284 Chapter 5. Solid State Detectors
the reader, the direct consequence of low charge carrier lifetime is the loss of signal.
Since this loss is not linearly dependent on the amount of deposited energy, it results
in nonlinear response of the detector. Another important point to note here is that
the charge collection also depends on the depth of the material. for example if the
charge is created near the collecting electrode of the detector, the loss of charge
will be minimal. On the other hand, if the same charge is produced away from the
collecting electrode, the signal loss will be higher.
Increasing the bias voltage might not always be practical or even desirable in
certain applications. A novel method tocompensatefor the low hole mobility is to
use the so calledohmic contactsat the electrodes. The advantage of this scheme
is that the holes get recombined with the electrons released into the material by
the ohmic contact. This hole recombination effectively stops the leakage current,
while the signal current is carried predominantly by the electrons. Ohmic contacts
therefore completely eliminate the need for operating the detector at high voltages
or external circuitry to compensate for low hole mobility.
5.1.H Thepn-Junction
The n- and p-type semiconductors can be joined together to create the so called
pn-junction (see Fig.5.1.23). These junctions have been found to be extremely use-
ful not only for building semiconductor electronics but also for radiation detector
technology. When a p- and a n-type semiconductors are brought together, a flow of
charges automatically starts to compensate for the imbalance in charge concentra-
tions across the junction. The electrons that are the majority charge carriers in the
n-type semiconductors flow towards the p-type material. Similarly the holes move
towards the n-type material. This process continues until the Fermi levels of the two
materials coincide with each other, as shown in Fig.5.1.23(b). As the electrons and
holes move in opposite directions and combine together to neutralize one another,
a central region devoid of any electrical charges is created. This region, generally
referred to as the depletion region, plays a central role in semiconductor radiation
detectors since this is where the incident radiation creates electron hole pairs. These
charges flow in opposite directions and constitute an electrical current that can be
measured. However the junction in this configuration can not be very effectively
used for radiation detection since firstly it is too thin and secondly the potential
difference across it is very small. The trick then is to widen this gap somehow and
establish a high enough electric field to allow the charges created by the radiation
to flow and constitute a measurable current. This is done by applying a reverse
bias across the junction. We will discuss the properties and characteristics of such
a junction in the next section.
Bringing an n type material in contact with a p type material produces an effective
electrostatic potential across the depletion region. The thickness of this depletion
region can be calculated from
W = xpd+xnd=[
2 V 0
q