Comparison of GaN/AlGaN/AlN/GaN HEMTs Grown on Sapphire with Fe-Modulation-Doped and Unintentionally Doped GaN Buffer: Material Growth and Device Fabrication

AlGaN/GaN high electron mobility transistors (HEMTs) grown on Fe-modulation-doped (MD) and unintentionally doped (UID) GaN buffer layers are investigated and compared. Highly resistive GaN buffers (10^9 \Omega\cdotcm) are induced by individual mechanisms for the electron traps' formation: the Fe MD buffer (sample A) and the UID buffer with high density of edge-type dislocations (7.24\times10^9 cm^-2, sample B). The 300 K Hall test indicates that the mobility of sample A with Fe doping (2503 cm^2V^-1s^-1) is much higher than sample B (1926 cm^2V^-1s^-1) due to the decreased scattering effect on the two-dimensional electron gas. HEMT devices are fabricated on the two samples and pulsed I–V measurements are conducted. Device A shows better gate pinch-off characteristics and a higher threshold voltage (-2.63 V) compared with device B (-3.71 V). Lower gate leakage current |I_\rm GS| of device A (3.32\times10^-7 A) is present compared with that of device B (8.29\times10^-7 A). When the off-state quiescent points Q_2 (V_\rm GQ2=-8 V, V_\rm DQ2=0 V) are on, V_\rm th hardly shifts for device A while device B shows +0.21 V positive threshold voltage shift, resulting from the existence of electron traps associated with the dislocations in the UID-GaN buffer layer under the gate. Under pulsed I–V and transconductance G_\rm m–V_\rm GS measurement, the device with the Fe MD-doped buffer shows more potential in improving reliability upon off-state stress.