Question

Consider a Si NPN BJT. Discuss how an change in one of the design parameters affect the emitter injection efficiency, the base transport factor, the common emmiter current gain, and the early voltage. Assume complete ionization. Further assume the carrier mobility and the minority carrier lifetime are independent of the change in design parameters.

(a) Increase emitter doping
(b) Increase base doping
(c) Increase base width
(d) Increase collector doping

Answer 1

a)Increase emitter doping:-

Emitter is a heavily doped region of the BJT transistor which provides majority carriers into the base region. Base region is a thin, lightly doped region and is sandwiched between emitter and collector. Majority carriers from the emitter pass through the base region and its flow can be externally controlled

Emitter injection efficiency in a BJT transistor defines the efficiency of majority carrier injection from emitter. It is the ratio of current due to emitter majority carriers to the total emitter current. It defines the injection capability of an emitter. Heavily doped region will have high injection factor.

b)Increase base doping:-

A BJT works by the carriers from the emitter "overwhelming" the amount of (opposite) carriers (in case of an NPN: holes) in the base region. Also, the base region is narrow. This makes the chance that a carrier originating from the emitter recombines in the base, small. If that chance was large (due to a highly doped base) then the base current (to fill up all those used-up holes) would be much larger !

So: the Base needs to have a lower doping level (compared to the emitter) so that the beta (current amplification) will be large, which is what we want.If you increase the base doping, the chances of an electron recombining in base region increases. As a result the collector current decreases.

C)Increase base width:-

In a BJT, if we change the reverse biasing voltage from collector to base junction, the depletion width changes which results in change in effective base width accordingly. This is known as base width modulation.

Now with increase in collector to base voltage, the effective base width decreases due to which the concentration gradient of the diffusion particles increases at forward biasing emittor to base junction. As a result of that to cause the same amount of collector current a very lesser value of base to emitter voltage is required. This effect was introduced by Sir James Early and hence this effect is also known as Early Effect.

d)Increase collector doping:-

1) If the collector doping is very high (E.g. 1e19), the collector resistance is low. However, the breakdown voltage of the BJT will be very small, which is BAD especially for POWER AMPLIFIER application.

2) If the collector doping is very low (e.g. 1e15), the collector resistance is very HIGH, which limits the available maximum collector current. However, the breakdown voltage for BJT is INCREASED.

The idea is, when you apply a collector voltage, most of the applied voltage dropped across the base-collector junction, which is in reverse-bias. And more importantly, the HIGHER the collector doping you have, more voltage drop acorss the BASE region [in case you don't understand, using simple potential divider, collector (collector doping > base doping) has small resistance comparing with base], which lowers the emitter-base barrier and makes the BJT go to the so called "punchthrough"