GaN HEMT and MMIC development at Fraunhofer IAF: performance and reliability

We present a systematic study of epitaxial growth, processing technology, device performance and reliability of our GaN HEMTs and MMICs manufactured on 3 inch SiC substrates. Epitaxy and processing are optimized for both performance and reliability. The deposition of the AlGaN/GaN HEMT epitaxial structures is designed for low background carrier concentration and a low trap density in order to simultaneously achieve a high buffer isolation and low DC to RF dispersion. Device fabrication is performed using standard processing techniques involving both electron-beam and stepper lithography. Gate lengths of 250 nm and 500 nm are employed for 10 GHz and 2 GHz applications, respectively. The developed HEMTs demonstrate excellent high-voltage stability, high power performance and large power added efficiencies. Devices exhibit two-terminal gate―drain breakdown voltages in excess of 160 V (current criterion 1 mA/mm) across the entire 3 inch wafer with parasitic gate and 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 width devices return a well-behaved relationship between bias-voltage and output-power as well as power-added- efficiencies beyond 60% up to U DS = 100 V. For a drain bias of 100 V an output-power-density around 22 W/mm with 26 dB linear gain is obtained. On large devices (32 mm gate width packaged in industry-standard ceramic packages) an output power beyond 100 W is achieved with a PAE above 50% and a linear gain around 15 dB. Dual-stage MMICs in microstrip transmission line technology yield a power added efficiency of 40% at 8.56 GHz for a power level of 11 W. A single-stage MMIC yields a PAE of 46% with 7 W of output power at V DS = 28 V. Reliability is tested on HEMT devices having a gate periphery of 8 x 60 μm at an operating bias of 50 V under both DC and RF conditions. About 10% drain-current change under DC-stress (50 mA/mm) is observed af- ter more than 1000 h of operation with an extrapolated drain- current degradation below 20% after 200000 h (more than 20 years) of operation. Under RF stress (2 GHz, 1 dB compression) the observed change in output power density is be- low 0.2 dB after more than 1000 h.