InAs Channel Inset Effects on the DC, RF, and Noise Properties of InP pHEMTs

GaInAs/InAs composite channels in InP-based pHEMTs enable wideband and/or low-noise performances because of their superior carrier transport properties. To date, the influence of the InAs inset design details on transistor performance has not been parametrized in the literature. We present a systematic study of the effects of the InAs channel inset thickness on transistor characteristics and cutoff frequencies versus temperature, and on the noise performance at 300 K. The epitaxial layer structures considered here incorporate 2 to 5-nm InAs insets in a fixed total composite channel thickness. All layers exhibit excellent electron mobilities (from 40200 to 54800 cm2/Vs at 77 K). Thicker InAs insets improve both the current gain cutoff frequency (f
_T
) and the maximum oscillation frequency (f
_MAX
). However, they also result in higher gate leakage currents and increased channel impact ionization. 50-nm gate length pHEMTs with a 5-nm InAs inset feature the highest simultaneous f
_T
/f
_MAX
≥ 390/675 (455/800) GHz at 300 (15) K for a low-noise bias but exhibit the poorest minimum noise figure NFMIN. Whereas higher f
_T
(and/or f
_MAX
) values have traditionally been associated with improved noise performances, this is no longer the case.