Network solids semiconductors

Even though silicon is metallic in appearance, it is not generally classified as a metal. The electrical conductivity of silicon is so much less than that of ordinary metals it is called a semiconductor. Silicon is an example of a network solid (see Figure 20-1)—it has the same atomic arrangement that occurs in diamond. Each silicon atom is surrounded by, and covalently bonded to, four other silicon atoms. Thus, the silicon crystal can be regarded as one giant molecule. 

All metals conduct electricity on account of the mobility of the electrons that bind the atoms together. Ionic, molecular, and network solids are typically electrical insulators or semiconductors (see Sections 3.f3 and 3.14), but there are notable exceptions, such as high-temperature superconductors, which are ionic or ceramic solids (see Box 5.2), and there is currently considerable interest in the electrical conductivity ol some organic polymers (see Box 19.1). 

Elemental semicondudDis, like Si and Ge, as well as compound semiconductors, like GaAs, InP, and CdTe, are important examples of covalent-network solids. In a semiconductor the filled bonding molecular orbitals make up the valence band, while the empty antibonding molecular orbitals make up the conduction band. The thence and conduction bands arc separated by an energy that is referred to as the band gap. The size of the band gap increases as the bond distance decreases, and as the difference in electronegativity between the two elements increases. 

COVALENT-NETWORK SOLIDS (SECTION 12.7) Covalent-network solids consist of atoms held together in large networks by covalent bonds. These solids are much harder and have higher melting points than molecular solids. Important examples include diamond, where the carbons are tetrahedrally coordinated to each other, and graphite, where the sp -hybridized carbon atoms form hexagonal layers. Semiconductors are solids that do conduct electricity, but to a far lesser extent than metals. Insulators do not conduct electricity at all. 

The standard method of adjusting the conductivity of a semiconductor and choosing the nature (electrons or holes) of the dominant, majority, carriers is by controlled doping. Dopants are incorporated into the solid s covalent bond network. This allows the construction of p-n junctions in which the concentration profiles of the dopants, and therefore the spatial dependence of the energy-level positions, remain stable despite the existence of high internal electric fields, p-n junctions have been the basic element of many electronic components [230]. 

The communications revolution also relies on a diverse set of CVD technologies. Some components are similar to those used in silicon microelectronics, but many are unique, involving complex epitaxial heterostructures of SiGe or compound semiconductor (e.g., AlGaAs) alloys that are required to yield high frequency (1-100 GHz) device operation. The communication revolution also relies on optoelectronic components, such as solid state diode lasers (another complex heterostructure device), and these devices are often grown by CVD. - Even the fiberoptic cables that transmit the optical component of the communications network are manufactured using a CVD technique to achieve the desired refractive index profile. ... 

FIGURE 6.3.12 Thermal sensor network. Temperature dependence of current is measured under voltage bias of 2 V and data normalized by current at room temperature is plotted as a function of 1000/T for three samples stand-alone thermal sensors, denoted by solid circles, consisting of double organic semiconductors (30-nm thick CuPc and 50-nm thick PTCDl), and single organic semiconductors (80-nm thick CuPc or 80-nm thick PTCDl, denoted by solid squares and open circles, respectively) sandwiching between ITO and Au electrodes. 

This section draws attention to some of the common structure types adopted by semiconductors. The diamond-type network (often referred to an adamantine solid structure) is adopted by Si and Ge the addition of dopants occurs without structural change. Related to this network is the zinc blende lattice and among compounds adopting this structure are GaAs, InAs, GaP, ZnSe, ZnTe, CdS, CdSe,... 

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