Mosfet

Metal Oxide Semiconductor FET (MOSFET)

Engr. Stephenn L. Rabano

MOSFET

• A MOSFET, apart from a JFET, has no pn junction structure. • Its gate is insulated from the channel by a silicon dioxide (SiO2) layer. • Two basic types:

– depletion (D) – enhancement (E)

• MOSFETs are sometimes called IGFETs (insulated gate FETs).

Depletion MOSFET (D-MOSFET) Basic Structure Representation

D-MOSFET (n-channel) Basic Construction

D-MOSFET

• Note that the drain and source are diffused into the substrate material and then connected by a narrow channel adjacent to the insulated gate. • D-MOSFETs can be operated in either of two modes - the depletion mode or the enhancement mode (the reason they are sometimes called depletion/enhancement MOSFETs). • Since the gate is insulated from channel, either a positive or a negative gate voltage can be applied.

Depletion Mode

• Visualize the gate as one plate of a parallel-plate capacitor and the channel as the other plate with the silicon dioxide insulating layer is the dielectric. • With a negative gate voltage, the negative charges on the gate repel conduction electrons from the channel, leaving positive ions in their place. • The n channel is then depleted of some of its electrons, decreasing the channel conductivity. • The greater the negative voltage on the gate, the greater the depletion of n-channel electrons.

n-channel D-MOSFET (VGS = 0 V; applied voltage VDD)

Depletion Mode Versus Enhancement Mode (n-channel D-MOSFET)

Depletion Mode

• At a sufficiently negative gate-to-source voltage, VGS(off), the channel is totally depleted and the drain current is zero. • Like the n-channel JFET, the n-channel DMOSFET conducts drain current for gate-tosource voltages between VGS(off) and zero. • D-MOSFETs conduct for values of VGS above zero.

Reduction in free carriers in channel due to a negative potential at the gate terminal

Enhancement Mode

• With a positive gate voltage, more conduction...