SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

9.5. MOSFET OPERATION 463

A typical n-channel

depletion mode

device

Ohmic

Region

Drain Current

ID

Drain Bias VD

Saturated Region

VG = +2.0 Volts

VG = +1.0

VG = 0.0

VG = -1.0

VG = -2.0

VG = -3.0

D

G S

substrate

A typical n-channel

enhancement mode

device

Ohmic

Region

Drain Current

ID

Drain Bias VD

Saturated Region

VG = +7.0 Volts

VG = +6.0

VG = +5.0

VG = +4.0

VG = +3.0

substrate

D

G S

(b)

(c)

(a)

p-substrate

Source Gate Drain

n+ n-channel n+

Oxide

Figure 9.18: (a) A schematic of a depletion MOSFET fabricated in ap-type substrate, with an
n-channel. (b) In the depletion mode, the device is ON at zero gate bias. To turn the device
OFF, a negative gate bias is required as shown. The device symbol is also shown. (c) The I-V
characteristics showing the device behavior in the enhancement mode. The device symbol is also
shown.