Semiconductors silicon crystal structure

Silicon is a semiconductor with an intrinsic conductivity of 4.3 x 10" Q" cm and a band gap of I.I2eV at 300K. It has a diamond crystal structure characteristic of the elements with four covalently bonded atoms. As shown in Fig. 2.1, the lattice constant, a, is 5.43 A for the diamond lattice of silicon crystal structure. The distance between the nearest two neighbors is V3a/4, that is, 2.35 A, and the radius of the silicon atom is 1.18 A if a hard sphere model is used. Some physical parameters of silicon are listed in Table 2.1. 

If instead, indium is the impurity in the silicon crystal structure, the opposite effect is produced. Such material contains a number of energy levels only 0.06 eV above the valence band the result is holes in the val ce bands. Such material is referred to as acceptor material. Silicon with acceptor material is called p-type silicon, since the holes are considered to be positively charged. Conduction is in this case by movement of holes. The addition of controlled amounts of impurity atoms thus provide charge carriers (as the electrons and holes collectively are called, c.f. 8.2) and produces the desired properties in semiconductor materials. 

Silicon can also be doped with a group 3A element, such as gallium, which has only three valence electrons. When gallium is incorporated into the silicon crystal structure, it results in electron holes, or empty molecular orbitals, in the valence band. The presence of holes also allows for the movement of electrical current because electrons in the valence band can move between holes. In this way, the holes move in the opposite direction as the electrons. This type of semiconductor is called a p-type semiconductor because each hole acts as a positive charge. 

The empirical pseiidopotential method can be illustrated by considering a specific semiconductor such as silicon. The crystal structure of Si is diamond. The structure is shown in figure Al.3.4. The lattice vectors and basis for a primitive cell have been defined in the section on crystal structures (ATS.4.1). In Cartesian coordinates, one can write G for the diamond structure as... 

J. E. Peters and P. D. Ownby. Far Infrared Transmission of Diamond Structure Semiconductor Single Crystals-Silicon and Germanium , Optical Engineering, Vol. 38, No. 11, pp. 1924-1931, November 1999. 

Silicon is the most popular material for photovoltaic (PV) power. Another material is gallium arsenide (GaAs), which is a compound semiconductor. GaAs has a crystal structure similar to that of silicon, but it consists of alternating gallium and arsenic atoms. It is well suited for PV applications since it has a high light absorption coefficient and only a thin layer of material is required, which reduces the cost. 

This capability of self-assembly to make ordered arrays of nanostructures is, in essence, nothing new. Crystallization of molecular or atomic species (whether it is the phase transition of liquid water into solid ice, or of liquid silicon into semiconductor-grade silicon crystal) is an example of self-assembly, as are the formation of surfactants in soap bubbles126, the crystallization of viruses for x-ray structure determination127, and the ordering of liquid crystals in displays128. The novelty of self-assembly is in the focus on the formation of matter structured rationally at scales less than 100 nm, and the realization that the only practical method of achieving these structure is to have the components assemble themselves spontaneously. 

Silicon crystallizes in the diamond structure,16 which consists of two interpenetrating face-centered cubic lattices displaced from each other by one quarter of the body diagonal. In zinc blende semiconductors such as GaAs, the Ga and As atoms lie on separate sublattices, and thus the inversion symmetry of Si is lost in III-V binary compounds. This difference in their crystal structures underlies the disparate electronic properties of Si and GaAs. The energy band structure in... 

Figure 12.9 Lewis structures for pure and doped silicon crystals, (a) Pure silicon showing excitation of two elcclron-hole pairs, (b). Si doped with P. an electron donor. Electrons can be excited from ihc donor band lo ihc coiuluclion band to form a n-type semiconductor, (c) Si doped wilh 15, an clci. iron acceplor. I dcclrons excited from the valence band leave positive holes which enable p type lomiui iivily.

A semiconductor such as phosphorus-doped silicon is called an n-type semiconductor because extra electrons (negatively charged) are present in the crystal structure. 

Intrinsic semiconductors. The group fourteen elements carbon, silicon, germanium, and tin can be found to adopt the diamond-type crystal structure shown in Figure 3 a. Other crystalline structures are also found for example, graphite and diamond are different crystal structures of the same element, carbon. Because of its size and orbital energies, carbon forms very... 

---