Physics and Engineering of Radiation Detection

262 Chapter 5. Solid State Detectors

which then gets emitted and may even escape from the detector. This emitted

electron is called Auger electron and the process is sometimes referred to as

Auger recombination. The excess energy can also go into increasing the lattice

vibrations of the crystal. These vibrations can travel through the crystal in

the form of heat carrying particles generally known asphonons.

Trap-Recombination:As we saw earlier, there are always crystal defects and

impurities in semiconductors. These defects and impurities produce energy lev-

els inside the forbidden gap. Such an energy level can act as a metastable elec-

tron trap such that when an electron falls into this level, it may remain there for

some time before eventually falling into the valence band. The net effect is still

recombination but the process is somewhat delayed as compared to the nor-

mal band-recombination. As shown in Fig.5.1.6, two photons can be produced

during this process with the combined energy equal to the energy released dur-

ing the band-recombination process. Also, as with band-recombination, the

process could be radiationless such that the excess energy gets transferred to

increase lattice vibrations. The trap-recombination process is generally known

as Shockley-Hall-Read (SHR) recombination.

Electron

Auger

Eg

Photon

Hole

Electron

Conduction Band

Valence Band

E

Figure 5.1.5: Band-recombination with

subsequent emission of an Auger electron.

The net effect of this recombination process is the removal of a charge pair from
the free charge population. Crystal imperfections and impurities produce intermedi-
ate energy levels that greatly enhance the recombination process. Radiation damage
to semiconductors, which we will discuss later in the chapter, also increase the re-
combination probability. To minimize the deteriorating effects of this process, it
must therefore be ascertained that the material has very few imperfections and also
has high radiation tolerance.
As described above, recombination of electrons and holes has different channels
and is therefore a fairly complicated process. However its overall effect can be charac-
terized by simple considerations of Poisson process. Byoverall effectwe specifically
mean the recombination rate, which has been seen to follow Poisson statistics. This
implies that the rate of change of number of charge pairs is proportional to the