Banha University

10x2.
25x10
10
25x450
5x10
3
19
Is = 1. 6x10
3
3
4
16
2
x10
3.
96x10
A/
cm
The diode current I = Is(e V/Vt – 1)
= 3.96x10 -16 (e 0.65/0.026 – 1) = 2.85x10 -5 A
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b) Describe the operation, the charge flow and the current components in npn
bipolar junction transistor in the forward active mode. What are the operating
modes of the transistor?
Solution
The bipolar transistor consists of three separately doped regions and two pn
junctions. There are two types of transistor structures an npn bipolar transistor and
a pnp bipolar transistor. The three terminal connections are called the emitter,
base, and collector. The width of the base region is small compared to the minority
carrier diffusion length.
The operation of an npn transistor in the forward active mode and the
charge flow
The base-emitter (B-E) pn junction is forward-biased, and the base-collector
(B-C) pn junction is reverse-biased in the forward-active operating mode. Since the
B-E junction is forward-biased so electrons from the emitter are injected across the
B-E junction into the base. Also holes from the emitter are injected across the B-E
junction into the emitter.
The injected electrons create an excess concentration of minority carriers in the
base. The B-C junction is reverse biased, so the minority carrier electron
concentration at the edge of the B-C junction is ideally zero. The large gradient in
the electron concentration means that electrons injected from the emitter will
diffuse across the base region into the B-C space charge region where the electric
field will sweep the electrons into the collector. Some of the injected electrons
recombine with the majority holes in the base. We want as many electrons as