Delay time analysis of AlGaAs/In xGa 1− xAs (0.15≤×≤0.45) pseudomorphic HJFETs with InGaAs channel grown under suppressed In-surface segregation conditions

Abstract

This study uses delay time analysis to investigate the dependence of current gain cutoff frequency ( f T ) on the In composition of an InGaAs layer grown by MBE under suppressed In surface segregation conditions for an AlGaAs/InGaAs hetero junction field effect transistor (HJFET). The experimental results show that the maximum f T is obtained at an In composition ( X In) of 0.30, in the range from 0.15 to 0.45. Delay time analysis reveals that the tendency of f T is determined mainly by the channel charging delay time ( τ char). To explain the dependence of τ char on In composition, the novel expression for τ char is proposed. From this expression, it is found that the decrease in f T at X In>0.30 is caused mainly by the increase in sheet resistance (1/ q N s μ) of the InGaAs layer. This shows that the improved electron mobility resulting from suppression of In surface segregation is useful in reducing τ char. Furthermore, use of an InGaAs layer thickness above 100 Å at X In≥0.30 can effectively reduce τ char. Delay time analysis also reveals that the saturation velocity is saturated at X In=0.30 and declines at X In>0.35. Consequently optimum In composition and InGaAs thickness are proposed to be 0.30 and 110 Å for an AlGaAs/InGaAs HJFET on a GaAs substrate.