The frequency at which the small signal short circuit current gain of an intrinsic MOS transistor drops to unity is called transit frequency(fT). The parameter fT is used assess the speed of an intrinsic MOS transistor
Transit frequency of MOS transistor
The small-signal equivalent circuit of a MOS transistor to compute fT is shown in Figure 1. (Assumption: )
At input node
At output node
At high frequencies,
At transit frequency (), the current gain is unity. From Eq.(4), transit frequency is given by
Substituting for gm in terms of gate bias voltage, in Eq-(6)
is the time required for the electron to transit from source to drain.
Impact of bias point on fT
From Eq-(7), transit frequency in terms of gate bias voltage and channel length is given by
– Increasing gate drive increases transit frequency
– Increasing gate length decreases transit frequency. So technology scaling favor the transit frequency.
From Eq-(6), transit frequency in terms of drain current and channel length is obtained by substituting
– Increasing bias current increases transit frequency (but square root dependence)
– Keeping bias current constant, decreasing length increases transit frequency ( )