Effect of electric-field-induced mobility degradation on the velocity distribution in a sub- mu m length channel of InGaAs/AlGaAs heterojunction MODFET

The electric field distribution, the velocity distribution, the effective saturation velocity, and the maximum operating frequency are evaluated for an InGaAs/AlGaAs MODFET by taking into account the electric-field-induced mobility degradation, which becomes important when the electric field in the channel exceeds in *=v th /2 mu o =1.7 kV cm -1 , where v th =3.9*10 7 cm s -1 is the thermal velocity of an electron. In the limit of small gate-length, the effective electron velocity is shown to approach the limiting value v th , the internal transconductance reaches the limit C o v th , and the saturation current is / Dsat =n sD qv th W, W being the gate width. The ratio of the carrier density at the drain end of the channel to that at the source end, at the onset of saturation, is found to be n sD /n so =1-(V c /V' g )((1+V' g /V c ) 1 2/-1), which approaches unity in the short-channel limit and V' g /2V c in the long-channel limit, in direct contrast to the well known pinch-off behaviour, where n sD vanishes. Here, V c =v th L/ mu o , where L is the gate length. An analysis of the effective saturation velocity, and maximum frequency of the MODFET as a function of the gate length is presented.