Physics and Engineering of Radiation Detection

5.1. Semiconductor Detectors 305

Nd1

Nd2

Nd1<Nd2

Fluence

Eff. Dopant Conc.

Figure 5.1.33: Dependence of effective

dopant concentration on particle fluence

in a silicon detector in high radiation en-

vironment for two different initial donor

concentration levels.

L.4 DepletionVoltage

Generally, radiation detectors are operated in fully depleted mode. The bias volt-
age is kept at several tens of volts above the depletion voltage to ensure complete
depletion. However with prolonged irradiation, the depletion voltage increases, re-
quiring increase in bias voltage. The principle mechanism is the change in effective
dopant concentration due to radiation damage. To overcome this effect, bias voltage
is increased.

L.5 ChargeTrappingandCarrierLifetime

Charge collection efficiency of the electrodes is directly proportional to the number
of free charges available. Perhaps the worst effect that incident radiation can cause
in a semiconductor detector is the creation of energy levels that trap the charges for
a long period of time. These could be either electron traps or hole traps according
to whether they are near the valence band or conduction band respectively.
If a charge traps in the hole, it can no longer be a part of the current and would
cause nonlinearity in the detector response. Note that the net result of traps is a
decrement in the average carrier lifetime in the bulk of the material.

L.6 Annealing............................

Annealing is thehealingprocess through which the radiation damage in a semicon-
ductor material diminishes with time. The underlying process responsible for this
reversal of damage is not fully understood. However it has been found that annealing
has a strong temperature dependence. The defect concentration can be represented
by an exponential of the form

N(t)=N 0 e−t/τ, (5.1.90)

whereN 0 is the initial defect concentration andτ is a function of the activation
energyEaand absolute temperatureT.Itisgivenby

τ=AeEa/kT, (5.1.91)

wherekis the Boltzmann’s constant. The constantAis determined experimentally.