264 Chapter 5. Solid State Detectors
in the material is 10^17 cm^3 and the lifetime of holes at this dopant level is 10μs.Solution:
The recombination rate constant of the holes having lifetimeτ=0. 1 μsis
given bykr =1
τ
=1
10 × 10 −^6
=10^5 s−^1.The percentage of holes absorbed after 4μscan be computed from equation
5.1.20 as follows.δN =N 0 −N
N
×100 =
(
1 −e−krt)
× 100
=
[
1 −exp{
−
(
105
)(
4 × 10 −^6
)}]
× 100
≈ 33%
5.1.F Charge Conductivity
The free charges in a semiconductor can drift under the influence of an externally
applied field. Just like metallic conductors, their movement can also be characterized
by the parameter calledconductivity, which quantifies the ability of the material to
conduct electric current. However as opposed to metallic conductors, there are a
number of mechanisms that can contribute to or suppress the conductivity of a
semiconductor. Some of these factors are described below.
Electron-Hole Recombination:An electron in the conduction band can fall
into the intermediate energy state and then recombine with a hole in the valence
band. The overall effect of this process is the removal of an electron-hole pair
from the free charge density and decrease in the current.Hole Emission:An electron from the valence band can jump to the interme-
diate level. Since this removes the electron from the valence band and leaves
behind a hole, it can also be viewed as the emission of a hole from the inter-
mediate energy level to the valence band.Electron Emission: An electron in the intermediate energy level can pro-
ceed to the conduction band and become part of the free charge density thus
increasing the current.Electron Trapping: An electron can fall into a slightly lower energy level
from the conduction band and get trapped there for some finite amount of
time. This trapping mechanism has the potential of introducing nonlinearity
in the response due to the time lag involved in electron trapping and release.