Tiêu đề 14. SEMICONDUCTOR ELECTRONICS MATERIALS, DEVICES AND SIMPLE CIRCUITS.pdf ✅

The word "electronics' is derived from electron + dynamics which means the study of the behavior of an electron under different conditions of externally applied field. This field of science deals with electronic devices and their utilization. An electronic device is a device in which conduction takes place by the movement of electron - through a vacuum, a gas or a semiconductor. Main application of electronic is computer which is used in every field. All electronics equipment required D.C. supply for operation (not A.C. supply). Energy Band The energy levels of an isolated atom are clearly defined. However, these individual energy levels overlap and undergo substantial modification when numerous such atoms combine to create a real solid. The energy values of electrons are not discrete but rather fall within a range. An energy band is thought to be formed by the accumulation of these densely grouped energy levels. Valence Band and Conduction Band are two terms used to describe these types of bands that form in solids. The Valence Band is made up of filled energy levels, while the Conduction Band is made up of partially filled or unfilled energy levels. A space known as the energy gap or forbidden gap typically separates the two bands. Classification Of Solids According to Energy Band Theory According to energy band theory, solids are conductor, semiconductor and insulator: Conductor In some solids conduction band and valence band are overlapped so there is no band gap between them, it means Eg = 0. Due to this a large number of electrons are available for electrical conduction and therefore its resistivity is low ( = 10–2 – 10–8U–m) and conductivity is high [ =102 – 108 (–m)–1 ]. Such materials are called conductors. For example, gold, silver, copper, etc. Insulator In some solids energy gap is large (Eg > 3eV). So, in conduction band there are no electrons and so no electrical conduction is possible. Here energy gap is so large that electrons cannot be easily excited from the valence band to conduction band to conduction band by any external energy (electrical, thermal or optical) Such materials are called as "insulator". Their U > 1011 –m and  < 10–11 (Q–m)–1 Semiconductor In some solids a finite but small band gap exists (Eg < 3eV). Due to this small band gap some electrons can be thermally excited to "conduction band". These thermally excited electrons can move in conduction band and can conduct current. Their resistivity and conductivity both are in medium range, ≃10–5 – 106 –m and ≃10–6 – 105 –m–1 Example of semiconducting materials Elemental semiconductor: Si and Ge Compound semiconductor Inorganic: CdS, GaAs, CdSe, InP etc. Organic: Anthracene, Doped pthalocyanines etc. Organic Polymers: Poly pyrrole, Poly aniline, polythiophene Properties Of Semiconductor Negative temperature coefficient (a.), with increase in temperature resistance decreases. Crystalline structure with CHAPTER – 14 SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND SIMPLE CIRCUITS SEMI C O N D U C T OR ELE CTR O NIC S: MATERI ALS, D E VICES A N D SIMPLE CIR CUITS
covalent bonding [Face centered cubic (FCC)]. Conduction properties may change by adding small impurities Position in periodic table - IV group (Generally} Forbidden energy gap (0.1 eV to 3 eV) Charge carriers: electron and hole. There are many semiconductors but few of them have practical application in electronics. Holes Due to external energy (temperature or radiation) when electron goes from valence band to conduction band (i.e., bonded electrons become free), vacancy of free e– creates in valence band. The electron vacancy called as "hole" which has same charge as electron but positive. This positively charged vacancy move randomly in semiconductor solid. Properties of holes ➢ It is missing electron in valence band. ➢ It acts as positive charge carrier. ➢ Its effective mass is more than electron. ➢ Its mobility is less than electron. Holes acts as virtual charge, although there is not physical charge on it. Intrinsic Semiconductor Pure semiconductors are in which the conductivity is caused due to charge carriers made available from within the material are called intrinsic semiconductors. There are no free charge carriers available under normal conditions. However, when the temperature is raised slightly, some of the covalent bonds in the material get broken due to thermal agitation and few electrons become free. In order to fill the vacancy created by absence of electron at a particular location, electron from other position move to this location and create a vacancy (absence of electron) at another place called hole. The movement/shifting of electrons and holes within the material results in conduction. An intrinsic semiconductor behaves as a perfect insulator at temperature 0 K Extrinsic Semiconductors Extrinsic semiconductors are semiconductors that are doped with specific impurities. The impurity modifies the electrical properties of the semiconductor and makes it more suitable for electronic devices such as diodes and transistors. While adding impurities, a small amount of suitable impurity is added to pure material, increasing its conductivity by many times. Extrinsic semiconductors are also called impurity semiconductors or doped semiconductors. The process of adding impurities deliberately is termed as doping and the atoms that are used as an impurity are termed as dopants. The impurity modifies the electrical properties of the semiconductor and makes it more suitable for electronic devices such as diodes and transistors. The dopant added to the material is chosen such that the original lattice of the pure semiconductor is not distorted. Also, the dopants occupy only a few of the sites in the crystal of the original semiconductor, and it is necessary that the size of the dopant is nearly equal to the size of the semiconductor atoms. Q. Find the maximum wavelength of electromagnetic radiation, which can create a hole-electron pair in germanium. Given that forbidden energy gap in germanium is 0.72eV. Sol. Here, Eg = 0.72eV = 0.72 × 1.6 × 10−19 The maximum wavelength of radiation, which can create a hole-electron pair in germanium is given by Eg = hc λ Or λ = hc Eg = 6.62×10−34×3×108 0.72×1.6×10−19 = 1.724 × 10−6 m Q. Distinguish between intrinsic and extrinsic semiconductors? Sol. A semiconductor free from all types of impurities is called an intrinsic semiconductor. At room temperature, a few covalent bonds break up and the electrons come out. In the bonds, from which electrons come out, vacancies are created. These vacancies in covalent bonds are called holes. In an intrinsic semiconductor, holes and electrons are equal in number and they are free to move about in the semiconductor. On the other hand, a semiconductor doped with a suitable impurity (donor or acceptor) so that it possesses conductivity much higher than that of pure semiconductor is called an extrinsic semiconductor. The extrinsic semiconductor may be of n-type or p-type.