0195136047.pdf

7.4 FIELD-EFFECT TRANSISTORS 373

D

S

+

+

−

−

G

vGS > 0

vDS > 0

iD

n+

n+

Electrons

Holes

(a)

Electric

field

D

n–type

channel

Depletion

region

S

+

−

G

vGS > VT

n+

n

n+

p

d

(b)

+

−

vDS > vGS − VT

iD

D

S

+

+

−

−

G

vGS > VT

vGD < VT

n+

n

n+

d p

(c)

Figure 7.4.6Internal physical picture in ann-channel enhancement MOSFET.(a)Movement of electrons
and holes due to electric field.(b)Formation ofn-type channel.(c)Pinched-down channel.

iD

vDS

vGS − VT BVDS

Ohmic

state

Breakdown

Constant–current

state

0

Figure 7.4.7IdealizediD–vDScurve of ann-channel enhance-

ment MOSFET with fixedvGS>VT.

50 V, at which the drain current abruptly increases and may damage the MOSFET due to heat if

operation is continued. Thegate breakdown voltage,at about 50 V, may also cause a sudden and

permanent rupture of the oxide layer.

Figure 7.4.8 shows the characteristics of a typicaln-channel enhancement MOSFET. In the

ohmic region wherevGS>VTandvDS<vGS−VT, the drain current is given by

iD=K

[

2 (vGS−VT)vDS−vDS^2

]

(7.4.6)

whereKis a constant given byIDSS/VT^2 having the unit of A/V^2 , andIDSSis the value ofiDwhen

vGS= 2 VT. The boundary between ohmic and active regions occurs whenvDS=vGS−VT. For

vGS>VTandvDS≥vGS−VT, in the active region, the drain current is ideally constant and

given by

iD=K(vGS−VT)^2 (7.4.7)

To account for the effect ofvDSoniD, however, a factor is added,

iD=K(vGS−VT)^2

(

1 +

vDS

VA

)

(7.4.8)

whereVAis a constant that is in the range of 30 to 200 V. Then-channel enhancement MOSFET

with characteristics depicted in Figure 7.4.8 has typical values ofVT=4V,VA=200 V, and

K= 0 .4 mA/V^2.

The small-signal equivalent circuit for low frequencies is of the same form as Figure 7.4.4

for a JFET, withgmandroevaluated from the equations