Near intrinsic semiconductor

It has already been remarked that when the hole concentration approaches the electron concentration as in a near-intrinsic semiconductor or a photo-conductive insulator, the diffusion length is no longer the minority-carrier dilfusion length Lp = ylDpXp, but rather an ambipolar or effective L. In this section we wish to show the relationship of the measured Lq to the minority-carrier diffusion length Lp in the context of a material with the photocon-ductive properties of undoped a-Si H. 

A p-i-n diode has a wide, lightly doped, near-intrinsic semiconductor between the usual p-type and n-type regions. The p and n regions are heavily doped and provide ohmic contacts, just as in a standard diode. The depletion region exists almost entirely within the intrinsic region - so it is much wider that in a conventional p-n diode. The wide depletion region means that the diode has a low capacitance when reverse biased, enhancing the frequency response. 

Since Nc is nearly equal to N, the Fermi level of intrinsic semiconductors is located midway in the band gap as shown in Fig. 2-16. All the equations given in the foregoing are valid under the condition that rii Nc or Ny. this condition is frilfilled with usual intrinsic semiconductors. 

Equation (6.31) indicates that the conductivity of intrinsic semiconductors drops nearly exponentially with increasing temperature. At still higher temperamres, the concentration of thermally excited electrons in the conduction band may become so high that the semiconductor behaves more like a metal. 

Chapter 4 discussed semiconductivity in terms of band theory. An intrinsic semiconductor has an empty conduction band lying close above the filled valence band. Electrons can be promoted into this conduction band by heating, leaving positive holes in the valence band the current is carried by both the electrons in the conduction band and by the positive holes in the valence band. Semiconductors, such as silicon, can also be doped with impurities to enhance their conductivity. For instance, if a small amount of phosphorus is incorporated into the lattice the extra electrons form impurity levels near the empty conduction band and are easily excited into it. The current is now carried by the electrons in the conduction band and the semiconductor is known as fl-type n for negative). Correspondingly, doping with Ga increases the conductivity by creating positive holes in the valence band and such semiconductors are called / -type (p for positive). 

The charged electrode exerts an electric field on the positive and negative ions in the electrolyte similarly, the sheet of charge on the OHP exerts an electric field on the holes and electrons in the intrinsic semiconductor so that relatively near the surface, electrons and holes are not present in equal numbers. 

Semiconductors are characterized by a forbidden energy gap, Eg, (band-gap) between the valence band (VB) maximum, Evb, and the conduction band (CB) minimum, Ecb- The magnitude of the band-gap is what diiferentiates semiconductors from insulators semiconductors have smaller band-gaps (<4 eV) than do insulators. Semiconductors are termed n-type if the majority charge carriers are electrons in the conduction band and p-type if the majority carriers are holes in the valence band. The Fermi level for most pure or intrinsic semiconductors lies near the middle of the band-gap [12]. The effect of doping is to shift the Fermi level closer to Eqb for n-type semiconductors and closer to vb for p-type semiconductors. For moderate dopant levels near room temperature this can be expressed quantitatively by Fqs. 4 and 5 ... 

The number of electrons that are able to make the jump between the valence and the conduction band depends on the temperature and on the energy gap between the two bands. In an intrinsic semiconductor, the Fermi level Ef, Figure 7-15), the energy at which an electron is equally likely to be in each of the two levels, is near the middle of... 

The Fermi level in an intrinsic semiconductor is located near or at the middle of the band gap. The position of the Fermi level can be manipulated by doping, which creates donor or acceptor levels in the band gap. Focusing on the semiconductor that is of interest in this study, i.e., Ti02, doping with cations of valence higher than + 4 (W, for example) leads to the creation of a donor level near the conduction band. As a result, the Fermi level moves upward close to the conduction band and the work function decreases. This has been verified experimentally by electrical conductivity [82] and work function measurements. 

Actually Eqs. (12) and (13) would apply as well to conventional semiconductors that were near-intrinsic. We now consider two extra limitations that are specific to our experimental method and to our material. 

Fermi level. An energy level that has 50 % probability of being occupied by an electron. The probability of occupancy decreases above the Fermi level (toward vacuum), and increases below the Fermi level. The Fermi level is near the middle of the band gap for an intrinsic semiconductor, near the conduction band for an n-type semiconductor, and near the valence band for a p-type semiconductor. 

Intrinsic semiconductors are undoped and the concentration of conduction band electrons equals that of valence band holes, i.e. n = p. In this case, the Fermi energy level is located near the centre of the bandgap. 

The structure was first fabricated in a vertical layout [363]. The middle layer (active region) may be fabricated in near-intrinsic p-type semiconductor (ji) or possibly v-type (in that case the exclusion contact is below, and the extraction contact above). The thickness of this layer must be smaller than the diffusion length of minority carriers (e.g., 1.5 pm, about one third of the absorption length) so that the exclusion and the extraction junction may function as a single unit. A typical thickness of the n" or of the p" region is about 10 pm. 

Similar to all other nonequilibrium devices, Auger suppression is significant only in starting concentrations of electrons and holes are comparable (near-intrinsic material), and negligible in strongly doped semiconductor. 

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