Physics and Engineering of Radiation Detection

5.1. Semiconductor Detectors 293

ie

ih

te th

pn

−

− +

x

d

0

Q

t

(a)

(b)

2q/3

q

Figure 5.1.28: (a) An almost fully depleted pn

junction. The incident radiation produces a charge

pair in the middle of the junction. The electron

and the hold move in opposite directions inducing

charges on the electrodes. (b) Time profile of the

charge induced by movement of an electron and

a hole produced in the middle of a fully depleted

pn junction. The electron mobility is almost three

times larger than that of the hole and therefore

only 1/ 3 rdof the initial charge of 2q/3 is due to

the hole. The second part of the signal (tetoth)

is exclusively due to the hole.

of holes, the output signal at initial stages is almost fully due to electrons. To see
this quantitatively let us assume that a charge pair is created in the center of the
depletion region, i.e.,x=d/2. The collection times of the electron and the hole in
this case are

te =

d^2

2 μeV 0

and

th =

d^2

2 μhV 0

≈

3 d^2

2 μeV 0

=3te.