0195136047.pdf

374 SEMICONDUCTOR DEVICES

vGS = VT = 4 V

vGS = 11 V

6 V

8 V

9 V

10 V

VT

25

20

Drain current

iD

, mA

15

10

(a) (b)

Cutoff

region

25 Pinch-off or active region vGS > VT

vDS ≥ vGS − VT

vGS > VT

Ohmic

region

vDS <

20 vGS − VT

Drain current

iD

, mA

15

10

5

0128

Gate-to-source voltage vGS, V

40128

Drain-to-source voltage vDS, V

4162420

5

Figure 7.4.8Characteristics of ann-channel enhancement MOSFET.(a)Transfer characteristic.(b)Static

characteristics.

gm=

∂iD

∂vGS

∣ ∣ ∣ ∣ ∣ Q

= 2 K(vGS−VT)

(

1 +

vDS

VA

)∣∣

∣

∣

∣

Q

∼= 2

√

KIDQ (7.4.9)

and

ro=

(

∂iD

∂vDS

∣ ∣ ∣ ∣ ∣ Q

)− 1

=

VA

K(vGS−VT)^2

∣ ∣ ∣ ∣ ∣ Q

∼= VA

IDQ

(7.4.10)

in which all definitions are the same as those used previously for the JFET.

EXAMPLE 7.4.2

Consider the basic MOSFET circuit shown in Figure E7.4.2 with variable gate voltage. The

MOSFET is given to have very largeVA,VT=4V,andIDSS=8 mA. DetermineiDandvDSfor

vGS=1, 5, and 9 V.

+

−

D

S

+

+

−

G

vGS

vDS

RD = 5 kΩ

VDD = 20 V

iD

Figure E7.4.2