0195136047.pdf

7.4 FIELD-EFFECT TRANSISTORS 377

iD = 0

S

+

−

G

vGS ≤ −VP

n+

n+ p

n+

n+ p

(b)

D

Depletion

regions

S

G

vGS < 0

n+

n

n+

p

d

(a)

vDS > vGS + VP

iD

S

G

vGS > −VP

vGD < −VP

(c)

d

DD

Figure 7.4.11Internal physical picture inn-channel depletion MOSFET.(a)Formation of depletion regions.
(b)Cutoff condition.(c)Active state.

Cut off vGS ≤ −VP

−VP

IDSS

25

20

Drain current

iD

, mA

15

10

(a) (b)

25

Ohmic

region

Active region

1 V

−1 V

−2 V

0 V

vDS < vGS + VP vDS^ ≥^ vGS + VP

vGS = +2 V

vGS > −VP

20

Drain current

iD

, mA

15

10

5

− 2 02

Gate-to-source voltage vGS, V

− 4064

Drain-to-source voltage vDS, V

281210

5

Figure 7.4.12Characteristics ofn-channel depletion MOSFET.(a)Transfer characteristic.(b)Static

characteristics.

iD=IDSS

(

1 +

vGS

VP

) 2 (

1 +

vDS

VA

)

(7.4.12)

whereVAandIDSSare positive constants, and the factor( 1 +vDS/VA)is added to account

approximately for the nonzero slope of theiD–vDScurves of a practical device, as was done in

Equation (7.4.2). The small-signal equivalent circuit for low frequencies is of the same form as

Figure 7.4.4 for a JFET.

EXAMPLE 7.4.3

Ann-channel depletion MOSFET, for whichIDSS=7mAandVP=4 V, is said to be operating

in the ohmic region with drain currentiD=1 mA whenvDS= 0 .8 V. Neglecting the effect of

vDSoniD, findvGSand check to make sure the operation is in the ohmic region.