5.1. Semiconductor Detectors 295
I.1 PhotovoltaicMode.......................
In this mode a very large load resistance is applied across the junction such that
essentially no current flows through the circuit. The results in the creation of a
potential difference across the diode. The incident radiation produces electron hole
pairs inside the depletion region, which move in opposite directions under the influ-
ence of the junction potential. Let us call this current the radiation induced reverse
currentir. The consequence of this flow of charges is the reduction in the junction
barrier, which starts another current but now in opposite direction toir. Let us call
this current the forward currentif. Since the diode is connected to essentially an
open circuit (very large load resistance) therefore almost no current flows through
the circuit. This implies that the forward current must be balanced by the reverse
current, or
if=ir. (5.1.85)
The change in the energy barrier due to flow of the reverse current changes the
potential difference across the diode. This potential difference can then be measured
with a sensitive device. Note that this operation mode does not require the junction
to be biased by an external source.
The good thing about this mode of operation is that essentially no leakage cur-
rent exists across the junction and therefore the noise is extremely small. However
noise is not the only important consideration for building a detection. There are
other factors, such as small thickness of the depletion region and non-linearity in
response, which make this mode undesirable for radiation detection purposes. The
most problematic aspect of this mode is that the output voltage is not a linear
function of the deposited energy and therefore the detector can not be easily used
for spectroscopic purposes. Because of such reasons, as noted earlier, this mode of
operation is not used for radiation detection.
Photovoltaic mode is most exclusively used to build solar cells.
I.2 Photoconductive Mode
This is the mode in which radiation detectors are generally operated. A high reverse
bias is applied across the diode, creating a large depletion region in the middle of
the junction. The incident radiation passing through the depletion region produces
electron hole pairs along its track. These charges move in opposite directions and
constitute an electrical current that can be measured. As the number of charge pairs
created by the incident radiation depends on the deposited energy, the measured
current is proportional to the energy carried by the radiation. For radiation detection
purposes, there are three main advantages of reverse biasing the pn-junctions.
Increase the size of the depletion region.Increase the signal to noise ratio.Decrease the capacitance.Charge pairs are not exclusively produced in the depletion region by the incident
radiation. In fact the electron hole pair production probability is the same whether
the region is depleted or not. The advantage of depletion region is that it has very few
free charge pairs and therefore the leakage current flowing through it in the absence