0195136047.pdf

368 SEMICONDUCTOR DEVICES

G

(gate)

Gate

D (drain)

S (source) G

(gate)

D (drain)

S (source)

Source

Metal

contact

Metal

contact

Conducting

channel

Depletion

region

(a)

Drain

Metal

contact

Gate

Source

Metal

contact

Metal

contact

Conducting

channel

Depletion

region

(b)

Drain

Metal

n contact

p n

p

Figure 7.4.2JFETs and their circuit symbols.(a)n-channel JFET.(b)p-channel JFET.

formed of ap-type semiconductor. The functions of source, drain, and gate are analogous to the

emitter, collector, and base of the BJT. The gate provides the means to control the flow of charges

between source and drain.

The junction in the JFET is reverse-biased for normal operation. No gate current flows because

of the reverse bias and all carriers flow from source to drain. The corresponding drain current is

dependent on the resistance of the channel and the drain-to-source voltagevDS.AsvDSis increased

for a given value ofvGS, the junction is more heavily reverse-biased, when the depletion region

extends further into the conducting channel. IncreasingvDSwill ultimately block orpinch offthe

conducting channel. After the pinch-off, the drain currentiDwill be constant, independent of

vDS. ChangingvGS(gate-to-source voltage) controls where pinch-off occurs and what the value

of drain current is.

It is the active region beyond pinch-off that is useful for the controlled-source operation, since

only changes invGSwill produce changes iniD. Figure 7.4.3 illustrates the JFET characteristics.

Part (a) shows the idealizedstatic characteristicswith two regions separated by the dashed line,

indicating theohmic(controlled-resistanceortriode)regionand theactive(controlled-source)

regionbeyond pinch-off. Note thatiDis initially proportional tovDSin the ohmic region where

the JFET behaves much like a voltage-variable resistance;iDdepends onvGSfor a given value

ofvDSin the active region. In a practical JFET, however, the curves ofiDversusvDSare not

entirely flat in the active region but tend to increase slightly withvDS, as shown in Figure 7.4.3(b);

when extended, these curves tend to intersect at a point of−VAon thevDSaxis. Another useful

characteristic indicating the strength of the controlled source is thetransfer characteristic, relating

the drain currentiDto the degree of the negative biasvGSapplied between gate and source; a cutoff

region exists, indicated by thepinch-off voltage−VP, for which no drain current flows, because

bothvGSandvDSact to eliminate the conducting channel completely.

Mathematically, the drain current in the active controlled-source region is approximately

given by [see Figure 7.4.3(a)]:

iD=IDSS

(

1 +

vGS

VP

) 2

(7.4.1)

whereIDSS, known as thedrain–source saturation current, represents the value ofiDwhenvGS=0.