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Unit 14 Semiconductor Physics

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Unit 14 Semiconductor Physics UNIT 14 SEMICONDUCTOR PHYSICS Structure Introduction Objectives Energy Bands in Solids Intrinsic and Extrinsic Semiconductors Conduction in Intrinsic Semiconductors Extrinsic Semiconductors p-n Junction Diode p-n Junction with no External Voltage Characteristics of Forward and Reverse Bias Diode as a Rectifier Other Applications of Diodes Solar Cell Photodiode Light Emitting Diode Zener Diode Summary Terminal Questions . Solutions and Answers 14.1 INTRODUCTION We are all familiar with simple applications of electronics like radio, television, calculators, personal computers etc. in, our day-to-day lives. If we look inside any electronic equipment, we will find resistors, capacitors, semiconductor diodes, transistors etc. Semiconductors have contributed immensely to the developments in science and technology. The invention of transistors in 1940s and that of integrated circuits (ICs) later led to great advancements in the capabilities of electronic devices and a wide variety of applications in different walks of life. 'This is because electronic components made up of se~niconductormaterials have advantages such as higher reliability, low power requirement and miniaturisation of devices. With recent semiconductor technology it has been possible to fabricate as many as one million transistors on a single 1 cm2 semiconductor chip. A unique property of semiconductors is a remarkable increase in their conductivity with doping. Their response to light and other electromagnetic radiations results in a drastic change in electrical and optical properties, leading to many useful applications as solar cell, photodiode, infrared detectors etc. Therefore, we begin this unit by discussing how to teach the physics of semiconductor devices and their uses. We first describe the basic features of semiconductor materials. Our experience shows that the formation of energy bands in solids is a difficult concept for students to understand. In Sec. 14.2 we suggest some ways of explaining this along with differences between insulators, metals and semiconductors on the basis of the energy band diagrams. Sec. 14.3 is devoted to an explanation of intrinsic semiconductors, the type of charge carriers (and their movement) present in them and the conduction mechanism at different temperatures. We also discuss the concept of doping of semiconductors with different types of impurity atoms to bring out a clear picture of carrier concentration and other properties of extrinsic semiconductors. Sec. 14.4 deals with the teaching ofp-n junction diode, the most simple but extremely useful electronic device made of semiconductors. By varying the doping levels in a simplep-n junction diodes, we can use them for a wide range of applications. In Sec. 14.5 we take up the application of semiconductorp-n junction diodes as a solar cell, photodiode, light emitting diode and zener diode. Semiconductor Physics Objectives After studying this unit, you should be able to - : Explain better to your students - the formation of energy bands in solids and difference in metals, insulators and semiconductors using energy band diagrams, - the fundamentals of semiconductor materials and mechanism for transport of charge carriers in them, - the formation of barrier potential in ap-n junction diode, - the conduction property of forward and reverse biasedp-n junction diode, - the working ofp-n junction diode as rectifiers (half wave and full wave), - construction and working of solar cells, photodiodes,,light emimng diodes and Zener diodes. Devise activities to help students acquire a beher understanding of the physics of semiconductor materials and devices; Assess how well your strategies have worked. 14.2 ENERGY BANDS IN SOLIDS As you are aware, an atom consists of a positively changed nucleus surrounded by electrons. You have taught your students about the energy levels of electrons in atoms. They have learnt that electrons in an atom can exist only in well defined quantized energy levels. Fig.14.1: Energy levels of the hydrogen atom Students find it difficult to understand the formation of energy bands from energy levels of atoms in a crystal. You can explain this as follows: There are billions of atoms in a crystal. You need to explain that when there are two identical atoms such as HZ,N2, 02,etc., each atomic energy level splits into two. When there are three identical atoms such 03,N3, each energy level splits into 3. And when there are N identical atoms in a solid, each atomic energy level splits into N closely spaced levels. The total energy spread may be only a few eV, so that we have as many as lo2' or 102' energy levels in a few eV. This is called an energy band. The first energy band is formed by the ground state energy levels. Atomic, Nuclear Physics and Electronics ENERGY t VALENCE BAND I======I st BAND Fig.14.2: Energy bands Closely spaced outermost filled energy levels of all atoms in the crystal form the outermost band. In an isolated atom, the outermost energy level is called valence shell or valence orbital. There is an infinite number of energy levels or bands, but a finite number of electrons, so some levels are filled while all the others are empty. The outermost filled levelhand refers to valence. Hence in a crystal the outermost filled band is called valence band. An important point to be emphasised here is the relation between interatomic distance and arrangement of energy bands in solids as shown in Fig. 14.3. Fig.14.3: Formation of energy bands Note from the figure, that when interatomic distance is more (denoted by dmin Fig. 14.3), then we have energy levels corresponding to isolated single atoms. As the interatomic distance decreases (denoted by distances.d and do in Fig. 14.3), atoms interact with each other and different atomic orbitals overlap each others. Thus, instead of discrete energy levels of single isolated atoms, we get energy bands. You may further explain the origin of energy bands by taking the example of a crystal consisting of N atoms of a semiconductor, say, silicon (Si). Electrons of an isolated Si atom (ls22s2 2p6 3s2 3p2)exist in discrete energy levels. In a crystal, the atoms are very close to each other (2 to 3 A). Electrons interact with each other and the atomic nuclei. The 3s 3p level in group IV atoms has accommodation for 8 electrons, though there are only 4 electrons. They form sp3 tetrahedral bonds, and the s and p cannot be separately distinguished. When N atoms come together, they form two bands of 4N levels each. So the lower of them is filled and the upper one empty. You can use Fig. 14.4 to show the relation between energy and inter-atomic separation (r) for Si. When the Si atoms are far apart (say r = dl) then the electrons in the outermost shell of one atom do not interact with those of the other and the energy levels of ls2 2s2, 2p6, 3s2, 3p2 of each of the N atoms remain unchanged. As the inter-atomic separation or distance is decreased from r = dl to r = d2 then we get two bands having a large number of closely spaced energy levels, with an energy gap between them. Sem;conductor Physics 1 4N states lsolated atoms e'eT\ ?I C.B. I 4Energygap j ' state /*--I 14~electrons + I d~ I + Interatomic separation Fig.14.4: Diagram showing formation of energy bands from energy levels when silicon atoms are brought closer to form a silicon crystal. [C.B. = CONDUCTION BAND and V.B. = VALENCE BAND) On further decreasing the inter-atomic distance to a point when Si atoms are at actual inter-atomic separation r = a, the 3s 3p energy levels are apportioned into two bands: a band of 4N energy levels completely filled with electrons and another band of 4N empty levels. Now there are as many energy levels in the band of lower energy as the number of valence electrons in the N atoms. - The lower energy band of 4N filled levels is composed of outermost energy I levels of all atoms having valence electrons, hence it is named valence band (VB). - The highest valence band energy is denoted by E,. - The electrons fiom the valence band (having" lower enerm)Wd r can aium~ n to umern. empty energy band (of higher energy) on gain of energy and become free (as loosely bound to the nucleus) and contribute to electrical conduction. Hence the upper energy band of 4N empty levels is called conduction band (CB). The lowest energy level in the conduction band is denoted by Ec. - These two bands are separated from each other by an energy gap, which is the difference between the energy levels Ec and E,. Above E, and below E,, there are a large number of closelv s~acedenerm levels. If this energy gap is very wide then it becomes almost impossible for electrons to jump from valence band to conduction band. Such materials are called insulators. I The metals, insulators and semiconductors can be differentiated from each other on the basis of energy band structure in them. The best criterion to distinguish between them is this: Let El be the topmost energy of an electron in a crystal such that all other electrons have energy lower than this at 0 K, and let E2 be the next allowed but empty level. If E2 -+El, that is they are two consecutive energy levels in an allowed band, then it is a conductor. If E2 - El is finite (non-zero), then it is a dielectric. (This definition will also work for semimetals, conducting polymers, etc.) Incidentally if the outermost atomic shell contains an odd number of electrons, such a solid must become a conductor, because there is accommodation for 2N 14N 16N .
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