A Uniform, Reproducible and Reliable GaN HEMT Technology for Efficient High-power Applications in Space
Waltereit, P¹; Bronner, W.¹; Quay, R.¹; Dammann, M.¹; Mueller, S.¹; Kiefer, R.¹; Walcher, H.¹; Kappeler, O.¹; Mikulla, M.¹; van Rijs, F.²; Roedle, T.²; Murad, S.²; Klappe, J.²; van der Wel, P.²; Thorpe, J.³; Behtash, R.³; Blanck, H.³; Riepe, K.^3
1Fraunhofer IAF; ²NXP Semiconductors; ³United Monolithic Semiconductors

GaN is a breakthrough material offering huge enhancements in power amplifier performance compared to GaAs. Favourable material properties like wide bandgap, high breakdown and operating voltage, high thermal conductivity, high temperature operation and inherent radiation hardness enable very high RF output power at high frequencies. GaN also offers several system level benefits - compact size, high Watt per Mass ratio, lower combiner losses, high bandwidths, high robustness, facilitated thermal management and more efficient, easier DC/DC conversion by lower current and operating voltage closer to system bus voltages. GaN devices will find applications in several space segments: earth observation (T/R-module, LNA and SSPA), navigation (high power SSPA), telecommunications (robust, high linearity receiver, LNA, mixer, switch and high power SSPA) and mobile communications and multimedia (HPA, SSPA).

We report on device performance and reliability of our 3-inch GaN HEMT (high-electron-mobility-transistor) technology. AlGaN/GaN HEMT structures are grown in-house on semi-insulating SiC substrates by metal-organic chemical-vapour-deposition with sheet resistance non-uniformities better than 3% for a structure having 22% Al in the AlGaN barrier. Device fabrication is performed using standard processing techniques involving both electron-beam and stepper lithography with special emphasis on uniformity and reproducibility.

The process technology exhibits an excellent uniformity across a single wafer as well as high reproducibility between individual wafers of the same or a different batch (typically 6-8 wafers): loadpull mapping at 2 GHz across all 21 cells on an entire 3-inch wafer yields a PAE of (60±2)% with only 2% scatter of the mean PAE from wafer to wafer. HEMTs demonstrate excellent high-voltage stability and large power added efficiencies. Devices with 0.5 mm gate length exhibit two-terminal gate-drain breakdown voltages in excess of 160 V across the entire 3-inch wafer with parasitic drain currents well below 1 mA/mm when biased up to 80 V drain bias under pinch-off conditions. Load-Pull measurements at 2 GHz on 800 µm gate periphery devices having 0.5 µm gate length return both a well-behaved relationship between bias-voltage and output-power as well as power-added-efficiencies beyond 60% up to UDS=80 V. For a drain bias of 88 V an output-power-density around 15 W/mm with 24 dB linear gain and more than 55% PAE is obtained. On large periphery devices (32 mm gate width) an output power beyond 125 W is obtained on these devices with a PAE above 50% and a linear gain around 15 dB.

Reliability tests on 8x60 mm gate periphery devices having a gate length of 0.5 µm indicate a promising device stability. Less than 10% drain-current degradation under 50 V DC-stress (50 mA/mm) is observed after more than 1000 h of operation. Under 50 V RF stress (2 GHz, 50 mA/mm) after an initial drop in output power the performance of the devices is well behaved for 300 h of operation and found to be stabilized.