Showing papers by "Stephen J. Pearton published in 2000"

The Blue Laser Diode: The Complete Story

Abstract: The story of Shuji Nakamura and the blue laser diode is remarkable. It is clear from this book that he enjoys this fact and wishes his readers to become familiar with his success. Nakamura was a little known researcher at a small but successful Japanese company, Nichia Chemical, on Shikoku, one of Japan's four main islands. One of their successful lines was phosphors for fluorescent lights. In 1989, Nakamura was given a few million dollars by the company's Chairman Nobuo Ogawa. Nakamura chose to research into blue light emitters using gallium nitride, a material that had been studied by Pankove at RCA some 20 years earlier and largely written off by the conventional semiconductor industry. In spite of many factors against progress, this second edition of The Blue Laser Diode testifies to the success of this gamble. The book is subtitled `The complete story'. This is an unlikely epithet for the book because there is still a long way to go. The book is written with a mixture of academic integrity and commercial trumpet blowing. There is too often a lack of detail and logical order. There is inadequate discussion of the case for and against other materials such as ZnSe. One feels that the commercial pressure not to give away all the answers about gallium nitride has triumphed over the wish for scientific disclosure to enable results to be repeated. The book clearly reports the two most significant difficulties faced by gallium nitride. First, it appeared from Pankove's work that it would not to be easy to find an appropriate p-type dopant that could make suitable p-n junctions. Two chapters consider this problem, starting with low energy electron beam irradiation and then in the second chapter considering thermal annealing in nitrogen. The writing and detail suggest that it is still a technology rather than science (or, perhaps more unkindly, cook-book recipes of time and temperature). The second important difficulty is that gallium nitride has too many dislocations for long-life laser action. Growth on sapphire with appropriate buffer layers is described as an initial step in reducing the dislocations. Later in the book, it is recognized that InxGa1-xN offers greater versatility, and this is considered in more detail along with InGaN/AlGaN double heterostructures. Regrettably it is not easy though to dig out from this book all the details of lattice matching that are required and how successful lattice matching has been in removing dislocations and increasing lifetime. Clearly the general trend of longer lifetimes means that there has been useful success. Blue laser diodes are now claimed to be commercially available with lifetimes measured in thousands of hours while blue light emitting diodes, with their lower current densities, are said to have lifetimes measurable in years. The book has a little for everyone. Applications are noted briefly as well as blow-by-blow accounts of the manufacturing technology of double heterostructure, multi-quantum well lasers and progress to room temperature operation. Applications range from the mundane traffic light, through full colour displays to 15-20 Gbyte optically read data storage discs. Interestingly it is the mundane applications that may have the biggest financial impact. A statistic that appears on the Internet is that if all the traffic signals in Japan could be switched to suitable LEDs then one could save the construction of at least one nuclear power plant. Although Nakamura is an admirer of Pankove's work, the writing and scientific style does not match that of Pankove. Nevertheless the book records a thorough solid achievement and as such there should be a similar solid basis for many readers in materials science and laser technology wishing to read this book. The book regrettably gives no indication why Nakamura has left Nichia for a Professorship at Santa Barbara after such magnificent early support by Nichia. Nor does the book explain why Nakamura's co-author Gerhard Fasol is undercutting the joint venture by selling for $15 over the web a 28 page summary about blue laser diodes using gallium nitride. However, if price is no consideration, there is also advertised on the web a 222 page report SC-23 from Strategies Unlimited entitled `Gallium Nitride 2000 - Technology Status, Applications, and Market Forecast' for a modest $3950. Clearly the present book cannot be `the complete story'. That will run for quite a time yet. John Carroll

Fabrication and performance of GaN electronic devices

Abstract: 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. Although the growth and processing technology for SiC, the other viable wide bandgap semiconductor material, is more mature, the AlGaInN system offers numerous advantages. These include wider 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) 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, including heterostructure field effect transistors (HFETs), Schottky and p–i–n rectifiers, heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs) and metal-oxide semiconductor field effect transistors (MOSFETs). This review focuses on the development of fabrication processes for these devices and the current state-of-the-art in device performance, for all of these structures. We also detail areas where more work is needed, such as reducing defect densities and purity of epitaxial layers, the need for substrates and improved oxides and insulators, improved p-type doping and contacts and an understanding of the basic growth mechanisms.

A Review of Dry Etching of GaN and Related Materials

Abstract: The characteristics of dry etching of the AlGaInN materials system in different reactor types and plasma chemistries are reviewed, along with the depth and thermal stability of etch-induced damage. The application to device processing for both electronics and photonics is also discussed.

GaN and related materials II

Abstract: 1, Laser Diodes 2. GaN and AlGaN Devices: Field Effect Transistors and Photodetectors 3. Growth and Doping of and Defects in III-Nitrides 4. Structural and Electronic Properties of AlGaN 5. Theory of Laser Gain in Group III-Nitride Quantum Wells 6. Electronic and Optical Properties of Bulk and QW Structure 7. Materials Theory Based Modelling of GaN Devices 8. Erbium Doping of III-V Nitrides 9. Thermodynamic and Electronic Properties of GaN and Related Alloys 10. GaN Device Processing 11. Contacts to GaN 12. Ion Implantation Advances in Group III-Nitride Semiconductors 13. Inductively Coupled Plasma Etching of III-V Nitrides 14. Low Energy Electron Enhanced Etching (LE4) of III-N Materials

Gd2O3/GaN metal-oxide-semiconductor field-effect transistor

Abstract: Gd2O3 has been deposited epitaxially on GaN using elemental Gd and an electron cyclotron resonance oxygen plasma in a gas-source molecular beam epitaxy system. Cross-sectional transmission electron microscopy shows a high concentration of dislocations which arise from the large lattice mismatch between the two materials. GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated using a dielectric stack of single crystal Gd2O3 and amorphous SiO2 show modulation at gate voltages up to 7 V and are operational at source drain voltages up to 80 V. This work represents demonstrations of single crystal growth of Gd2O3 on GaN and of a GaN MOSFET using Gd2O3 in the gate dielectric.

High voltage GaN Schottky rectifiers

Abstract: Mesa and planar GaN Schottky diode rectifiers with reverse breakdown voltages (V/sub RB/) up to 550 and >2000 V, respectively, have been fabricated. The on-state resistance, R/sub ON/, was 6 m/spl Omega//spl middot/cm/sup 2/ and 0.8 /spl Omega/ cm/sup 2/, respectively, producing figure-of-merit values for (V/sub RB/)/sup 2//R/sub ON/ in the range 5-48 MW/spl middot/cm/sup -2/. At low biases the reverse leakage current was proportional to the size of the rectifying contact perimeter, while at high biases the current was proportional to the area of this contact. These results suggest that at low reverse biases, the leakage is dominated by the surface component, while at higher biases the bulk component dominates. On-state voltages were 3.5 V for the 550 V diodes and /spl ges/15 for the 2 kV diodes. Reverse recovery times were <0.2 /spl mu/s for devices switched from a forward current density of /spl sim/500 A/spl middot/cm/sup -2/ to a reverse bias of 100 V.

Inductively coupled plasma-induced etch damage of GaN p-n junctions

Abstract: Plasma-induced etch damage can degrade the electrical and optical performance of III-V nitride electronic and photonic devices. We have investigated the etch-induced damage of an Inductively Coupled Plasma (ICP) etch system on the electrical performance of mesa-isolated GaN pn-junction diodes. GaN p-i-n mesa diodes were formed by Cl{sub 2}/BCl{sub 3}/Ar ICP etching under different plasma conditions. The reverse leakage current in the mesa diodes showed a strong relationship to chamber pressure, ion energy, and plasma flux. Plasma induced damage was minimized at moderate flux conditions ({le} 500 W), pressures {ge}2 mTorr, and at ion energies below approximately -275 V.

GaN n- and p-type Schottky diodes: Effect of dry etch damage

Abstract: The reverse breakdown voltage (V/sub B/) and forward turn-on voltage (V/sub F/) of n- and p-GaN Schottky diodes were used to examine the effects of Cl/sub 2//Ar and Ar plasma damage. Even short plasma exposures (4 secs) produced large changes in both V/sub B/ and V/sub F/, with ion mass being a critical factor in determining the magnitude of the changes. The damage depth was established to be 500-600 /spl Aring/ and the damaged material could be removed in boiling NaOH solutions, producing a full recovery of the diode properties. Annealing at 700 to 800/spl deg/C under N/sub 2/ produced only a partial recovery of V/sub B/ and V/sub F/.

Low Temperature Silicon Nitride and Silicon Dioxide Film Processing by Inductively Coupled Plasma Chemical Vapor Deposition

Abstract: High‐density plasma technology is becoming increasingly attractive for the deposition of dielectric films such as silicon nitride and silicon dioxide. In particular, inductively coupled plasma chemical vapor deposition (ICPCVD) offers a great advantage for low‐temperature processing over plasma‐enhanced chemical vapor deposition (PECVD) for a range of devices including compound semiconductors and magnetic heads. In this paper, the development of low temperature (<200°C) silicon nitride and silicon dioxide films utilizing ICP technology is discussed. The material properties of these films have been investigated as a function of ICP source power, radio‐frequency chuck power, chamber pressure, gas chemistry, and temperature. The ICPCVD films are compared to PECVD films in terms of wet etch rate, stress, and other film characteristics. Two different gas chemistries, and , were explored for the deposition of ICPCVD silicon nitride. The ICPCVD silicon dioxide films were prepared from . The wet etch rates of both silicon nitride and silicon dioxide films are significantly lower than films prepared by conventional PECVD. This implies that ICPCVD films prepared at these low temperatures are of higher quality. The advanced ICPCVD technology can also be used for efficient void‐free filling of high aspect ratio (3:1) sub‐micrometer trenches. © 2000 The Electrochemical Society. All rights reserved.

Creation of high resistivity GaN by implantation of Ti, O, Fe, or Cr

Abstract: Implantation of n- and p-type GaN with Ti+, O+, Fe+, or Cr+ was found to produce defect levels which pinned the Fermi level in these materials at EC−(0.20–0.49) eV (n type) or EV+0.44 eV (p type). Maximum sheet resistances of ∼1012 Ω/□ (n type) and ∼1010 Ω/□ (p type) were obtained after implantation and annealing in the range of 300–600 °C. At higher annealing temperatures, the sheet resistance decreased to near the unimplanted values (3×104 Ω/□ in p type, 7×102 Ω/□ in n type). The evolution of the sheet resistance with annealing temperature is consistent with damage-related trap sites removing carriers from the conduction or valence bands.

Ultradeep, low-damage dry etching of SiC

Abstract: The Schottky barrier height (Phi(B)) and reverse breakdown voltage (V-B) of Au/n-SiC diodes were used to examine the effect of inductively coupled plasma SF6/O-2 discharges on the near-surface elec ...

Advanced selective dry etching of GaAs/AlGaAs in high density inductively coupled plasmas

Abstract: We report a breakthrough for selective etching of GaAs over AlxGa1−xAs, x=0.2, layer with a high density plasma source. This result is particularly important for III–V devices such as heterojunction bipolar transistors (HBTs) or high electron mobility transistors (HEMTs). For example, fabrication of HBTs requires a process for selective etching of a GaAs contact layer while stopping on AlGaAs layer. Inductively coupled plasma (ICP) etching with BCl3/SF6/N2/He chemistries showed extremely high selectivity of GaAs over AlGaAs (>200:1) and a photoresist (>10:1). This process also produced excellent sidewall passivation on GaAs with reasonably high rate (>1500 A /min.). Both scanning electron microscope and atomic force microscope data showed AlGaAs etch stop layer was quite smooth after processing. We found that He played a key role in enhancing selectivity and obtaining smooth AlGaAs surfaces. When used with resist masks, addition of N2 into BCl3/SF6 plasma helped formation of passivation on the sidewall an...

Schottky diode measurements of dry etch damage in n- and p-type GaN

Abstract: n- and p-type GaN was exposed to inductively coupled plasma of N2, H2, Ar, or Cl2/Ar, as a function of source power (0–1000 W) and rf chuck power (20–250 W). For n-GaN, there was a strong reduction in diode reverse breakdown voltage and an increase in forward and reverse currents, while for p-GaN the reverse breakdown voltage increased. These results are consistent with creation of point defects with shallow donor nature that increase the conductivity of initially n-type GaN or decrease the conductivity of p-type GaN. Annealing at 750 °C under N2 produced significant recovery of the electrical properties, while wet etch removal of 500–600 A of the surface produced a full recovery. For completed n-type mesa diodes exposed to Ar or Cl2/Ar discharges, the low bias forward currents increased by several orders of magnitude. The exposed surfaces became N2 deficient in all cases.

Strong surface disorder and loss of N produced by ion bombardment of GaN

Abstract: The damage buildup in wurtzite GaN films under light (12C) and heavy (197Au) ion bombardment at temperatures from −196 to 550 °C is studied by Rutherford backscattering/channeling spectrometry. A strong surface peak of lattice disorder in addition to the expected damage peak in the region of the maximum of nuclear energy loss has been observed for all implant conditions of this study. Capping of GaN with SiOx and SixNy layers prior to implantation somewhat reduces but does not eliminate surface disordering. This suggests that nitrogen loss is not the main reason for the observed enhanced surface disorder, but, rather, the GaN surface acts as a strong sink for migrating point defects. However, pronounced loss of N during ion bombardment is observed for high dose implantation when the near-surface region is amorphized. Moreover, after amorphization, annealing at temperatures above about 400 °C leads to complete decomposition of the near-surface layer.

Temperature dependence of GaN high breakdown voltage diode rectifiers

Abstract: The temperature dependence of reverse breakdown voltage (VRB) and forward turn-on voltage (VF) of GaN Schottky diode rectifiers is reported. The VRB values display a negative temperature coefficient (−0.92 V K−1 for 25–50°C; −0.17 V K−1 for 50−150°C), indicative of surface- or defect-assisted breakdown. The VF values decrease with increasing temperature. The room temperature breakdown voltage is approximately a factor of three lower than the theoretical maximum expected based on avalanche breakdown, and the current performance of GaN rectifiers is comparable to that of Si at the same on-resistance.

Contact resistivity and transport mechanisms in W contacts to p- and n-GaN

Abstract: The electrical properties of W contacts to both n- and p-GaN were investigated. W contacts to p-type GaN doped with Mg to a level of 1018 cm−3 were annealed for 1 min at temperatures from 350 to 900 °C. The contact resistivity was found to decrease with increasing annealing temperature parallel to an increase in the GaN sheet resistance. The contacts were rectifying after all of the heat treatments. Measurements at higher temperatures (up to 400 °C) resulted in I–V characteristics becoming almost linear and a decrease in contact resistivity with temperature down to 10−2 Ω cm2. These results are attributed to the ionization of more Mg acceptors as the temperature increases. In this system thermionic emission combined with tunneling through deep energy levels was found to be the transport mechanism. W contacts to heavily Si implanted (Nd∼1020 cm−3) n-GaN annealed at 750–1050 °C for 10 s produced ohmic behavior with no significant dependence of the contact resistivity on the annealing temperature. The observed weak dependence of the contact resistivity on the measurement temperature is attributed to the dominance of the field emission mechanism.

Plasma damage in p-GaN

Abstract: The effect of Inductively Coupled Plasma H2 or Ar discharges on the breakdown voltage of p-GaN diodes was measured over a range of ion energies and fluxes. The main effect of plasma exposure is a decrease in net acceptor concentration to depths of 400–550 A. At high ion fluxes or energies there can be type conversion of the initially p-GaN surface. Post etch annealing at 900°C restores the initial conductivity.

A unified global self-consistent model of a capacitively and inductively coupled plasma etching system

Abstract: Based on the concept of independent control of ion flux and ion-bombardment energy, a global selfconsistent model was proposed for etching in a high-density plasma reactor. This model takes account of the effect on the plasma behavior of separate rf chuck power in an Inductively Coupled Plasma etching system. Model predictions showed that the chuck power controls the ion bombardment energy but also slightly increases the ion density entering the sheath layer, resulting in an increase in etch rate (or etch yield) with increasing this rf chuck power. The contribution of the capacitive discharge to total ion flux in the ICP etching process is less than about 6% at rf chuck powers lower than 250 W. As a model system, etching of InN was investigated. The etch yield increased monotonically with increasing the rf chuck power, and was substantially affected by the ICP source power and pressure. The ion flux increased monotonically with increasing the source power, while the dc-bias voltage showed the reverse trend.

Surface and bulk leakage currents in high breakdown GaN rectifiers

Abstract: GaN Schottky diode rectifiers with contact diameters 125–1100 μm were fabricated on thick (4 μm) epi layers. At low reverse bias voltages the leakage current was proportional to contact perimeter size while at voltages approximately half the breakdown value, the reverse current was proportional to contact area. These results suggest that surface leakage dominated at low biases, while at higher biases the main contribution was from bulk leakage. The reverse leakage currents were several orders of magnitude higher than the theoretical values, while the forward turn-on voltages were approximately a factor of two higher than the theoretical value.

GaN PN junction issues and developments

Abstract: Critical nitride-based p-n junction issues relating to wide bandgap bipolar device performance include minority carrier lifetime, defect related current characteristics and ohmic contact properties. Recent developments in p-GaN deposition processes resulted in GaN p-i-n UV photodetectors with improved deep UV responsivity, visible light rejection and shunt resistance characteristics. From the device data, the electron diffusion length in p-GaN doped at 1·1018 cm−3 was estimated to be 790 A, and the minority carrier lifetime in the p-GaN was estimated to be 24 ps to 0.24 ns. Improved junction electrical characteristics were achieved using MBE deposition on GaN buffers grown by MOCVD. NiAu ohmic contacts were also made to p-GaN with specific contact resistances less than 10−4 Ω·cm2.

Advanced Processing of GaN for Electronic Devices

Abstract: This review focuses on understanding and optimization of several key aspects of GaN device processing. A novel rapid thermal processing up to 1500°C, in conjunction with AlN encapsulation, has been developed. The activation processes of implanted Si or Group VI donors, and common acceptors in GaN by using this ultrahigh-temperature annealing, along with its effects on surface degradation, dopant redistribution, and damage removal have been examined. 1400 degrees has proven to be the optimum temperature to achieve high activation efficiency and to repair the ion-induced lattice defects. Ion implantation was also employed to create high-resistivity GaN. Damage-related isolation with sheet resistances of 1012 Ω/square in n-GaN and 1010 Ω/square in p-GaN has been achieved by implant of O and transition metal elements. The effects of surface cleanliness on characteristics of GaN Schottky contacts have been investigated, and the reduction in barrier height was correlated with removing the native oxide that form...

Direct-current characteristics of pnp AlGaN/GaN heterojunction bipolar transistors

Abstract: AlGaN/GaN pnp heterojunction bipolar transistors were fabricated using a low-damage dry-etch process, and the dc characteristics measured up to 250 °C. In the common–base mode, the collector current was approximately equal to the emitter current under all conditions. Although not optimized for power operations, the devices were tested up to power densities of ∼50 kW cm−2. The dc current gain was in the range 20–25 at room temperature. The pnp configuration avoids the problem of high base sheet resistance encountered with npn-AlGaN/GaN devices.

GaN/AlGaN HBT fabrication

Abstract: Discrete GaN/AlGaN heterojunction bipolar transistors (HBTs) were fabricated on material grown by both metal organic chemical vapor deposition and molecular beam epitaxy. For both types of material, DC current gains of ∼10 were achieved in 90 μm emitter diameter devices measured at 300°C. Some of the key processing steps, such as ohmic contact annealing temperature and mesa fabrication by low damage dry etching, are described, together with secondary ion mass spectrometry measurements of the dopant and background impurity profiles.

High breakdown voltage Au/Pt/GaN Schottky diodes

Abstract: Au/Pt/GaN Schottky diode rectifiers were fabricated with reverse breakdown voltage (VRB) up to 550 V on vertically depleting structures and >2000 V on lateral devices The figure-of-merit (VRB)2/RON, where RON is the on-state resistance, had values between 42 and 48 MW cm−2 The reverse leakage currents and forward on-voltages were still somewhat higher than the theoretical minimum values, but were comparable to SiC Schottky rectifiers reported in the literature These devices show promise for use in ultrahigh-power switches

Electrical properties and defect states in undoped high-resistivity GaN films used in high-power rectifiers

Abstract: The electrical properties and spectra of deep centers in high-resistivity undoped GaN samples used to fabricate high-power Schottky diode rectifiers with breakdown voltages exceeding 2000 V are reported. It is shown that the Fermi level in such material is pinned by defects with energy levels close to Ec−0.6 eV. Thermally stimulated current measurements revealed the presence of unidentified traps with activation energies of 0.3 and 0.7 eV. The important role of hole traps with energy levels Ev+0.3 eV and Ev+0.9 eV was confirmed by measurements of temperature quenching of photocurrent and by photoinduced transient current spectroscopy measurements. Prominent persistent photoconductivity was observed even for temperatures above 300 K. Imaging of the sample using microcathodoluminescence showed the existence of cellular nonuniformities with characteristic dimensions of the cells of about 3 μm.

Effect of Mg ionization efficiency on performance of Npn AlGaN/GaN heterojunction bipolar transistors

Abstract: A drift-diffusion transport model has been used to examine the performance capabilities of AlGaN/GaN Npn heterojunction bipolar transistors (HBTs). The Gummel plot from the first GaN-based HBT structure recently demonstrated is adjusted with simulation by using experimental mobility and lifetime reported in the literature. Numerical results have been explored to study the effect of the p-type Mg doping and its incomplete ionization in the base. The high base resistance induced by the deep acceptor level is found to be the cause of limiting current gain values. Increasing the operating temperature of the device activates more carriers in the base. An improvement of the simulated current gain by a factor of 2 to 4 between 25 and 300 C agrees well with the reported experimental results. A preliminary analysis of high frequency characteristics indicates substantial progress of predicted rf performances by operating the device at higher temperature due to a reduced extrinsic base resistivity.

Hydrogen in wide bandgap semiconductors

Abstract: Publisher Summary This chapter focuses on the prevalence of hydrogen in the growth and processing ambients for wide bandgap semiconductors and the effect of this hydrogen. The different configurations of hydrogen in semiconductors and the techniques for hydrogen incorporation and the diffusion behavior of different forms of hydrogen in diamond, SiC, GaN and II–VI compounds has been discussed in the chapter. It is well-established that atomic hydrogen, the much more mobile species relative to molecular hydrogen, can enter the semiconductor during growth or device processing steps and neutralize the electrical activity of both n- and p-type dopants and a variety of defect and impurity states. In GaN for example, the unintentional hydrogen passivation of Mg acceptors prevent observation of p-type doping for a time, delaying the achievement of light-emitting diodes and eventually lasers. Hydrogen plays an important role in all the wide bandgap semiconductors, producing strong passivation of acceptors in all the materials, and donor passivation also in some cases. The hydrogen is unintentionally incorporated during growth and processing and remains in the material until quite high temperatures. Reactivation of dopants occurs in the range 400–700°C depending on the material.

Cl2-based dry etching of GaN films under inductively coupled plasma conditions

Abstract: Dry etching of undoped, n- and p-type GaN films was carried out in Cl2-based inductively coupled plasmas (ICPs) using different rf excitation frequencies of 100 kHz and 13.56 MHz, in which the rf chuck power source operates. The etch rates with lower frequency of 100 kHz are somewhat greater than those with a higher frequency of 13.56 MHz due to higher ion bombarding energy with lower frequency. The highest etch rates with the 100 kHz frequency were obtained at moderately high ICP power of 700 W: ∼9300 A/min of n-GaN, ∼5300 A/min of p-GaN, and ∼7100 A/min of undoped GaN. The 13.56 MHz frequency of rf chuck power source produced maximum etch rates of ∼7900 A/min of n-GaN, ∼5800 A/min of p-GaN, and 6100 A/min of undoped GaN at 20 mTorr, 700 W ICP, and 150 W rf power. The surface roughness was relatively independent of the chuck power up to 150 W in 13.56 MHz and showed fairly smooth morphology (rms 1.1–1.3 nm), while etching at higher rf power (>200 W) produced rougher surface.

High temperature characteristics of GaN-based heterojunction bipolar transistors and bipolar junction transistors

Abstract: GaN Bipolar Junction Transistors and GaN/AlGaN Heterojunction Bipolar Transistors were characterized at temperatures up to 250–300°C and power densities >10 kW cm −2 . The breakdown voltage in both types of devices decreased at higher temperatures, with less degradation in the BJTs. At low current levels, the offset voltages were 2–3 V but increased at higher currents.