schematic of the planar diodes is shown in Figure 18. The devices were tested at room temperature under a Fluorinert® ambient.

On the GaN diodes, we also examined the use of three different edge termination methods, namely p-guard rings formed by Mg ⁺ implantation at the edge of the rectifying contact, use of p-type floating field rings of width 5 µm to extend the depletion boundary along the edge of a SiO₂ passivation layer and finally, use of junction barrier controlled Schottky rectifiers (a rectifier with integrated p–n junction grid in its drift region). In all of these edge-terminated diodes the Schottky metal was extended over a SiO₂ layer at the edge to minimize field crowding.

Figure 19 shows current–voltage (I–V) characteristics from two different diodes. The GaN device employed 30 µm wide p-guard rings. This was found to be the most effective edge termination method for these structures, producing an increase in V_RB of ~800 V over devices without any passivation or edge termination, i.e., breakdown occurred at 2.3 kV in the control diodes and 3.1 kV in devices with guard rings. The use of guard rings or floating field rings each produced improvements in V_RB over the control diodes, with increases in the range 200–800 V. By sharp contrast, junction barrier control was unsuccessful in our structures, leading to decreases in V_RB of 300–400 V. We believe this is due to Schottky barrier lowering because of the depletion regions around each section of the grid not completely overlapping in our initial design. The best on resistance (R_ON) achieved for GaN diodes was 0.8 Wcm², producing a figure-of-merit (V_RB)²/R_ON of 15.5 MW cm^–2. Figure 19 also shows an I–V characteristic from an Al_0.25Ga_0.75N rectifier, without any edge termination or surface passivation. In this case V_RB was 4.3 kV, which is far in excess of the values reported previously for GaN rectifiers, i.e., 350–450 V [116,117]. The on resistance of the AlGaN diodes was higher than for pure GaN, due to higher ohmic contract resistance. The lowest R_ON achieved was 3.2 W×cm², leading to a figure-of-merit of ~5.5 MW cm^–2.

Figure 20 shows the variation of V_RB with Al percentage in the AlGaN active layers of the rectifiers. In this case we are using the V_RB values from diodes without any edge termination or surface passivation. The calculated band gaps as a function of Al composition are also shown, and were obtained from the relation:

where x is the AlN mole fraction and b is the bowing parameter with value 0.96 eV [67]. Note that V_RB does not increase in linear fashion with band gap. In a simple theory, V_RB should increase as (Eg)^1.5, but it has been empirically established that factors such as impact ionization coefficients and other transport parameters need to be considered and that consideration of Eg alone is not sufficient to explain measured V_RB behavior. The fact that V_RB increases less rapidly with Eg at higher AlN mole fractions may indicate increasing concentrations of defects that influence the critical field for breakdown.

The reverse I–V characteristics of all of the rectifiers showed I µ V^0.5 over a broad range of voltage (50–2000 V), indicating that Shockley–Read–Hall recombination is the dominant transport mechanism. The current density in all devices was in the range 5–10×10^–6 A cm^–2 at 2 kV. At low biases (25 V) the reverse current was proportional to the perimeter of the rectifying contact, suggesting that surface contributions are the most important in this voltage range. For higher biases, the current was proportional to the area of the rectifying contact. Under these conditions, the main contribution to the reverse current is from under this contact, i.e., from the bulk of the material. It is likely that the high defect density in heteroepitaxial GaN is a primary cause of this current. The forward I–V characteristics showed that the current density was proportional to exp (–eV/2kT) at lowest voltages (up to current densities of ~5×10^–4 A cm^–2) and to exp (–eV/1.5kT) at higher voltages (current densities in the range 10^–3–1.5×10² A cm^–2). These results are consistent with Shockley–Read–Hall recombination as the dominant mechanism at low bias, followed by diffusion current at higher voltage. Qualitatively similar behavior has been reported previously for SiC Schottky rectifiers.

When pushed beyond breakdown, the diodes invariably failed at the edges of the rectifying contact, as shown in Figure 21. As described earlier, the use of metal field plate contact geometries with SiO₂ as the insulator and either guard rings or floating field rings significantly increased V_B. These rectifiers generally did not suffer irreversible damage to the contact upon reaching breakdown and could be re-measured many times.

In summary, Schottky rectifiers on high resistivity Al_XGa_1-X N epi layers produced reverse breakdown voltages up to 4.3 kV for Al_0.25Ga_0.75N diodes without edge termination. The current transport mechanisms were investigated as a function of bias voltage, with Shockley-Read-Hall recombination being dominant over a broad range of conditions. Minimizing electric field crowding at the corners of the rectifying contact was effective in increasing the breakdown voltage. The AlGaN materials system appears promising for high voltage applications.

3.3 Temperature dependence and current transport mechanisms in Al_XGa_1–XN Schottky rectifiers

P-i-n rectifiers are expected to have larger reverse blocking voltages than Schottky rectifiers, but inferior switching speeds and higher forward turn-on voltages. GaN Schottky rectifiers with reverse breakdown voltage (V_RB) to 3.1 kV have been demonstrated when p⁺ guard rings and metal overlap onto a dielectric are employed as edge termination techniques. Use of Al_0.25Ga_0.75N instead of GaN produced V_RB values up to 4.3 kV.

Since this type of device is intended for elevated temperature operation, there is a need to understand the current transport mechanisms, the origin of the reverse leakage current and the magnitude and sign of the temperature coefficient for V_RB. In this section all of these properties are investigated. Over a broad range of voltages, the reverse leakage current is proportional to the diameter of the rectifying contact indicating that surface periphery leakage is the dominant contributor. The temperature coefficient for V_RB was found to be negative for both GaN and AlGaN, even in edge-terminated devices.

The GaN and Al_0.25Ga_0.75N layers were found to be resistive (~10⁷ W cm). Each was grown on c-plane Al₂O₃ substrates by metal organic chemical vapor deposition using conventional precursors and growth temperatures of 1040 (GaN) or 1100 °C (Al_0.25Ga_0.75N). The layer thicknesses were 2.5–3 µm. Schematics of the completed rectifiers are shown in Figure 22. The GaN devices employed p⁺ guard rings formed (7 µm wide) by Mg⁺/P⁺ implantation, n⁺ source/drain region formed by Si⁺ implantation (annealing was performed at 1150 °C for 10 s under N₂) and overlap of the rectifying contact onto a SiO₂ passivation layer. The AlGaN devices did not use any edge termination techniques. The contacts on all rectifiers were formed by lift-off, with the ohmic metallization annealed at 700 °C for 30 s under N₂. The rectifying contact diameters were 45–125 µm with a separation of 124 µm between these contacts and the ohmic contacts.

Current–voltage (I–V) characteristics from both types of rectifiers are shown in Figure 23 as a function of measurement temperature. The most obvious feature of the data is that there is a negative temperature coefficient for V_RB. The only previous information for GaN-based devices comes from GaN/AlGaN heterostructure field effect transistors in which a value of +0.33 V×K^–1 was found [68], and from linearly graded GaN p⁺pn⁺ junctions, in which a value of +0.02 V K–1 was determined [58]. In both cases the V_RB values were more than an order of magnitude lower than in the present diodes.

Figure 24 shows the variation of V_RB with temperature. The data can be represented by a relation of the form:

where = –6.0±0.4 V K^–1 for both types of rectifiers. However, in Schottky and p-i-n rectifiers we have fabricated on more conducting GaN, with V_RB values in the 400–500 V range, the values were consistently around –0.34 V K^–1. Therefore, in present state-of-the-art GaN rectifiers, the temperature coefficient of V_RB appear to be a function of the magnitude of V_RB. Regardless of the origin of this effect, it is clearly a disadvantage for GaN. While SiC is reported to have a positive temperature coefficient for V_RB there are reports of rectifiers that display negative b values [69]. One may speculate that particular defects present may dominate the sign and magnitude of b, and it will be interesting to fabricate GaN rectifiers on bulk or quasibulk substrates with defect densities far lower than in heteroepitaxial material.

The forward turn-on voltage V_F of a Schottky rectifier can be written as

where n is the ideality factor, k is Boltzmann's constant, T is the absolute sample temperature, e the electronic charge, J_F the forward current density (usually taken to be 100 A cm^–2) at V_F, A** the Richardson constant, f_B the barrier height (~1.1 eV in this case), and RON the on-state resistance. The typical best V_F values were ~5 V for GaN and ~7.5 V for Al_0.25Ga_0.75N, with best R_ON values of 50 and 75 mW cm², respectively. The ideality factors derived from the forward I–V characteristic were typically ~2 for both GaN and Al_0.25Ga_0.75N for biases up to ~2/3 of V_F. This is consistent with recombination being the dominant current transport in this bias range. At high voltages, n was typically ~1.5 for both types of rectifiers, indicating that diffusion currents were dominant. Beyond ~2×V_F, series resistance effects controlled the current. This behavior is often reported for SiC junction rectifiers, while Schottky rectifiers in that materials system show ideality factors of 1.1–1.4. In our GaN devices, the higher ideality factors may reflect the high compensation levels in the material.

Figure 25 shows the reverse current (I_R) at –100 V reverse bias for GaN rectifiers of different contact diameter, for three different measurement temperatures. Since I_R µ contact diameter, this indicates that under these conditions the reverse current originates from surface periphery leakage. Similar results were obtained for the GaN rectifiers as shown in Figure 26. The activation energy for this periphery leakage was ~0.13 eV, which may represent the most prominent surface state giving rise to the current. At voltages approximately 90% of the breakdown values, the reverse current was proportional to contact area, indicating that bulk leakage is dominant under these conditions.

In conclusion, the temperature dependence of V_RB has been measured in high breakdown GaN and AlGaN Schottky rectifiers. The temperature coefficient is negative, which is a significant disadvantage for devices intended for high temperature operation, and there are indications that it is a function of V_RB. The forward current conduction makes a transition from recombination to diffusion currents. The reverse leakage current originates from surface components around the rectifying contact at modest voltages. This current is thermally activated with an energy of 0.13 eV. The yield of acceptable devices (i.e., with V_RB at least 90% of the maximum found on a wafer and R_ON within 50% of the best values obtained) was rather small (~15%), so there is still much development needed on both materials and processing.

3.5 Lateral Al_xGa_1–xN power rectifiers with 9.7 kV reverse breakdown voltage

There have been advances in developing GaN and AlGaN power rectifiers, which are key components of inverter modules for power flow control circuits. Vertical geometry GaN Schottky rectifiers fabricated on conducting materials typically show reverse breakdown voltages (V_B) 750 V whereas lateral devices on insulating GaN and AlGaN have V_B values up to 4.3 kV.

Since the predicted breakdown field strength in GaN is of order 2–3×10⁶ V×cm^–1 [59,70], there appears to be much room for improvement in rectifier performance and a need to understand the origin of reverse leakage currents, breakdown mechanisms, and the effect of contact spacing on V_B. In this section we report on the variation of VB with Schottky-to-ohmic contact gap spacing in Al_xGa_1–xN diodes (x = 0–0.25) employing p-guard rings and extension of the Schottky contact edge over an oxide layer for edge termination. V_B values up to 9700 kV were achieved for Al_0.25Ga_0.75N rectifiers, with breakdown still occurring at the edges of the Schottky contact. The reverse leakage current just before breakdown is dominated by bulk contributions, scaling with the area of the rectifying contact.

The rectifiers were fabricated on resistive (~10⁷ Wcm) layers of 2.5–3 µm thick GaN or AlGaN grown on c-plane Al₂O₃ substrates at 1040–1100 °C by metalorganic chemical vapor deposition. To create n⁺ regions for ohmic contacts, Si⁺ ions were implanted at 5×10¹⁴ cm^–2, 50 keV, and activated by annealing at 1150 °C for 10 s under N₂. It is important to control both the heating and cooling rates to avoid cracking of the AlGaN layer. Mg⁺ implantation at 5×10¹⁴ cm^–2, 50 keV was used to create 30 µm diameter p-guard rings at the edge of the Schottky barrier metal. The rectifying contact diameter was 124 µm in most cases, while the distance of this contact from the edge of the ohmic contact was varied from 30–100 µm. The Schottky metal was extended over a SiO₂ layer deposited by plasma-enhanced chemical vapor deposition in order to minimize field crowding. Ohmic contacts were created by lift off of e-beam evaporated Ti/Al/Pt/Au annealed at 750 °C for 30 s under N₂. The Schottky contacts were formed by lift off of e-beam evaporated Pt/Ti/Au. A schematic of the completed rectifiers is shown in Figure 27. Current–voltage (I–V) characteristics were recorded on a HP4145 parameter analyzer, with all testing performed at room temperature under a Fluorinert® ambient.

Figure 28 shows the measured V_B values for GaN and Al_0.25Ga_0.75N rectifiers as a function of the gap spacing between the rectifying and ohmic contacts. For gaps between 40 and 100 µm, V_B is essentially linearly dependent on the spacing, with slopes of 6.35×10⁵ V×cm–1 for Al_0.25Ga_0.75N and 4.0×10⁵ V×cm–1 for GaN. We assume the deviation from these values at shorter spacing is due to the fact that the p-guard ring almost covers this region. In vertical geometry diodes V_B is related to the maximum electric field strength at breakdown E_M, through the relation [57]:

where W_B is the depletion width at breakdown. In our laterally depleting devices the surface quality will dominate the onset of breakdown, which is reflected in the lower breakdown field observed. However, given the current state of defect densities in epitaxial GaN, the lateral geometry seems the most promising, for the time being, for achieving very high V_B values. Quasi-substrates of GaN, produced by thick epi-growth on mismatched substrates and subsequent removal of this template, are soon to be commercially available. In some cases the background doping in these is as low as 7.9×10¹⁵ cm^–3 which makes feasible the use of these thick (200 µm) freestanding GaN films for vertically depleting rectifiers.

Figure 29 shows some I–V characteristics from the 100 µm gap spacing GaN and Al_0.25Ga_0.75N rectifiers. The best forward turn-on voltages, V_F (defined as the forward voltage at a current density of 100 A cm^–2) was ~15 V for GaN and ~33 V for Al_0.25Ga_0.75N. These are much higher than the values obtained on more conducting GaN films, where V_F is typically 5–8 V. Note, however, that the ratio V_B/V_F is still very high for the resistive diodes, with values ranging from 294 to 423. The specific on-state resistance for a rectifier is given by

where e is the GaN permittivity, µ the carrier mobility, S and W_S are substrate resistivity and thickness, and R_C is the contact resistance. The best on-state resistances we achieved were 0.15 W×cm² for GaN and 1 W×cm² for Al_0.25Ga_0.75N, leading to figure of merits (V_B)²/R_ON of 268 MW cm^–2 and 94 MW cm^–2, respectively. At low reverse voltages (2000 V), the magnitude of the reverse current was proportional to contact diameter. As the diodes approached breakdown the reverse current was proportional to contact area, suggesting bulk leakage becomes dominant.

The variation of V_B with Al percentage in the AlGaN layer of the rectifiers is shown in Figure 30, along with the calculated bandgaps. V_B does increase with increasing bandgap Eg, but is not proportional to (Eg)^1.5 as expected from a simple theory. The presence of bulk and surface defects will have a strong influence on V_B, and these are not well controlled at this stage of AlGaN rectifier technology.

To place our results in context, Figure 31, Figure 32 and Figure 33 show a compilation of R_ON, reverse leakage current and forward turn-on voltages versus V_B data for state-of-the-art SiC and GaN Schottky diode rectifiers, respectively, together with theoretical curves for Si, 6H, and 4H–SiC and hexagonal GaN. Our results for high breakdown GaN devices show the on resistances and forward turn-on voltages are still well above the theoretical values and more work is needed to understand current conduction mechanisms, the role of residual native oxides on contact properties, and impact ionization coefficients in GaN.

In conclusion, lateral geometry Al_xGa_1-xN Schottky rectifiers employing edge termination show reverse breakdown voltages up to 9.7 kV. These breakdown voltage scale with contact spacing and the rectifiers appear promising for high power electronics applications.

3.5 Vertical and Lateral GaN Rectifiers on Free-Standing GaN Substrates

Although the GaN-based power rectifiers on sapphire substrate show impressive results, there are still numerous short-comings in these devices, including higher reverse leakage current than expected from thermionic emission, high forward turn-on voltages, negative temperature coefficients for reverse breakdown voltage, non-uniformities and the low thermal conductivity of the sapphire substrate.

Recently there have been initial reports of reverse recovery characteristics of GaN Schottky rectifiers fabricated on free-standing substrates. Those substrates have the advantages of higher thermal conductivity than sapphire and the potential for higher forward current densities and reverse breakdown voltages than lateral rectifiers fabricated in insulating substrates.

We investigated the effect of contact dimension and current flow direction (lateral versus vertical) on the on-state resistance and breakdown voltage of Gan Schottky rectifiers fabricated on free-standing GaN substrates. There is a dramatic effect of contact diameter on V_B, with the latter ranging from 6 to 700C as the diameter was decresred from 7mm to 75mm. At the lower end of this range the on-state resistance (R_ON) are exceptionally low (1.71~3.01 mW×cm^-2), producing maximum figure-of-merit (V_B²/R_ON) above 100 MW×cm^-2.

The 200 mm thick GaN quasi-substrates were grown by hydride vapor phase epitaxy on sapphire substrate, lifted-off by laser heating and then etched and polished as shown in Figure 34. The measured n-type doping concentration was ~10¹⁷ cm^-3. Mg⁺ implantation at 5´10¹⁴ cm^-2, 50keV, followed by annealing was used to create 30mm diameter p-guard rings at the edge of the Schottky contacts. The rectifying contact diameter was 75mm for the small-area device and 7mm for the large-area devices. On these latter structures the Schottky metal was extended over a SiO₂ layer deposited by rf (13.56 MHz) plasma enhanced chemical vapor deposition using SiH₄ and N₂O as the precursors. Full-area back ohmic contacts were placed on the N-face using e-beam evaporation of Ti/Al/Pt/Au. On the small-area devices we also placed ohmic contacts on the top (Ga-face) surface so that we could compare results from the lateral and vertical geometries (Figure 35). The top Schottky contacts were e-beam deposited Pt/Ti/Au in both large and small area devices. In the latter case, the Schottky-ohmic metal spacing was 30 mm. Schematics of the completed structures are shown in Figure 36. Current-voltage (I-V) characteristics were recorded on an HP 4145B parameter analyzer at 25 °C for the forward part of the characteristics, while Tektronix 370A curve tracer was used for the reverse characteristics measurement.

Figure 37 shows the I-V characteristic from the large area rectifiers. The reverse breakdown voltage (V_B) is only ~6V and is obviously far below anything of practical use. The on-state resistance (R_ON) was 3.4 W-cm² for these devices. The low V_B is in stark contrast to the values achieved in smaller devices, as described below. Since the defect density in the quasi-substrate was ~10⁵ cm^-2 as measured by combined photo-chemicals etching and atomic force microscopy, the large area rectifiers are highly likely to include one or more defects. Other workers have found that reverse bias leakage in GaN Schottky diodes occurred primarily at defects and dislocations. The figure-of-merit V_B²/R_ON had a value of 10.7 W-cm^-2 for the large area rectifiers while maximum current of ~500mA could be achieved before sample heating became a problem.

I-V characteristics from the small-area rectifiers, measured in the lateral geometry are shown in Figure 38. The V_B was ~250V, with an excellent R_ON of 1.7 mW-cm². This on-state resistance is the lowest reported for any GaN rectifiers and shows that continued improvements in surface cleaning and contact technologies for this materials system have led to a rapid maturation of our understanding of how to process these devices. The value of V_B²/R_ON was 36.5MW-cm^-2. Note that remarkable improvement in the electrical characteristics in the