ABSTRACT
In Chapter 2 it was explained how irradiation of a semiconductor material can result in the creation of a hole-electron pair, provided enough energy is given to the electron to raise it into the conduction band. Since the material starts out with very few mobile electrons, i.e., as a poor conductor, the absorption of even a few quanta can cause a disproportionately large change in conduction properties. The nature of these changes depends on the type of semiconductor involved, whether conduction occurs predominantly by electrons (n-type material) or by holes (p-type material) and how rapidly either type of charge carrier is removed by annihilation, that is, a return of the excited electron to its vacant place or an equivalent one in the valence band. This return can be delayed by the presence of impurities which produce traps for either holes or electrons, where they may stay for long or short periods depending on the general state of the material and the presence of any applied potentials. Impurities that favor conduction of electrons, by trapping holes, are called donors; those that trap electrons and promote charge conduction by holes, i.e., positive charges, are called acceptors. By adding impurities of one type or another to an intrinsic semiconductor, such as very pure silicon, one can produce n-type or p-type silicon. Similar effects can be obtained with germanium; semiconducting Si and Ge form the basic ingredients of modern semiconductor electronics and microcircuitry. Both silicon and germanium belong to the central Group IV in the Periodic Table with four electrons and four vacancies to fill an electronic shell. Elements in Groups V and VI with more electrons can serve as donor impurities, those in Groups II or III as acceptor impurities. Binary compounds composed of near-equal proportions of donor and acceptor elements by themselves, such as GaAs, InSb, CdTe, HgI2, BiTe and InP, also exhibit semiconductor properties. Because most of these materials show significant changes in conductivity while irradiated, they 176all could serve, to varying extent, as radiation detectors. Table 6.1 lists the major properties of some semiconductors of interest. Properties of Semiconductors
Si
Ge
Ga As
Cd Te
In Sb
Atomic Number (Z)
14
32
31,33
48,34
49,51
Density (ρ) (300°K) (g/cm3)
2.33
5.33
5.32
6.20
5.78
Band Gap (Eg) (eV)
1.08
0.75
1.43
1.5
0.17
Mean Energy per Carrier Pair (W) (eV)
3.65
2.95
4.2
4.43
0.6
Intrinsic Resistivity (300°K) (Ωcm)
2.3 × 105
47
Electron Mobility (300°K) (μ n) (cm2/Vsec)
1,350
3,800
8,500
~650
78,000
Hole Mobility (300°K) (μ p) (cm2/Vsec)
480
1,800
420
~45
750
Carrier Lifetime (τ) (μsec)
100–3,000
1,000
0.1
40
~0.1
