Comparison of carrier velocity in 0.25 mu m gate-length conventional, pseudomorphic, and lattice-matched modulation-doped FETs at 300 K and 77 K

The microwave characterization of 0.25- mu m gate-length MODFETs at 300 and 77 K is reported. Microwave S-parameter measurements of MODFETs from 0.45 to 20 GHz were used to extract their HF equivalent network parameters. After allowing for the parasitic gate and drain-to-gate feedback capacitances, the intrinsic unity-current gain frequency was determined. Using a theoretical model, the effective carrier saturation velocity and average velocity ( nu /sub av/) were obtained for the conventional AlGaAs/GaAs, the pseudomorphic AlGaAs/InGaAs/GaAs, and lattice-matched InAlAs/InGaAs/InAlAs on InP samples at 300 K and 77 K. The extracted values of nu /sub a/ range from 0.82*10/sup 7/ cm/s to 3.84*10/sup 7/ cm/s, respectively, depending on the type of MODFET and operating temperature. These high values of nu /sub av/ indicate that the velocity overshoot effect is present in the devices tested.<
>