Depletion Mode MOSFET

Depletion Mode MOSFET

The depletion MOSFET is similar to Enhancement type MOSFET except for the difference as mentioned before; the channel exists even for zero gate to source voltage. In order to control the conductivity of channel the gate terminal is reverse biased. The channel is depleted of charge carriers to reduce effective channel width. A depletion layer forms below the Oxide layer which is the region of uncovered charges. The drain and transfer characteristics of depletion MOSFET are similar except for the inverse effect of gate voltage on drain current. In N-type depletion mode MOSFET the control voltage Vgs is negative whereas in Enhancement type MOSFET control voltage Vgs is positive.



Depletion mode Symbol

The circuit symbol of depletion type MOSFET is as shown in the figure (a) and (b). In fig (b) circuit symbol is simplified with as source is often shorted to body and effect of body terminal on volt ampere characteristics can be neglected. The thick shaded area represents there exists a physical channel even for zero gate bias.

In depletion mode MOSFET the saturation drain current with gate shorted to source is given by

Idss = K’n * W * (Vt2/2) *L

Where W = is the lateral width of oxide layer

            L = is the effective length of oxide layer

            Vt = is threshold voltage

 K’n = is Transconductance of the device given by K’nµn * Cox

µn is the mobility of electrons

Cox is the capacitance of oxide layer per unit area of oxide layer given by Cox = εox / tox

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