Einstein College of Engineering

www.vidyarthiplus.com UNIT I PN DIODE AND ITS APPLICATIONS INTRODUCTION The current-voltage characteristics is of prime concern in the study of semiconductor devices with light entering as a third variable in optoelectronics devices.The external characteristics of the device is determined by the interplay of the following internal variables: 1. Electron and hole currents 2. Potential 3. Electron and hole density 4. Doping 5. Temperature Semiconductor equations The semiconductor equations relating these variables are given below: Carrier density: where is the electron quasi Fermi level and is the hole quasi Fermi level. These two equations lead to In equilibrium = = Constant Current: There are two components of current; electron current density and hole current density . There are several mechanisms of current flow: (i) Drift (ii) Diffusion (iii) thermionic emission (iv) tunneling Einstein College of Engineering www.vidyarthiplus.com www.vidyarthiplus.com The last two mechanisms are important often only at the interface of two different materials such as a metal-semiconductor junction or a semiconductor-semiconductor junction where the two semiconductors are of different materials. Tunneling is also important in the case of PN junctions where both sides are heavily doped. In the bulk of semiconductor , the dominant conduction mechanisms involve drift and diffusion. The current densities due to these two mechanisms can be written as where are electron and hole mobilities respectively and are their diffusion constants. Potential: The potential and electric field within a semiconductor can be defined in the following ways: All these definitions are equivalent and one or the other may be chosen on the basis of convenience.The potential is related to the carrier densities by the Poisson equation: - where the last two terms represent the ionized donor and acceptor density. Continuity equations These equations are basically particle conservation equations: Einstein College of Engineering www.vidyarthiplus.com www.vidyarthiplus.com Where G and R represent carrier generation and recombination rates.Equations (1-8) will form the basis of most of the device analysis that shall be discussed later on. These equations require models for mobility and recombination along with models of contacts and boundaries. Analysis Flow Like most subjects, the analysis of semiconductor devices is also carried out by starting from simpler problems and gradually progressing to more complex ones as described below: (i) Analysis under zero excitation i.e. equilibrium. (ii) Analysis under constant excitation: in other words dc or static characteristics. (iii) Analysis under time varying excitation but with quasi-static approximation dynamic characteristics. (iv) Analysis under time varying excitation: non quasi-static dynamic characteristics. Even though there is zero external current and voltage in equilibrium, the situation inside the device is not so trivial. In general, voltages, charges and drift-diffusion current components at any given point within the semiconductor may not be zero. Einstein College of Engineering www.vidyarthiplus.com www.vidyarthiplus.com Equilibrium in semiconductors implies the following: (i) steady state: Where Z is any physical quantity such as charge, voltage electric field etc (ii) no net electrical current and thermal currents: Since current can be carried by both electrons and holes, equilibrium implies zero values for both net electron current and net hole current. The drift and diffusion components of electron and hole currents need not be zero. (iii) Constant Fermi energy: The only equations that are relevant (others being zero!) for analysis in equilibrium are: Poisson Eq: In equilibrium, there is only one independent variable out of the three variables : If one of them is known, all the rest can be computed from the equations listed above. We shall take this independent variable to be potential. The analysis problem in equilibrium is therefore determination of potential or equivalently, energy band diagram of the semiconductor device. This is the reason why we begin discussions of all semiconductor devices with a sketch of its energy band diagram in equilibrium. Einstein College of Engineering www.vidyarthiplus.com www.vidyarthiplus.com Energy Band Diagram This diagram in qualitative form is sketched by following the following procedure: 1. The semiconductor device is imagined to be formed by bringing together the various distinct semiconductor layers, metals or insulators of which it is composed. The starting point is therefore the energy band diagram of all the constituent layers. 2. The band diagram of the composite device is sketched using the fact that after equilibrium, the Fermi energy is the same everywhere in the system. The equalization of the Fermi energy is accompanied with transfer of electrons from regions of higher Fermi energy to region of lower Fermi energy and viceversa for holes. 3. The redistribution of charges results in electric field and creation of potential barriers in the system. These effects however are confined only close to the interface between the layers. The regions which are far from the interface remain as they were before the equilibrium Analysis in equilibrium: Solution of Poisson‘s Equation with appropriate boundary conditions - Non-equilibrium analysis: The electron and hole densities are no longer related together by the inverse relationship of Eq. (5) but through complex relationships involving all three variables Y , , p The three variables are in general independent of each other in the sense that a knowledge of two of them does not lead automatically to a knowledge of the third. The concept of Fermi energy is no longer valid but new quantities called the quasi-Fermi levels are used and these are not in general constant. For static or dc analysis, the continuity equation becomes time independent so that only ordinary differential equations need to be solved. For dynamic analysis however, the partial differential equations have to be solved increasing the complexity of the analysis. Analysis of Semiconductor Devices There are two complementary ways of studying semiconductor devices: (i) Through numerical simulation of the semiconductor equations. (ii) Through analytical solution of semiconductor equations. There are a variety of techniques used for device simulation with some of them starting from the drift diffusion formalism outlined earlier, while others take a more fundamental approach starting from the Boltzmann transport equation instead. In general, the numerical approach gives highly accurate results but requires heavy computational effort also. Einstein College of Engineering www.vidyarthiplus.com www.vidyarthiplus.com The output of device simulation in the form of numerical values for all internal variables requires relatively larger effort to understand and extract important relationships among the device characteristics. The electrons in the valence band are not capable of gaining energy from external electric field and hence do not contribute to the current. This band is never empty but may be partially or completely with electrons. On the contrary in the conduction band, electrons are rarely present. But it is possible for electrons to gain energy from external field and so the electrons in these bands contribute to the electric current. The forbidden energy gap is devoid of any electrons and this much energy is required by electrons to jump from valence band to the conduction band. In other words, in the case of conductors and semiconductors, as the temperature increases, the valence electrons in the valence energy move from the valence band to conductance band. As the electron (negatively charged) jumps from valence band to conductance band, in the valence band there is a left out deficiency of electron that is called Hole (positively charged). Depending on the value of Egap, i.e., energy gap solids can be classified as metals (conductors), insulators and semi conductors. Semiconductors Conductivity in between those of metals and insulators. Conductivity can be varied over orders of magnitude by changes in temperature, optical excitation, and impurity content (doping). Generally found in column IV and neighboring columns of the periodic table. Elemental semiconductors: Si, Ge. Compound semiconductors: Binary : GaAs, AlAs, GaP, etc. (III-V). ZnS, ZnTe, CdSe (II- VI). SiC, SiGe (IV compounds). Ternary : GaAsP. Quaternary : InGaAsP. Si widely used for rectifiers, transistors, and ICs. III-V compounds widely used in optoelectronic and high-speed applications. Einstein College of Engineering www.vidyarthiplus.com www.vidyarthiplus.com Applications Integrated circuits (ICs) SSI, MSI, LSI, and VLSI. Fluorescent materials used in TV screens II-VI (ZnS). Light detectors InSb, CdSe, PbTe, HgCdTe. Infrared and nuclear radiation detectors Si and Ge. Gunn diode (microwave device) GaAs, InP. Semiconductor LEDs GaAs, GaP. Semiconductor LASERs GaAs, AlGaAs. Energy Gap Distinguishing feature among metals, insulators, and semiconductors. Determines the absorption/emission spectra, the leakage current, and the intrinsic conductivity. Unique value for each semiconductor (e.g. 1.12 eV for Si, 1.42 eV for GaAs) function of temperature. Impurities Can be added in precisely controlled amounts. Can change the electronic and optical properties. Used to vary conductivity over wide ranges. Can even change conduction process from conduction by negative charge

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