0195136047.pdf

360 SEMICONDUCTOR DEVICES

npn C

(collector)

C (collector)

(a)

E (emitter)

E

(emitter)

B (base)

+

+

−

−

(base)

B

iC

iB

vCE

vBE

pnp C

(collector)

C (collector)

(b)

E (emitter)

E

(emitter)

B (base)

+

+

−

−

(base)

B

iC

iE iE

iB

vCE

vBE

Figure 7.3.1Bipolar junction transistors.(a)npnBJT structure and circuit symbol.(b)pnpBJT structure

and circuit symbol.

emitter is open-circuited), calledreverse saturation current,results. BJTs biased in the active

region are shown in Figure 7.3.2.

It can be shown that the currents in a BJT are approximately given by

iE=ISEeVBE/VT=iB+iC=

1

α

IC−

1

α

ICBO (7.3.1)

iC=iE+αICBO (7.3.2)

iB=( 1 −α)iE−ICBO=

1 −α

α

iC−

1

α

ICBO (7.3.3)

whereISEis the reverse saturation current of the BEJ,ICBOis the reverse saturation current of

the CBJ,α(known ascommon-base current gainorforward-current transfer ratio, typically

ranging from about 0.9 to 0.998) is the fraction ofiEthat contributes to the collector current, and

VT=kT /qis the thermal voltage (which is the voltage equivalent of temperature, having a value

of 25. 861 × 10 −^3 V whenT=300 K). Note that the symbolhFBis also used in place ofα.

Another important BJT parameter is thecommon-emitter current gain,denoted byβ(also

symbolized byhFE), which is given by

β=

α

1 −α

(

or α=

β

1 +β

)

(7.3.4)

which ranges typically from about 9 to 500, being very sensitive to changes inα. In terms ofβ,

one can write

iC=βiB+

ICBO

1 −α

=βiB+(β+ 1 )ICBO=βiB+ICEO (7.3.5)

whereICEO=(β+ 1 )ICBOis thecollector cutoff currentwhen the base is open-circuited (i.e.,

iB=0).

Figure 7.3.3 illustrates common-base static curves for a typicalnpnsilicon BJT. In a common-

emitter configuration in which transistors are most commonly used, where the input is to the base

and the output is from the collector, the input and output characteristics are shown in Figure 7.3.4.