Semiconductor Module 2 PDF

Introduction To Semiconductor General Classification of Materials : The term conductor is applied to any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminals. An insulator is a material that offers a very low level o...

Introduction To Semiconductor General Classification of Materials : The term conductor is applied to any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminals. An insulator is a material that offers a very low level of conductivity under pressure from an applied voltage source. A semiconductor, therefore, is a material that has a conductivity level somewhere between the extremes of an insulator and a conductor. Scientific Principle of Conduction Valence Band The highest occupied energy band is called the valence band. Most electrons remain bound to the atoms in this band.  Conduction Band The conduction band is the band of orbitals that are high in energy and are generally empty. It is the band that accepts the electrons from the valence band. Classification of Materials based on Energy Band : Intrinsic Semiconductors An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form. Examples of such semiconductors are: pure germanium and silicon which have forbidden energy gaps of 0.72 eV and 1.1 eV respectively. The energy gap is so small that even at ordinary room temperature; there are many electrons which possess sufficient energy to jump across the small energy gap between the valence and the conduction bands. Alternatively, an intrinsic semiconductor may be defined as one in which the number of conduction electrons is equal to the number of holes. Intrinsic Silicon At any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position. Intrinsic Silicon The electron in the lattice knocked loose from its position leaves behind an electron deficiency called a "hole". Intrinsic Semiconductors At T=0 Kelvin, intrinsic semiconductor will behave as an insulator. An increase in temperature of a semiconductor can result break up of covalent bond, i.e. 1 electron n 1 hole will be generated with break of each bond. This means electron will jump from valence band to conduction band and become free electron and hole will remain in valence band.  Doping Doping (Process of adding an impurity to increase the conductivity of Semiconductor material ) can produce 2 types of semi-conductors depending upon the element added. Extrinsic Semiconductors A semiconductor material that has been subjected to the doping process is called an extrinsic material. There are two extrinsic materials : n-type and p-type. N-Type Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. N-Type It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. N-type silicon is a good conductor. Electrons have a negative charge, hence the name N-type.  N-Type Material The n-type is created by introducing those impurity elements that have five valence electrons (pentavalent), such as antimony, arsenic, and phosphorus. Diffused impurities with five valence electrons are called donor atoms. In an n-type material the electron is called the majority carrier and the hole the minority carrier. P-Type Doping Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. P-Type Doping The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. P-type silicon is a good conductor. P type material The p-type material is formed by doping a pure germanium or silicon crystal with impurity atoms having three valence electrons, such as boron, gallium, and indium. The diffused impurities with three valence electrons are called acceptor atoms. In a p-type material the hole is the majority carrier and the electron is the minority carrier. Majority and Minorities carriers : P-N Junction In the n-type region there are extra electrons and in the p-type region, there are holes from the acceptor impurities. P-N Junction In the p-type region there are holes from the acceptor impurities and in the n-type region there are extra electrons. P-N Junction When a p-n junction is formed, some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes. P-N Junction Filling a hole makes a negative ion and leaves behind a positive ion on the n-side. A space charge builds up, creating a depletion region. P-N Junction This causes a depletion zone to form around the junction (the join) between the two materials. This zone controls the behavior of the diode. Diode A diode is the simplest possible semiconductor device.  P-N junction Diode In the absence of an applied bias voltage, the net flow of charge in any one direction for a semiconductor diode is zero This causes a depletion zone to form around the junction (the join) between the two materials. This zone controls the behavior of the diode.  Reverse biased p-n junction A semiconductor diode is Reverse-biased when the association p-type and negative and n-type and positive has been established. The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction.  Forward biased p-n junction A semiconductor diode is forward-biased when the association p-type and positive and n-type and negative has been established. Forward biasing the p-n junction drives holes to the junction from the p-type material and electrons to the junction from the n-type material. Diode Current Is is reverse saturation current k = 11,600/η, η=1 for Ge and η=2 for Si for relatively low levels of diode current and η=1 for Ge and Si for higher levels of diode current T is temp.  When forward-biased, there is a small amount of voltage necessary to get the diode going. In silicon, this voltage is about 0.7 volts. This voltage is needed to start the hole-electron combination process at the junction. Ideal Diode