Determination of effective saturation velocity in n+self-aligned GaAs MESFET's with submicrometer gate lengths

The dependence of effective saturation velocity on gate length in n⁺self-aligned GaAs MESFET's with submicrometer gate lengths has been determined by comparing experimentalI-Vcharacteristics with that obtained from one-dimensional analysis and two-dimensional simulation. The experimentalI-Vcharacteristics have been precisely matched to the theoretical ones calculated by two-dimensional simulation with a quasi-static (effective) velocity-electric-field relationship and reasonable doping profiles. The effective saturation velocity determined by best fit is 2.3 × 10⁷cm/s, and is independent of the gate length in 0.3- to 1.0-µm range. Though this high value gives evidence of the velocity overshoot effects, the constant characteristic disagrees with the expectation of the simulations based on nonstationary electron transport. On the contrary, the saturation velocity determined by using one-dimensional analysis decreases with an increase in the gate length. This dependence is explained by taking into account the channel pinchoff mechanism for drain current saturation before velocity saturation.