Biomedical Engineering Reference
In-Depth Information
The junction region over which the charge imbalance occurs is also called the
depletion region, because any mobile charges initially there moved out when the
two sides were joined. The depletion region acts, therefore, like a high-resistivity
parallel-plate ionization chamber, making it feasible to use it for radiation detec-
tion. Ion pairs produced there will migrate out, their motion giving rise to an
electrical signal. The performance of such a device is greatly improved by using
a bias voltage to alleviate recombination and noise problems. The biased junction
becomes a good rectifier, as described next.
Consider the n-p junction device in Fig. 10.21(a) with the negative side of an
external bias voltage
V
applied to the n side. When compared with Fig. 10.20, it is
seen that the applied voltage in this direction lowers the potential difference across
the junction region and causes a relatively large current
I
to flow in the circuit. Bias
in this direction is called forward, and a typical current-voltage curve is shown at
the right in Fig. 10.21(a). One obtains a relatively large current with a small bias
voltage. When a reverse bias is applied in Fig. 10.21(b), comparison with Fig. 10.20
shows that the potential difference across the junction region increases. Therefore,
a much smaller current flows—and in the opposite direction—under reverse bias,
as illustrated on the right in Fig. 10.21(b). Note the vastly different voltage and cur-
rent scales on the two curves in the figure. Such n-p junction devices are rectifiers,
passing current readily in one direction but not the other.
Fig. 10.21
(a) Forward- and (b) reverse-biased n-p junctions
and typical curves of current vs. voltage. Note the very different
scales used for the two curves. Such an n-p junction is a good
rectifier.
