SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

284 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

In the common-base configuration, asVCBis increased, the breakdown is essentially similar
to that of a singlep-njunction discussed in chapter 4. The current coming from the emitter has
little effect on the breakdown. However, in the common-emitter configuration, as soon as the
impact ionization process starts, say, in annpnBJT, the secondary holes are injected into the
base and act as a base current, leading to increased emitter current and eventual current runaway
as the process snowballs.

6.6.6 LowInjectionEffectsandCurrentGain ..................

In our calculations for the BJT junction currents we have assumed that in the space charge
region, the junctions are “ideal,” i.e., there is no current flow due to recombination-generation
effects. In chapter 5, we discussed how non-ideal effects arising from recombination genera-
tion in the depletion region alter the current flowing in the junction. This effect is particularly
important under low injection (i.e., low values ofVEB) conditions.
If we examine the forward-biased EBJ for a device operating in the forward active mode, the
base current will have an “ideal” current component and a “non-ideal” current component arising
from generation-recombination. We can write

IB=

eADenoe

L

exp

(

eVEB

kBT

)

+

eAniWEBJ

2 τ

exp

(

eVEB

2 kBT

)

where the second term is due to recombination in the emitter-base junction depletion region
(WEBJ). The recombination time isτ. The base current may be written as

IB=ISexp

(

eV

mkBT

)

wheremis the junction ideality factor.
The collector current is not greatly influenced by the recombination-generation process. At
low injection the recombination-generation part of the base current dominates and as a result,
the current gainβis reduced. As the injection (VEBvalue) is increased, the recombination part
becomes negligible and the value ofβreaches its ideal value calculated earlier.
In section 6.3.3 we discussed the Ebers-Moll model for bipolar transistors. This model does
not account for some of the issues discussed in this section. A more advanced model that includes
more realistic effects is the Gummel-Poon model. Three important effects are incorporated in
the Gummel-Poon model:

• Recombination current in the emitter depletion region under low injection levels.


• Reduction of current gain at high injection levels.


• Finite output conductance in terms of an Early voltage,VA.

6.6.7 CurrentCrowdingEffect..........................

The picture we have developed for the BJT is a one-dimensional picture. In reality, the base
current flows along the directions perpendicular to the emitter, as can be seen from figure 6.6.