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SEMICONDUCTOR PHYSICS

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In this chapter, a brief description of the basic concepts governing the flow of current in a pn junction are discussed. Both intrinsic and extrinsic semiconductors are discussed. | Attia John Okyere. Semiconductor Physics. Electronics and Circuit Analysis using MATLAB. Ed. John Okyere Attia Boca Raton CRC Press LLC 1999 1999 by CRC PRESS LLC CHAPTER TEN SEMICONDUCTOR PHYSICS In this chapter a brief description of the basic concepts governing the flow of current in a pn junction are discussed. Both intrinsic and extrinsic semiconductors are discussed. The characteristics of depletion and diffusion capacitance are explored through the use of example problems solved with MATLAB. The effect of doping concentration on the breakdown voltage of pn junctions is examined. 10.1 INTRINSIC SEMICONDUCTORS 10.1.1 Energy bands According to the planetary model of an isolated atom the nucleus that contains protons and neutrons constitutes most of the mass of the atom. Electrons surround the nucleus in specific orbits. The electrons are negatively charged and the nucleus is positively charged. If an electron absorbs energy in the form of a photon it moves to orbits further from the nucleus. An electron transition from a higher energy orbit to a lower energy orbit emits a photon for a direct band gap semiconductor. The energy levels of the outer electrons form energy bands. In insulators the lower energy band valence band is completely filled and the next energy band conduction band is completely empty. The valence and conduction bands are separated by a forbidden energy gap. conduction band 1.21 eV gap valence band w c o u _Q CD O O O c CD conduction band 0.66 eV gap valence band co c o u _Q CD O O O c CD conduction band 5.5 eV gap valence band Figure 10.1 Energy Level Diagram of a Silicon b Germanium and c Insulator Carbon 1999 CRC Press LLC In conductors the valence band partially overlaps the conduction band with no forbidden energy gap between the valence and conduction bands. In semiconductors the forbidden gap is less than 1.5eV. Some semiconductor materials are silicon Si germanium Ge and gallium arsenide GaAs . Figure 10.1 shows the energy level .