L15 SemiconductorsDiodes 1

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EEE 1

Transcript of L15 SemiconductorsDiodes 1

  • Semiconductors: Diodes

    EEE 1: Lecture 15

  • Semiconductors

    Conductors any material that will support a generous flow of charge when a voltage source of limited magnitude is applied across its terminals

    Insulators material that offers a very low level of conductivity under pressure from an applied voltage source

    Semiconductors material whose conductivity level is in between that of an insulator and a conductor

  • Resistivity

    =

    measured in units of -cm

    Copper (conductor) = 10-6

    Mica (insulator) = 1012

    Germanium (semiconductor) = 50

    Silicon (semiconductor) = 50x103

  • Semiconductor materials

    Intrinsic materials impurities reduced to a very low level

    Ge and Si show a reduction in resistance with increase in temperature (negative temperature coefficient)

    Opposite for conductors

  • Energy gap

    Energy required to be absorbed by an electron in order to break away from the atomic structure to enter the conduction band

    The energy is measured in electron volts (eV)

    W = QV = 1.6E-19 C * 1V

    1eV = 1.6E-19 J

  • Energy gap

    Silicon = 1.1eV

    Germanium = 0.67eV

    Gallium Arsenide = 1.41eV

  • Extrinsic Materials

    Semiconductor materials subjected to doping processes to alter their electrical characteristics

    N-type, P-type

    Still electrically neutral

  • N-type extrinsic materials

    Excess of electrons

    Example includes the doping with a antimony impurity (pentavalent)

    Energy gap is significantly less than the intrinsic versions

    Majority carrier electron

    Minority carrier holes

  • P-type extrinsic material

    Common doping materials are boron, gallium, and indium (3 valence electrons)

    Excess of holes

    Majority carrier holes

    Minority carrier electrons

  • Ideal Diode

    Symbol

    P Anode

    N Cathode

  • Ideal Diode

    Depletion region/layer region where the holes and electrons immediately combine

  • Diode Voltage

    Application of voltage across diodes

    VD = 0 V no bias

    VD > 0 V forward bias

    VD < 0 V reverse bias

  • No Bias

    Holes in n-type will be in the depletion region and will pass into p-type material

    Electrons in the n-type must overcome the attractive forces of the positive ions in the n-type material and the shield of negative ions in the p-type

    A large number of electrons means there is a small number of electrons with sufficient kinetic energy to cross into p-type material

  • No Bias

    Applying the same assumptions from the p-type side, will result in a net electron/hole movement of zero

    A charge barrier is formed at the depletion region, restricting movement of majority carriers but help movement of minority carriers

    The small number of minority carriers will trigger a cancellation of vectors and zero current flow

  • Reverse Bias

    The application of a positive voltage at the n-type side will cause the majority carriers to be drawn to the terminal, leaving the minority carriers to build up an even larger depletion region, reducing majority carrier flow through it to zero Same condition for the p-type side

    There will be the same minority carrier flow through the depletion region, causing a small-scale current flow from n-type to p-type material Saturation current (Is)

    - VD +

  • Forward Bias

    Applying a negative voltage at the n-type terminal will cause an attraction of the minority carriers and will repel the majority carriers causing pressure to recombine with the majority carriers of the p-type material

    Minority carrier flow remains unchanged, but majority carrier flow is significantly larger as the charge barrier has

    been reduced

    + VD -

  • Diode Equation

    Diode current is defined by the following equation

    = 1

    Where

    - IS = reverse saturation current

    - K = 11600/ (=1 for Ge, =2 for Si)

    - TK = temperature in Kelvin

  • Diode Characteristics

  • Zener Region

    If the reverse biasing voltage is large enough, the diode will reach a point of operation wherein the diode current will increase rapidly in the opposite direction

    Zener potential (VZ)

    The velocity of minority carrier movement increases which triggers a large enough value of kinetic energy to release additional carriers

    Avalanche breakdown region Avalanche current

  • Diode Characteristics

  • Diode Equivalent Circuits

    Piecewise-Linear Equivalent

    Simplified Equivalent

    Ideal Equivalent

  • Diodes

    Piecewise-Equivalent Model

    Biasing Diode Voltage (VD)

    Diode Current (ID)

    Effective Resistance

    Reverse < VT 0 mA Infinite

    Forward >= VT (VD VT)/rav A rav

    VT - Threshold Voltage VT = 0.7 V for Si VT = 0.2 for Ge

  • Diodes

    Simplified Diode Model

    Biasing Diode Voltage (VD)

    Diode Current (ID)

    Effective Resistance

    Reverse < VT 0 mA Infinite

    Forward >= VT >= 0 A 0

    VT - Threshold Voltage VT = 0.7 V for Si VT = 0.2 for Ge

  • Diodes

    Biasing Diode Voltage (VD)

    Diode Current (ID)

    Effective Resistance

    Reverse < 0 V 0 mA Infinite

    Forward > 0 V >= 0A 0

    As a switch, Forward Bias == ON Reverse Bias == OFF

    Ideal Diode Model

  • Example

    For the series diode configuration, determine VD, VR, and ID. Is the diode forward or reverse biased?

    600mV1 R1

    1.2k

    + VR -

    + VD - ID

  • Example

    Determine Vo, ID1, ID2, and IA. Both D1 and D2 are Silicon diodes

    D2D1R1

    100

    10

    V1

    + VD -

    ID2 ID1

    IA