Extrinsic n- and p-type semiconductors

Semiconductors demand the use of silicon of extreme purity. The native element does not occur naturally and silica (Si02) and silicate minerals are its principal sources. Silicon can be extracted from silica by reduction with carbon in an electric furnace, but the product is far too impure for the semiconductor industry. A number of purification methods are used, but of these, two are important for producing single crystals of Si. 

The principle of the Czochralski process is to draw single crystals of Si from the molten element. The thermal 

Gmelin Handbook of Inorganic Chemistry (1984), Silicon Part A1 History , System number 15, Springer-Verlag, Berlin, p. 51. 

See also Section 13.6 (hydrides of group 14 elements) and Section 27.6 (chemical vapour deposition). 

Extrinsic semiconductors contain dopants a dopant is an impurity introduced into a semiconductor in minute amounts to enhance its electrical conductivity. 

The semiconducting properties of Si and Ge can be enhanced by doping these elements with atoms of a group 13 or group 15 element. Doping involves the introduction of only a minutely small proportion of dopant atoms, less than 1 in 

and extremely pure Si or Ge must first be produced. The reduction of Si02 in an electric furnace gives Si, and the Czochralski process (see Box 6.3) is used to draw single crystals of Si from the melt. We describe how dopants are introduced into semiconductors in Section 28.6. 

Kltifers, R. Staudigl and P. Stallhofer (2003) Angewandte Chemie International Edition, vol. 42, 

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Figure 11.2 Energy diagram for various semiconductors. Shown are examples for pure (intrinsic) and doped (extrinsic) n- and p-types. In each case the Fermi level is shown as a dotted line...

Dowden (27) in a theoretical approach similar to that used for metals, has examined the probability of positive ion formation on intrinsic and extrinsic semiconductors. The energy of activation of this process in intrinsic semiconductors is considered to be proportional to ) where / is the ionization potential of the activated complex (/ + Ab ) the activation energy decreases as (exit work function) increases and as AF decreases, — defining the Fermi level. In the case of n- and p-type semiconductors, the Fermi level will... 

The n- and p-type semiconductors are extrinsic semiconductors, and their precise properties are controlled by the choice and concentration of dopant. Semiconductors are discussed further in Section 27.6. 

Figure 6.1 Schematic band structures of solids (a) insulator (kT ,) (b) intrinsic semiconductor (kT ,) (c) and (d) extrinsic semiconductors donor and acceptor levels in n-type and p-type semiconductors respectively are shown, (e) compensated semiconductor (f) metal (g) semimetal top of the valence band lies above the bottom of the conduction band.

Extrinsic semiconductors (both n- and p-type) are produced from materials that are initially of extremely high purity, commonly having total impurity contents on the... 

The position of the Fermi level in an extrinsic semiconductor depends upon the dopant concentrations and the temperature. As a rough guide the Fermi level can be taken as half way between the donor levels and the bottom of the valence band for n-type materials or half way between the top of the valence band and the acceptor levels for p-type semiconductor, both referred to 0 K. As the temperature rises the Fermi level in both cases moves toward the center of the band gap. 

Extrinsic Semiconductors are materials that contain donor or acceptor species (called doping substances) that provide electrons to the conduction band or holes to the valence band. If donor impurities (donating electrons) are present in minerals, the conduction is mainly by way of electrons, and the material is called an n-type semiconductor. If acceptors are the major impurities present, conduction is mainly by way of holes and the material is called a p-type semiconductor. For instance in a silicon semiconductor elements from a vertical row to the right of Si of... 

Thus, it follows that the Fermi level of p-1ype semiconductors ascends from an energy level near ea toward the middle of the band gap with decreasing acceptor concentration, N. From Eqns. 2-22 and 2-24, we obtain in general that the Fermi level is located at levels higher for n-type semiconductors and lower for p-type semiconductors than the middle of the band gap. As described in the foregoing, the concentration of electrons, n, in the conduction band is different from the concentration of holes, p, in the valence band in extrinsic semiconductors... 

The presence of an impurity such as an As or a Ga atom in silicon leads to an occupied level in the band gap just below the conduction band or a vacant level just above the valence band, respectively. Such materials are described as extrinsic semiconductors. The n-type semiconductors have extra electrons provided by donor levels, and the p-type semiconductors have extra holes originating from the acceptor levels. Band structures of the different types of semiconductors are shown in Fig. 4.3.4. 

In intrinsic semiconductors, considerable effects are possible, provided that the concentration of charge carriers remains small in comparison with the values of 10 and 10 just cited. Depending on whether the donor or acceptor type of defect is predominant, transformation may result into n- or p-type extrinsic semiconductor. 

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