Physics and Engineering of Radiation Detection

5.1. Semiconductor Detectors 285

wherexpdandxndare the widths of the depletion regions onp-andn-sides respec-
tively,NAandNDare the acceptor and donor doping densities,qis the unit charge
of electron,is the permitivity of the medium andV 0 is the potential difference.

n−type p−type

++

++

+

−−

−

−−

n−type p−type

Ec

Ev

EF

EF

depletion region

E

EF

Eccc

Ev

(a)

(b)

Figure 5.1.23: Semiconductors

and their energy levels. (a)

Separate n- and p-type crystals.

(b) Formation of pn-junction.

When n- and p-type crystals

are brought in contact, flow of

charges starts which continues

till the Fermi levels EF of the

two materials do not coincide.

Before we go on any further an important point should be mentioned. The
characterization of the depletion region asdevoid of any charges is not entirely
correct. No matter how good a semiconductor material is, there are always crystal
imperfections and impurities, which introduce energy levels inside the band gap.
Such energy levels can be exploited by electrons to jump out of the valence band
and eventually go up into the conduction band. The result is the creation of an
electron hole pair. Even if we assume that the material does not have any crystal
imperfections and impurities, still some electrons can attain enough energy through
thermal agitation to jump to the conduction band. In summary, the depletion region
is not really completely devoid of free charges. However the number of such charges
is very small and the corresponding current is extremely low. If a reverse bias is
applied and there were no free charges in the depletion region, then there should not
be any current flowing through the circuit. However since some free charge pairs are
present, a very small current is always observed. This current is generally known as
darkorreversecurrent (see Fig.5.1.24).
Let us now see what happens if we apply forward bias to the junction. In such a
case, as our intuition suggests, a large current starts flowing, which increases rapidly
with increasing voltage. This property of the pn junction orsemiconductor diodeis
extensively used to design electronic devices such as switches and solar cells. For
radiation detection purposes, the pn junction is always reversed biased.