Adam William Saxler

40-W/mm Double Field-plated GaN HEMTs

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 GHzl. Recently, multiple field plates were pursued for further improvementsv 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.

Wide bandgap semiconductor devices and MMICs for RF power applications

High power densities of 5.2 W/mm and 63% power added efficiency (PAE) have been demonstrated for SiC MESFETs at 3.5 GHz. Wide bandwidth MMICs have also been demonstrated with SiC MESFETs, yielding 37 W at 3.5 GHz. Even higher power densities have been obtained with GaN HEMTs, showing up to 12 W/mm under pulsed conditions. Hybrid amplifiers using GaN HEMTs on SiC substrates have demonstrated a pulsed output power level of 50.1 W, with 8 dB gain and PAE of 28% at 10 GHz, and CW power levels of 36 W have also been obtained. A wide bandwidth GaN MMIC amplifier had a peak pulsed power level of 24.2 watts, with a gain of 12.8 dB and PAE of 22% at 16 GHz.

Applications of SiC MESFETs and GaN HEMTs in power amplifier design

Very high power densities have been shown for both SiC MESFET and GaN HEMT devices. Both of these active devices benefit from the high breakdown voltages afforded by their wide-bandgap semiconductor properties. The GaN device also benefits from current densities as high as 1 A/mm. This high power density, along with good efficiency and linearity, provide an excellent base for future military and commercial power amplifier applications. High power densities are possible using narrow band power-matching networks. Although the gain-bandwidth limitation is exacerbated due to the high-impedance load lines required, high power design is possible even over multi-octave bandwidths.

3.5-watt AlGaN/GaN HEMTs and amplifiers at 35 GHz

Sub-0.2-/spl mu/m AlGaN/GaN HEMTs were successfully scaled to 1.05 mm gate-width with minor gain reduction. On-chip single-stage amplifiers exhibited gains of 8 dB and 7.5 dB, as well as output powers of 3.6 W and 3.5 W, at 30 GHz and 35 GHz, respectively. This multi-watt output power at millimeter-wave frequencies well exceeded previous state-of-the-art for a GaN HEMT and is comparable to that from 6-7 times larger GaAs-based devices.

Linearity performance of GaN HEMTs with 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.

8-watt GaN HEMTs at millimeter-wave frequencies

Field-plated short-gate-length GaN HEMTs were developed for superior large-signal performance at millimeter-wave frequencies. 100-mum-wide devices achieved 8.6 W/mm power density at 40 GHz. Scaled-up, pre-matched 1.05-mm-wide devices generated 5.4 & 5.2 W output power with associated PAE of 36 & 31 % at 30 and 35 GHz, respectively. A 1.5-mm-wide device produced 8 W at 30 GHz with 31 % PAE, representing the state-of-the-art for GaN HEMTs at millimeter-wave frequencies

Field-plated GaN HEMTs and amplifiers

Field-plates remarkably enhanced large-signal performance of GaN HEMTs by reducing trapping effect and increasing breakdown voltages. Power densities exceeding 30W/mm at 4GHz were demonstrated with gate-connected field plates. Further development of source-connected field pates boosted large-signal gain by 5-7dB, while maintaining the benefit of the field plates. Short-channel GaN HEMTs with field plates also showed promise at millimeter-wave bands. Amplifiers with 1.08-mm-wide device periphery generated 5W at 35GHz.

III-Nitride Epitaxial Material on Large-Diameter Semi-Insulating SiC Substrates for High Power RF Transistors

Metalorganic chemical vapor deposition was employed to deposit high quality, highly uniform III-Nitride transistor structures on 100 mm diameter semi-insulating 4H-SiC substrates. Electron mobility was over 2000 cm/Vs at room temperature. Sheet resistivity uniformity was as low as 0.75%. Typical standard deviations were about 1% in most properties including sheet resistivity, carrier concentration, mobility, and AlGaN composition. Additionally, wafers maintained their flat shape after deposition of these structures. Wafer bow and warp were typically less than 20 μm for optimized structures and <5 μm for the best wafers.