Small-signal characteristics of graded AlGaN channel PolFETs

We present small-signal measurements on graded AlGaN channel polarization-doped field-effect transistors (PolFETs) that show constant current gain cutoff frequency, f
_T
, and power gain cutoff frequency, f
_max
, profiles as a function of current density or gate bias. AlGaN/GaN high electron mobility transistors (HEMTs) are suitable candidates for mm-wave and THz amplifiers. However their transconductance, current gain and power gain profiles reach a peak value at low current density, and then drop off significantly as the current density is increased. Reduction in power gain with gate voltage is detrimental for linearity under large signal operation, and translates into intermodulation distortion. Transconductance and current gain are directly proportional to the channel saturation velocity, vsat. It has been shown experimentally that the saturation velocity of degenerate two-dimensional electron gas (2DEG) decreases as the electron density is increased [1]. Grading the Al composition in the channel leads to a polarization gradient in the graded region which in turn induces a 3D bulk charge [2-4]. This results in a distributed electron density with lower peak value as compared to a 2DEG, therefore the effective channel velocity is expected to be constant as a function of gate bias.