Page 167 - Electrical Properties of Materials

Measurement of semiconductor properties 149

(a) Detector

Light from
monochromator Thin semiconductor
sample

) b ( ) c (

Transmission Transmission

Fig. 8.18
(a) General arrangement of an optical
0.8 0.9 1.0 1.1 1.3 1.2 1.1 1.0 transmission measurement and the
Wavelength, (μm) Wavelength, (μm) result for (b) GaAs and (c) Si.

from the valence into the conduction band. Most of the photons are then ab-
sorbed, and the transmission is close to zero. As the wavelength increases, there
will be a particular value (λ = c/f = hc/E g ) when band-to-band transitions are
no longer possible. The absorption then suddenly declines, and correspond-
ingly, transmission sharply increases as shown in Fig. 8.18(b) for a thin GaAs
sample. The point where the sudden rise starts may be estimated from the fig-
ure as about 880 nm, which corresponds to an energy gap of 1.41 eV, which is
just about right. Figure 8.18(b) is typical for the so-called direct-gap semicon-
ductors which have an E–k energy band structure [illustrated in Fig. 8.11 and
again in Fig. 8.19(a)] where the maximum of the valence band is at the same k
value as the minimum of the conduction band. There are quite a number of direct-
Silicon and germanium are indirect-gap semiconductors as shown in gap semiconductors. In fact, most
Fig. 8.10 and also in Fig. 8.19(b). The measured transmission as a function of the III–V and II–VI compounds
belong to that family.

) a ( E ) b ( E

hf
Electron v
Light Light
E Electron
g
Fig. 8.19
k k Photon absorption by (a) a direct- and
(b) an indirect-gap semiconductor.