0195136047.pdf

372 SEMICONDUCTOR DEVICES

The input resistance of a MOSFET is even higher than that of the JFET (typically on the order

of 10^10 to 10^15 ) because of the insulating layer of the gate. As in JFETs, the conductive gate

current is negligibly small in most applications. The insulating oxide layer can, however, be

damaged easily due to buildup of static charges. While the MOSFET devices are often shipped

with leads conductively tied together to neutralize static charges, users too must be careful in

handling MOSFETs to prevent damage due to static electricity. The MOSFETs are used primarily

in digital electronic circuits. They can also provide controlled-source characteristics, which are

utilized in amplifier circuits.

ENHANCEMENTMOSFETS

Figure 7.4.5 illustrates the cross-sectional structure of ann-channel enhancement MOSFET

and its symbol showing as anormally offdevice when used for switching purposes. When the

gate-to-source voltagevGS >0, an electric field is established pushing holes in the substrate

away from the gate and drawing mobile electrons toward it, as shown in Figure 7.4.6(a). When

vGSexceeds thethreshold voltage VTof the MOSFET, ann-type channelis formed along the gate

and adepletion regionseparates the channel from the rest of the substrate, as shown in Figure

7.4.6(b). WithvGS>VTandvDS> 0, electrons are injected into the channel from the heavily doped

n+source region and collected at then+drain region, thereby forming drain-to-source currentiD,

as shown in Figure 7.4.6(b). Note that none of the electrons comes from thep-type portion of the

substrate, which now forms a reverse-biased junction with then-type channel. As the gate voltage

increases aboveVT, the electric field increases the channel depth andenhancesconduction. For

a fixedvGSand smallvDS, the channel has uniform depthd, acting like a resistance connected

between the drain and source terminals. The MOSFET is then said to be operating in theohmic

state.

With a fixedvGS>VT, increasingvDSwill reduce the gate-to-drain voltagevGD(=vGS−

vDS), thereby reducing the field strength and channel depth at the drain end of the substrate. When

vDS>(vGS−VT), i.e.,vGD<VT,apinched-downcondition occurs when the electron flow is

limited due to the narrowed neck of the channel, as shown in Figure 7.4.6(c). The MOSFET is

then said to be operating in aconstant-current state, wheniDis essentially constant, independent

ofvDS.

Figure 7.4.7 illustrates the MOSFET behavior explained so far. WhenvGS≤VT, however,

the field is insufficient to form a channel so that theiD–vDScurve for the normally off state is

simply a horizontal line atiD=0. Thedrain breakdown voltageBVDSranges between 20 and

D (drain)

S (source)

D

S

+

+

− −

(gate) G

G

vGS

vDS

iD

Insulating

(SiO 2 )

oxide layer

Metallic

film

Heavily

doped

(a) (b)

p–type

semiconductor

substrate (body)

p

n+

n+

Figure 7.4.5n-channel enhancement MOSFET.(a)Cross-sectional structure.(b)Symbol.