619Microwave Transistors
The transistor is on, and a channel is created that allows current to
ow between the drain and the source. The MOSFET operates like a
resistor, controlled by the gate voltage relative to both the source and
drain voltages. The current from the drain to the source is given as
I C Z L V V V V
d n
= µ
ox gs th ds ds
2
/ { /2 }( ) ( ) ( ) )− −
(14.63)
where
μ
n
is the charge-carrier effective mobility, Z is the gate width,
L is the gate length and C
ox
is the gate oxide capacitance per unit
area. The transition from the exponential sub-threshold region to
the triode region is not sharp.
3. Saturation or active mode: This case is shown in Figure 14.29c wherein
V
gs
> V
th
and V
ds
> (V
gs
– V
th
). The switch is turned on,