High-Frequency Limits of Heterostructure Field-Effect Transistors (HFETs)

Abstract

This paper present a simplified theoretical basis for an understanding of the high frequency limits of the HFETs, and compares the performance of InP based short gate-length devices reported by various groups. For this comparison, the average velocity (V^av) in each device is extracted using the effective gate-length that includes the laterally extended gate-to-drain depletion length. The results indicate that V_av increases approximately from 2.46 x 10⁷ to 3.0 x 10⁷ cm/sec when the gate-length is reduced from 0.15 μm to 0.05 μm. This comparison clearly suggests that although reduction of the gate-length can increase the unity current-gain frequency (f_τ), it does not necessarily lead to a high unity power-gain frequency (f_max). The latter requires a low gate series resistance (r_gg'), a low source series resistance (r_ss'), and a very low drain-to-gate feedback capacitance c_dg. Since all these parameters are adversely affected by the reduction of the gate-length, the challenge of realizing practical HFETs with useful power gain of 4dB at and beyond 300 GHz, appears to be a formidable one.