6.2. BIPOLAR TRANSISTOR: A CONCEPTUAL PICTURE 251
have
IC=BIEn (6.2.1)
whereIEnis the electron part of the emitter current and the factorBis called the base transport
factor. In the absence ofe-hrecombination, the emitter current is made up of electrons injected
from then-top-sides (IEn) and holes injected fromp-ton-sides (IEp). Since the BCJ is reverse
biased, the collector current is related only to the electrons injected and we define the emitter
efficiencyγeas
γe=IEn
IEn+IEp(6.2.2)
For optimum devices,γeandBshould be close to unity. The ratio between the collector and
emitter currents is the current transfer ratio,α
IC
IE=
BIEn
IEn+IEp=Bγe=α (6.2.3)This ratio is close to unity in good bipolar devices. In figure 6.5 we show a typical circuit for
a BJT in the forward-bias active mode. A change in the base current alters the minority carrier
densitynpin the base and causes a large change in the collector current. The ratio between the
collector current and thecontrolling base current is of great importance since this represents the
current amplification. The base current is made up of the hole current injected into the emitter
(IEp) and the hole current due to the recombination in the base with injected electrons from the
emitter (= (1−B)IEn). Thus
IB=IEp+(1−B)IEn (6.2.4)The base-to-collector current amplification factor, denoted byβis then
β=IC
IB
=
BIEn
IEp+(1−B)IEn=
B(IEn/IE)
1 −B(IEn/IE)=Bγe
1 −Bγe(6.2.5)
This gives for the current gain
β=α
1 −α(6.2.6)
The factorβcan be quite large for the bipolar transistor. In the next section we will discuss the
mathematical derivation of the device characteristics.