Carrier motion electric fields
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Carrier Motion - Electric Fields. ECE 2204. Movement of Electrons and Holes. Nearly free electrons can easily move in a semiconductor since they are not part of a chemical bond between atoms.

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Carrier Motion - Electric Fields

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Carrier Motion - Electric Fields

ECE 2204


Movement of Electrons and Holes

  • Nearly free electrons can easily move in a semiconductor since they are not part of a chemical bond between atoms.

  • Valence electrons are shared between atoms. It turns out that a valence electron can also exchange places with another valence electron that is being shared with a different atom.


Since valence electrons can move, holes can move also.


Carrier Mobility and Velocity

  • Mobility - the ease at which a carrier (electron or hole) moves in a semiconductor

    • Symbol: mn for electrons and mp for holes

  • Drift velocity – the speed at which a carrier moves in a crystal when an electric field is present. The electric field is the force applied to the carrier.

    • For electrons: vd= mnE

    • For holes: vd = mpE


Carrier mobility

  • The ease at which electrons and holes can move depends on the semiconductor material.

    • Nearly free electrons in direct semiconductors are faster than nearly free electrons in indirect semiconductors. Extremely high speed electronic devices are usually made from these materials.


Direction of Carrier Motion

  • Suppose we consider a piece of intrinsic semiconductor to be a resistor (which it is) and attach a dc voltage source to it.

    • Let say that the length of the semiconductor is L, its width is W, and the height is Z.

    • The magnitude of the voltage source is Va.


L

W

Z

Va

Va


Resistance

  • The equation for resistance that we used in ECE 2004 is shown below.

  • R is resistance in W.

    • is resistivity with units of W-cm.

  • L is the distance that the current has to flow as it enters and leaves the

  • resistor.

  • WZ is the cross-sectional area A of the material.


  • Resistivity and Conductivity

    • Fundamental material properties


    Questions

    • Since the resistance of the semiconductor depends on its geometry

      • What do you expect to happen to the resistance of the Si bar if L increases?

      • How about as either W and H increases?


    Current

    Current that is a result of an applied electric field is called a drift current.


    Drift Currents


    EC

    Energy Diagram

    e

    eVa

    EF

    EV

    Slopes on the energy diagram indicate that an electric field is present at that location.

    h

    e

    In

    h

    Ip

    Va


    Questions

    • Assume that the electron and hole mobilities are constant.

      • What happens to the resistance of the Si bar as the temperature increases?

    • Suppose there were bars of Si, Ge, and GaAs that had exactly the same dimensions.

      • At a particular temperature (say 300K), which bar has the lowest resistance?


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