Semiconductor Diodes Lecture 1 EEME-2752 Electronics Engineering Type of Materials Conductors Materials that offer low resistance to the flow of current Semi conductors Materials that have a value of resistance between that of a conductor and an insulator Insulators Material that offers high resistance to the flow of current 2 Electronics Engineering 26-Feb-19 Semiconductor Materials Three semiconductor materials are commonly used in the construction of semiconductor devices Germanium (Ge) Silicon (Si) Gallium arsenide (GaAs) Semiconductors can be in intrinsic or extrinsic form depending upon the level of impurity present Intrinsic semiconductors (without impurity) Extrinsic semiconductors (with impurity) 3 Electronics Engineering 26-Feb-19 Intrinsic Materials Semiconductor materials have four valance electrons Pure Silicon or Germanium crystal have four valance electrons of one atom bound in covalent bond with four neighbouring atoms 4 Electronics Engineering 26-Feb-19 Intrinsic Materials Covalent bonding of Silicon atoms 5 Electronics Engineering 26-Feb-19 Energy Levels The farther the electron is from the nucleus, the higher the energy state • 6 The electron that has left the parent atom has higher energy than any electron in the atomic structure Electronics Engineering 26-Feb-19 Energy Levels Conductivity of conductors decreases with the increase in temperature and conductors have positive temperature coefficient Conductivity of semiconductors increases with the increase in temperature and semiconductors have negative temperature coefficient 7 Electronics Engineering 26-Feb-19 Extrinsic Materials A semiconductor material that has been subjected to the process of doping is called extrinsic material Extrinsic materials can be classified into p-type materials and n- type materials depending on the impurity used for doping Pentavalent impurity atoms are used to create n-type materials and trivalent impurity atoms are used to create p-type materials 8 Electronics Engineering 26-Feb-19 N-Type Materials Pentavalent impurity atoms ( Arsenic, Phosphorous, Antimony) are added to increase the number of conduction band electrons The fifth electron loosely bound is free to move within the n-type material The impurity atom are called donor atoms 9 Electronics Engineering 26-Feb-19 P-Type Materials Trivalent impurity atoms( Boron, Indium, Gallium) are added to increase the number of holes The vacancy of one electron creates a hole which is treated as a positive carrier that can move through p-type material The impurity atoms are called acceptor atoms 10 Electronics Engineering 26-Feb-19 Majority and Minority Carriers In p-type material holes are majority carriers and electrons are minority carriers In n-type materials electrons are majority carriers and holes are minority carriers 11 Electronics Engineering 26-Feb-19 Semiconductor Diode p-n junction diode is formed by joining p-type and n-type materials together The electrons and holes across the junction combine to form a region free of carriers This region around the junction is called depletion region 12 Electronics Engineering 26-Feb-19 Semiconductor Diode In case there is no voltage applied across the diode no current will flow through the diode Biasing is the process of application of external voltage across the two terminals of the device to extract a response 13 Electronics Engineering 26-Feb-19 Semiconductor Diode REVERSE BIAS (VD < 0) Current will flow through the diode due to minority carriers Current that exists in reverse bias condition is called reverse saturation current (IS) The reverse saturation is very small usually in microamperes or nanoamperes 14 Electronics Engineering 26-Feb-19 Semiconductor Diode FORWARD BIAS (VD > 0) There will be the flow of current due to majority carriers across the junction The increase in bias voltage will result in an exponential rise the current 15 Electronics Engineering 26-Feb-19 16 Electronics Engineering 26-Feb-19 Forward Bias Voltage The point at which the diode changes from no-bias condition to forward bias condition knee point This occurs when holes and electrons are provided with sufficient energy to cross the junction The energy comes from external voltage source The forward bias voltage required for Gallium arsenide diode is 1.2 V Silicone diode is 0.7 V Germanium diode is 0.3 V 17 Electronics Engineering 26-Feb-19 Diode Resistance Due to the nonlinear characteristic curve of the diode its resistance vary with the operating point The type of applied voltage or signal will determine the resistance level of the diode There are three types of diode resistance DC or Static Resistance AC or Dynamic Resistance Average AC resistance 18 Electronics Engineering 26-Feb-19 Diode Resistance DC (Static) Resistance Operating point will not change with time if an dc voltage is applied across the diode Diode resistance can be found by VD RD = ID Lower the current through the diode higher will be the resistance 19 Electronics Engineering 26-Feb-19 Diode Resistance AC (Dynamic) Resistance Application of a time varying signal will move the operating point The straight line drawn tangent to the curve at Q-point will define dynamic resistance Vd rd = I d 20 Electronics Engineering 26-Feb-19 Diode Resistance Average AC Resistance Resistance determined by the straight line between two intersections established by maximum and minimum value of the input voltage rav 21 Vd = I d Electronics Engineering pt − pt 26-Feb-19 Determine the ac resistance at ID = 2mA Determine the ac resistance at ID = 25mA Determine the DC resistance at both points 22 Electronics Engineering 26-Feb-19 Diode Models Ideally the diode conducts current in only one direction and acts like an open in the opposite direction 23 Electronics Engineering 26-Feb-19 Diode Models Piecewise Linear model Approximate the non-linear characteristics of a diode with two linear segments 24 Electronics Engineering 26-Feb-19 Diode Models Simplified model Average resistance is small enough and can be ignored to simplify the diode model When the VD exceeds VK the diode will start to conduct 25 Electronics Engineering 26-Feb-19 Diode Models Ideal diode model If VK is ignored in comparison the applied voltage the diode model can be reduced to ideal model An ideal diode behaves as a switch and conducts in one direction 26 Electronics Engineering 26-Feb-19 Diode Models 27 Electronics Engineering 26-Feb-19