Current—Voltage characteristics of ungated GaAs FET's

We develop a model which describes the current-voltage characteristics of GaAs saturated resistor loads (or ungated FET's) with uniform and nonuniform (ion-implanted) doping profiles. The results of the calculation are in good agreement with the experimental data for 1-, 2-, and 3-µm GaAs ungated FET's. Our model allows us to determine the values of the electron saturation velocity νsand of the surface built-in voltage VSbifrom the measured current-voltage characteristics of ungated loads. For long devices (with 3-µm length) we obtainνₛ ≈ 1.20-1.21 × 10⁵m/s andVSbi ≈ 0.46-0.47V, in good agreement with expected values. For shorter (1 µm) devices, the measured values of νsare considerably higher (1.64-1.73 × 105m/s). This may be considered as evidence of velocity enhancement in short structures due to ballistic or overshoot effects.