Two-dimensional transient simulation of the turn-off behavior of a planar MOS-transistor

Abstract

A two-dimensional transient simulation of the turn off behavior of a short channel NMOS transistor with L eff = 0.68 μm and d ox = 100 A ̊ fs, has been carried out with the simulation program BAMBI. For the first time an external drain resistor is accounted for in this fast switching calculation. The computation starts with a steady state bias point in the triode region. The gate voltage is pulsed from V gs = 5 V to V gs = 0 V immediately. The paper presents the transient response of all physical quantities during the time after the switching. The simulation shows how the inversion layer is slowly reduced from both the source and the drain side whereas the lateral space-charge regions of both reverse biased pn-junctions build up quickly. Even at a time when lateral current has already vanished, electrons still remain in the channel region. They diffuse into the bulk, although the channel length is very short in comparison to the bulk depth. The electrons in the bulk slowly vanish by recombination. It should be pointed out, that our results are based on a totally selfconsistent solution.