Wide bandgap semiconductors show superior material properties enabling potential power device operation at higher temperatures, voltages, and switching speeds than current Si technology. As a result, a new generation of power devices is being developed for power converter applications in which traditional Si power devices show limited operation. The use of these new power semiconductor devices will allow both an important improvement in the performance of existing power converters and the development of new power converters, accounting for an increase in the efficiency of the electric energy transformations and a more rational use of the electric energy. At present, SiC and GaN are the more promising semiconductor materials for these new power devices as a consequence of their outstanding properties, commercial availability of starting material, and maturity of their technological processes. This paper presents a review of recent progresses in the development of SiC- and GaN-based power semiconductor devices together with an overall view of the state of the art of this new device generation.

A Survey of Wide Bandgap Power Semiconductor Devices

The rapid development of the RF power electronics requires the introduction of wide bandgap material due to its potential in high output power density, high operation voltage and high input impedance GaN-based RF power devices have made substantial progresses in the last decade This paper attempts to review the latest developments of the GaN HEMT technologies, including material growth, processing technologies, device epitaxial structures and MMIC designs, to achieve the state-of-the-art microwave and millimeter-wave performance The reliability and manufacturing challenges are also discussed

GaN-Based RF Power Devices and Amplifiers

Gallium-nitride power transistor (GaN HEMT) and integrated circuit technologies have matured dramatically over the last few years, and many hundreds of thousands of devices have been manufactured and fielded in applications ranging from pulsed radars and counter-IED jammers to CATV modules and fourth-generation infrastructure base-stations. GaN HEMT devices, exhibiting high power densities coupled with high breakdown voltages, have opened up the possibilities for highly efficient power amplifiers (PAs) exploiting the principles of waveform engineered designs. This paper summarizes the unique advantages of GaN HEMTs compared to other power transistor technologies, with examples of where such features have been exploited. Since RF power densities of GaN HEMTs are many times higher than other technologies, much attention has also been given to thermal management-examples of both commercial “off-the-shelf” packaging as well as custom heat-sinks are described. The very desirable feature of having accurate large-signal models for both discrete transistors and monolithic microwave integrated circuit foundry are emphasized with a number of circuit design examples. GaN HEMT technology has been a major enabler for both very broadband high-PAs and very high-efficiency designs. This paper describes examples of broadband amplifiers, as well as several of the main areas of high-efficiency amplifier design-notably Class-D, Class-E, Class-F, and Class-J approaches, Doherty PAs, envelope-tracking techniques, and Chireix outphasing.

/pdf/a-review-of-gan-on-sic-high-electron-mobility-power-4mjzeqemry.pdf

A Review of GaN on SiC High Electron-Mobility Power Transistors and MMICs

Impurity-based p-type doping in wide-band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting-diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide-band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide-band-gap bipolar electronic devices of the future.

https://science.sciencemag.org/content/sci/327/5961/60.full.pdf

Polarization-Induced Hole Doping in Wide–Band-Gap Uniaxial Semiconductor Heterostructures

We report a novel approach in fabricating high-performance enhancement mode (E-mode) AlGaN/GaN HEMTs. The fabrication technique is based on fluoride-based plasma treatment of the gate region in AlGaN/GaN HEMTs and post-gate rapid thermal annealing with an annealing temperature lower than 500/spl deg/C. Starting with a conventional depletion-mode HEMT sample, we found that fluoride-based plasma treatment can effectively shift the threshold voltage from -4.0 to 0.9 V. Most importantly, a zero transconductance (g/sub m/) was obtained at V/sub gs/=0 V, demonstrating for the first time true E-mode operation in an AlGaN/GaN HEMT. At V/sub gs/=0 V, the off-state drain leakage current is 28 /spl mu/A/mm at a drain-source bias of 6 V. The fabricated E-mode AlGaN/GaN HEMTs with 1 /spl mu/m-long gate exhibit a maximum drain current density of 310 mA/mm, a peak g/sub m/ of 148 mS/mm, a current gain cutoff frequency f/sub T/ of 10.1 GHz and a maximum oscillation frequency f/sub max/ of 34.3 GHz.

High-performance enhancement-mode AlGaN/GaN HEMTs using fluoride-based plasma treatment

GaN high-electron-mobility-transistors (HEMTs) on SiC were fabricated with field plates of various dimensions for optimum performance Great enhancement in radio frequency (RF) current-voltage swings was achieved with acceptable compromise in gain, through both reduction in the trapping effect and increase in breakdown voltages When biased at 120 V, a continuous wave output power density of 322 W/mm and power-added efficiency (PAE) of 548% at 4 GHz were obtained using devices with dimensions of 055/spl times/246 /spl mu/m/sup 2/ and a field-plate length of 11 /spl mu/m Devices with a shorter field plate of 09 /spl mu/m also generated 306 W/mm with 496% PAE at 8 GHz Such ultrahigh power densities are a dramatic improvement over the 10-12 W/mm values attained by conventional gate GaN-based HEMTs

30-W/mm GaN HEMTs by field plate optimization

Field plate technologies have dramatically raised the benchmarks of GaN-based high-electron-mobility transistors (HEMTs). Greater than 30 W/mm power density was demonstrated with gate-connected field plates'. The drawback of additional feedback capacitances added by the field plates was then addressed using source-termination, achieving 21dB large-signal gain and 20-W/mm power density at 4 GHz"l. Recently, multiple field plates were pursued for further improvements""v I. Here we present double field-plated GaN HEMTs with increased power density and robustness. The devices in this study consisted of a Cree HPSI SiC substrate, a 2-4 ptm thick insulating GaN buffer, a thin AlN interlayer and an Al0.26Gao.74N barrier layer. The GaN buffer was doped with Fe for enhanced resistivity and the AlN interlay was included to achieve a high charge-mobility product without the complication of increasing the Al mole fraction of the top AlGaN layer. The device has a first field plate (FP1) integrated with the gate for both reduced gate resistance and elimination of electron trapping. The task of further tailoring the electric field and attaining a higher breakdown voltage is accomplished by a second field plate (FP2), placed on the drain side of the first field plate. FP2 is electrically connected to the source of the HEMT to minimize feedback capacitance. When designed properly, the double field-plated devices can offer a more optimal electric field distribution, improving performance and robustness. Targeting high-power operation at C band, the length of FP1 was set at LF1=0.3-0.5 ptm and FP2 at LF2=0.9-1.2 ptm. The SiN dielectric thickness under FP1 and FP2 was 100 nm and 200 nm, respectively. The device fabrication steps were similar to previous reports,"" except for the gate formation, where the integrated gate and FP1 were deposited on the SiN layer with a previously etched gate opening. Devices of four configurations were fabricated for a direct comparison. Device A had no field plate. Device B had double field plates, both connected to the gate. Device C had double field plates, FP, connected to the gate and FP2 connected to the source. Device D had a single field plate connected to the source. The gate length was about 0.55 ptm and gate-drain separation was 3.5 ptm. Typical devices showed -4 V pinch-off voltage and >1.2 A/mm full channel current. While circuit element extraction from S-parameters revealed practically the same current gain cut-off frequency of 30-35 GHz for the intrinsic devices, the maximum stable gains (MSG) varied based on the extrinsic parasitics. In particular, with LF1=0.3 pim and LF2=0.9 pim, MSG values at 10-GHz and 41 V for devices A, B, C and D were 15.6 dB, 11.2 dB, 16.7 dB and 17.1dB, respectively. It is expected that device B with both field plates connected to the gate has a high feedback capacitance, hence a much lower MSG than the non-field-pate device A. With FP2 connected to the source, however, device C actually exhibited higher MSG than the non-field-pate device. This is attributed to the Faraday shielding effect by the source field plate, which reduces the feedback capacitance. Although device D, with a single field plate connected to the source, showed 0.4-dB higher gain than device C, the less-optimum electric field distribution made it less robust and more prone to degradation at high operation voltages. Power measurements were performed with a load-pull system at 4 GHz. As intended, device C showed the best combination of output power, gain, power-added efficiency and robustness. A 246-pim-wide device with LF1=0.5 pim and LF2=1 .2 pim was able to be biased at 135 V and achieved a continuous-wave (CW) power density of 41.4 W/mm, along with 16-dB associated gain and 60% PAE. This is a significant improvement over previous result of 32.2 W/mm, 14 dB associated gain and 54.8% PAE by single-field-plated GaN HEMTs. Initial reliability tests showed that the double-fieldplated device had no degradation after 100-hour RF operation at 80 V while generating CW output power of 25 W/mm. In summary, a double-field-plate structure has been developed to extend the performance limit of microwave GaN HEMTs. The first field plate offers a high gate conductance and prevents the onset of trapping; while the 2nd field plate maximizes operation voltage without additional feedback capacitances. 41.4 W/mm CW power density was obtained, establishing a new state-of-the-art for microwave devices.

40-W/mm Double Field-plated GaN HEMTs

A semiconductor device, and particularly a high electron mobility transistor (HEMT), having a plurality of epitaxial layers and experiencing an operating (E) field. A negative ion region in the epitaxial layers to counter the operating (E) field. One method for fabricating a semiconductor device comprises providing a substrate and growing epitaxial layers on the substrate. Negative ions are introduced into the epitaxial layers to form a negative ion region to counter operating electric (E) fields in the semiconductor device. Contacts can be deposited on the epitaxial layers, either before or after formation of the negative ion region.

Robust transistors with fluorine treatment

GaN HEMTs with field-plates connected to the source terminal have been developed for high-gain, high-voltage operation at microwave frequencies Due to the reduced feedback capacitance compared to the gate-terminated field-plate structures, improvement in large-signal gain of 5-7 dB is obtained Superior performance including 21-dB associated gain, 20-W/mm output power and 60% power-added-efficiency at 4 GHz and 118V bias, is achieved simultaneously This translates to an extremely high voltage-frequency-gain product approaching 10 kV-GHz

High-gain microwave GaN HEMTs with source-terminated field-plates

Recently, electric field modification with GaN-based high-electron-mobility-transistors (HEMTs) using field plates (FP) has resulted in dramatically enhanced power performance. Power densities up to 32 W/mm at 4 GHz have been demonstrated with power-added-efficiency (PAE) of 55%. When scaled to a large periphery, a total output power of 149 W was obtained at 2 GHz. Modern communication applications also require high linearity for power devices. Here we present the linearity performance of GaN-channel HEMTs with various FP lengths at biases up to 108V.

T. Wisleder

Papers

30-W/mm GaN HEMTs by field plate optimization

40-W/mm Double Field-plated GaN HEMTs

Robust transistors with fluorine treatment

High-gain microwave GaN HEMTs with source-terminated field-plates

Linearity performance of GaN HEMTs with field plates