0195136047.pdf

376 SEMICONDUCTOR DEVICES

Figure 7.4.9p-channel enhancement MOSFET.(a)Cross-sectional structure.(b)Symbol.

MOSFETs, like JFETs, arenormally ontransistors, in which the field effect reduces conduction

by depleting the built-in channel. Figure 7.4.11(a) shows the formation of depletion regions due

to electron-hole recombinations with a negative gate voltage. WithvGS≤−VP, whereVPis the

pinch-off voltage, the depletion regions completely block the channel, makingiD=0, as shown in

Figure 7.4.11(b), which corresponds to the cutoff condition. WithvGS>−VPandvGD<−VP,

so thatvDS>vGS+VP, the channel becomes partially blocked or pinched down when the device

operates in its active state.

Figure 7.4.12 shows the characteristics of a typicaln-channel depletion MOSFET. With

VP =3V,iD=0 forvGS≤−3V.If−3V<vGS≤0, the device operates in the depletion

mode; ifvGS>0, it operates in the enhancement mode. The equations describing the drain current

are of the same form as for the JFET.

In the ohmic region, whenvDS<vGS+VP,

iD=IDSS

[

2

(

1 +

vGS

VP

)(

vDS

VP

)

−

(

vDS

VP

) 2 ]

(7.4.11)

In the active region, whenvDS≥vGS+VP,

D (drain)

S (source)

D

S

+

+

− −

(gate) G

G

vGS

vDS

iD

Heavily

doped

Oxide

layer

Metallic

film

(a) (b)

D

S

−

−

+ +

G

vSG

vSD

iD

(c)

n–channel

n p

n+

n+

Figure 7.4.10Depletion MOSFETs.(a)Structure ofn-channel depletion MOSFET.(b)Symbol ofn-channel

depletion MOSFET.(c)Symbol ofp-channel depletion MOSFET.