Performance evaluation of GaAs based MODFETs

Three key aspects of AlGaAs/InGaAs/GaAs MODFETs, namely current-handling capability, signal delay, and saturated output regime, are experimentally evaluated and correlated with the heterostructure configuration. Gate lengths down to 0.1 μm and corresponding cutoff frequencies above 140 GHz are employed. The three aspects are found to be closely interrelated. By the incorporation of an InGaAs quantum well the two-dimensional-electron-gas (2DEG) density of the materials system can be considerably extended. However, this results in only limited improvement for the FET current-handling capability above a 2DEG density of 2×10¹² cm^-2. The main effect on the MODFET current gain cutoff frequency is through the reduction of the input delay as demonstrated with 0.1-μm-gate-length devices. Extracting the same intrinsic delay time means that the electron dynamics in the channel of AlGaAs/GaAs and pseudomorphic MODFETs is very comparable. This is consistent with the fact that there is no significant change in the effective electron mass, although changes in the intervalley scattering dynamics are still expected. Open-circuit voltage gain and output conductance are strongly related to the space-charge-layer configuration on top of the channel. This is related to the recess configuration; however, for a large voltage gain a high structural aspect ratio is generally needed