294 Chapter 5. Solid State Detectors
This shows that the hole takes approximately three times longer than the electron
to reach the opposite electrode. The charge induced by the electron after timeteis
Qe=q
2. (5.1.83)
The hole also keeps on moving during this time. The total charge induced by the
hole during the electron drift is given by
Qh = ihte= qμhV 0
d^2d^2
2 μeV 0
= qμh
2 μe
≈
q
6, (5.1.84)
where we have used the relationμe≈ 3 μh. The cumulative charge induced after
timeteis then
Q(t=te)=q
2+
q
6=
2 q
3
The hole keeps on moving even after the electron has been collected such that after
timeththe total induced charge is equal toq.
Q(t=th)=
q
2+
q
6+
q
3=qThe time profile of the induced charge is shown in Fig.5.1.28(b).
H.3 FrequencyResponse
The way semiconductor detectors respond to different frequencies depends not only
on the particular geometry and construction of the detectors but also their associated
electronics. The discussion of the effect of electronic components on the frequency
response will be deferred to the chapter on electronics. The detector related effects
are mainly due to the transit time of charge pairs in the depletion region. This will
be discussed here.
The response time of a detector depends on how quickly the charges are collected
by the readout electrodes after their generation by the radiation. Most detectors are
built and operated such that the charge carriers quickly attain saturation velocity
after generation.
5.1.I ModesofOperationofapn-Diode
The pn-diodes can be operated in essentially two different modes: photovoltaic
mode and photoconductive mode. Although the semiconductor detectors are almost
always operated in the photoconductive mode but it is also theoretically possible
and practically feasible to build a detector that operates in the photovoltaic mode.
These two modes will be described in the next two sections. However, since the
photovoltaic mode does not have much practical significance as far as radiation
detectors are concerned, we will not spend much time discussing it.