Bias Circuit

B.2 Base-Width Dependence on Junction Voltage
In real transistors, the base width depends on the voltage applied across a pn junction. The
effect is known as base-width modulation. A diagrammatic representation of the effect is
illustrated in Fig. B.6. Not shown in the diagrams in Unit B.1 of the pnp structure are the
depletion regions, which are shown shaded in Fig. B.6. These are the transition regions that
are depleted of free carriers, and in which the barrier is formed that impedes electron and hole
flow to the p regions and n region, respectively. The base width (n region) is actually the width
between the depletion regions. In the diagram of Fig. B.6, we define wBo for VBC = 0 and wB for
VBC > 0.
Figure B.6. Diagrammatic pnp structure showing the effect of base –
collector voltage on base width.
The magnitude of saturation current, IS, in (B.3) is specifically defined for VBC = 0 or wBo. In a
circuit application, though, VBC will in general be nonzero and the base width can vary as
shown; in the example of Fig. B.6, the base width is wB for a given applied VBC. To a
reasonable approximation, the relationship between the effective saturation current with a
nonzero VBC can be accounted for with the form.
Equation B.4
where is the effective saturation current for a reverse-applied base – collector voltage. The
approximate form assumes that VBC