THE IMPACT OF ELECTRON-TRANSPORT REGIMES ON THE LINEARITY OF ALGAAS/N+-INGAAS HFETS

The metal-insulator-doped-channel FET (MIDFET) is well-suited to telecommunications power applications requiring high device linearity. We have explored the transport physics limiting linearity in the MIDFET through a systematic study of the gate length (L(g)) scaling of g(m) and f(T) vs V(GS), V(B), and I(D,MAX) in pseudomorphic Al0.38Ga0.62As/n+-In0.15Ga0.85As MIDFETs with L(g) between 1.7 and 50 mum. Our devices reveal three distinct regimes, with g(m), f(T), and f(max) rising linearly with V(GS) just above threshold, flattening into broad plateaus at higher V(GS), and finally declining. The L(g) scaling of the slopes of both g(m) and f(T) vs V(GS) demonstrates that the MIDFET is mobility-limited for small V(GS). For larger V(GS), the scaling of the plateau values of both g(m) and f(T) shows that devices with L(g) < 4 mum enter velocitiy saturation (v(sat)) limited transport. This regime, together with a large, L(g)-independent drain-source breakdown voltage of 19V, results in the MIDFET's linear, high-power capability. For sufficiently high V(GS), both g(m) and f(T) decline due to v(sat) in the extrinsic device in short gate devices (L(g) < 4 mum) and to gate leakage in longer gate devices. Our results provide key insights for optimizing the MIDFET design for many important telecommunications applications.