Semiconductors degenerate

The diamond films specific resistance (p) depends on the boron content and varies, e.g., from some 104 2 cm at a boron concentration of 1018 cm 3 to a value of tenths and even thousandths of 2 cm for boron concentration as high as 1021 cm 3. Correspondingly, diamond changes its nature, starting as a dielectric, then sequentially converting to a semiconductor, degenerate semiconductor, and finally a quasimetal. 

These surprising results can be understood on the basis of the electronic structure of a graphene sheet which is found to be a zero gap semiconductor [177] with bonding and antibonding tt bands that are degenerate at the TsT-point (zone corner) of the hexagonal 2D Brillouin zone. The periodic boundary... 

Another class of conducting oxides are degenerate semiconductors, obtained by heavy doping with suitable foreign atoms. Two oxides, n-Sn02 (doped with Sb, F, In) and n-ln203 (doped with Sn), are of particular interest. These are commercially available in the form of thin optically... 

We shall assume that the electron and hole gases on the surface of semiconductor are not degenerate. Then, by definition, ... 

Degenerate semiconductors can be intrinsic or extrinsic semiconductors, but in these materials the band gap is similar to or less than the thermal energy. In such cases the number of charge carriers in each band becomes very high, as does the electronic conductivity. The compounds are said to show quasi-metallic behavior. 

Since the electron state density near the Fermi level at the degenerated surface (Fermi level pinning) is so high as to be comparable with that of metals, the Fermi level pinning at the surface state, at the conduction band, or at the valence band, is often called the quasi-metallization of semiconductor surfaces. As is described in Chap. 8, the quasi-metallized surface occasionally plays an important role in semiconductor electrode reactions. 

Electrical measurements of the luiOs (ITO) films gave temperature-independent resistivities (p) of 2 X 10 (10 ) ff-cm, carrier concentrations (N ) of 2 X 10 ° (10 ) cm, and mobilities (pi) of 3 (17) cm V sec The temperature independence of the resistivity indicated that the films, even the nominally undoped ones, were degenerate semiconductors. 

The resistivity of pure CdO was 3 X 10 O-cm (CdO is normally a degenerate n-type semiconductor), which increased approximately linearly (on a semilog scale) with increasing solution Zn content up to 10 O-cm (at 60% Zn) and then tailed off to a value of ca. 10 O-cm for very Zn-rich films. 

CdO, a degenerate n-type semiconductor, was chemically deposited on single-crystal p-type Si [40]. The junction showed clear diode behavior, and, although no photovoltaic effect was observed, photocurrent was generated under reverse bias. From the spectral response of the photocurrent, almost all of the current generation occurred in the Si. 

Bandgap measurements for Cu sulphides and selenides are complicated by the fact that these semiconductors are normally degenerate, with high free-carrier absorption in the near-infrared and possible Moss-Burstein shifts (due to saturation of the top of the valence band by holes) in the optical gap. It is quite possible that variations in bandgaps in these materials are due to differences in stoichiometry, phase, and doping rather than to any quantum size effect. Only studies where crystal size can be estimated and are possibly in the quantum size range are given here. 

In detection by degenerate superposed states, the detector is a gas of hydrogenic atoms or ions or a layer of donor doped isotropic semiconductor (e.g., GaAs and CdTe). An electrostatic field is applied in the z direction, combining the degenerate hydrogenic states 2y) and 2po) into the Stark states ... 

Fig. 1.13 Electron density of states N(E) in a cubic material F denotes the Fermi energy (a) normal metal (b) semimetal (c) insulator, (d) n-type degenerate semiconductor.

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