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Schottky diodes JFETs - MESFETs - MODFETshadley/psd/lectures17/nov29.pdf · metal - semiconductor...

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Institute of Solid State Physics Technische Universität Graz Schottky diodes JFETs - MESFETs - MODFETs
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  • Institute of Solid State PhysicsTechnische Universität Graz

    Schottky diodes

    JFETs - MESFETs -MODFETs

  • Quasi Fermi level

    exp

    exp

    Fn cc

    B

    v Fpv

    B

    E En Nk T

    E Ep N

    k T

    When the charge carriers are not in equilibrium the Fermi energy can be different for electrons and holes.

  • Institute of Solid State Physics

    metal - semiconductor contacts Technische Universität Graz

    Photoelectric effectWorkfunctionElectron affinity Interface states Schottky barriersSchottky diodes Ohmic contacts Thermionic emission Tunnel contacts

  • Photoelectric effect

    hf0 = e at threshold

    workfunction f

    curr

    ent

    threshold frequency f0

  • Singh

    There is a dipole field at the surface of a metal. This electric field must be overcome for an electron to escape.

  • work function - electron affinity

    If s < m, the semiconductor bands bend down.

    If s > m, the semiconductor bands bend up.

  • Singh

  • p-type

    Schottky contact / ohmic contact

    specific contact resistance:

    Ohmic contact: linear resistance

    Schottky contact

    12 -cmc

    JRV

    metal

    metal

    EF,s

    EF,s

    EF,m

    EF,m

    Walter Schottky

  • n-type

    Schottky contact / ohmic contact

    specific contact resistance:

    Ohmic contact: linear resistance

    Schottky contact

    12 -cmc

    JRV

    metal

    metal

    EF,s

    EF,s

    EF,m

    EF,m

  • Interface states

  • http://www.springermaterials.com/navigation/#n_240905_Silicon+%25

  • Schottky barrier

    2 bin

    D

    V VW x

    eN

    0

    Dn

    r

    eNE x x

    2

    2

    Dn

    eN xV xx

  • CV measurements

    -2 F m

    2A

    p bi

    e NCx V V

    2 bip

    A

    V Vx

    eN

    2

    21 biA

    V VC e N

    GaAs has larger Eg and Vbi

    V

    1/C

    2

    ( )ln vbi b BA

    N TeV k TN

  • Thermionic emission

    1901 Richardson

    Current from a heated wire is:

    2 expRB

    eJ A Tk T

    Some electrons have a thermal energy that exceeds the work function and escape from the wire.

    Owen Willans Richardson

  • Vacuum diodes

    diode

  • Thermionic emission

    EF

    Fermi function

    ( ) exp exp exp expF FB B B B

    E E E E Ef Ek T k T k T k T

    The density of electrons with enough energy to go over the barriers expB

    Ek T

    bie V V

  • Thermionic emission

    expthB

    eVnk T

    expsm thB

    eVI nk T

    ( 0)ms smI I V

    e 1BeVk T

    sm ms msI I I I

  • Schottky barrier

    Ism > ImsIsm ~ 0Ims constant

  • Thermionic emission

    e 1BeVk T

    sm ms sI I I I

    Nonideality factor = 1

  • Thermionic emission

    * 2 exp bs RB

    eI AA Tk T

    A = AreaAR* = Richardson constant

    n-Si AR* = 110 A K-2cm-2

    Thermionic emission dominates over diffusion current in a Schottky diode.

    p-Si AR* = 32 A K-2cm-2n-GaAs AR* = 8 A K-2cm-2p-GaAs AR* = 74 A K-2cm-2

  • Schottky diodes

    Majority carrier current dominates.

    nonideality factor = 1.

    Fast response, no recombination of electron-hole pairs required.

    Used as rf mixers.

    Low turn on voltage - high reverse bias current

    e 1BeVk T

    sI I

  • Tunnel contacts

    For high doping, the Schottky barrier is so thin that electrons can tunnel through it.

    metal p+ p

    metal n+ n

    Tunnel contacts have a linear resistance.

    Degenerate doping at a tunnel contact

  • Contacts

  • Transport mechanisms

    DriftDiffusionThermionic emissionTunneling

    All mechanisms are always present.

    One or two transport mechanisms can dominate depending on the device and the bias conditions.

    In a forward biased pn-junction, diffusion dominates.

    In a tunnel contact, tunneling dominates.

    In a Schottky diode, thermionic emission dominates.

  • Institute of Solid State Physics

    JFETs - MESFETs - MODFETs

    Technische Universität Graz

    Junction Field Effect Transistors (JFET) Metal-Semiconductor Field Effect Transistors (MESFET)Modulation Doped Field Effect Transistors (MODFET)

    n

  • JFET

    For NA >> ND2 ( )bi

    nD

    V VxeN

    Depletion mode2 bi

    nD

    Vh xeN

    Enhancement mode2 bi

    nD

    Vh xeN

    conducting at Vg = 0

    nonconducting at Vg = 0

    n-channel JFET

    n

  • n-channel (power) JFET

    depletion zone

  • Power SiC JFET

    p

    p

    n

  • np p

    source

    gate

    drain

    n-channel JFET

    depletion region

    JFETs are often discrete devices

    p+

    n+

    n+

  • MESFET

    Depletion layer created by Schottky barrier

    2 ( )bin

    D

    V VxeN

    Metal-Semiconductor Field Effect Transistors

    n

    Fast transistors can be realized in n-channel GaAs, however GaAs has a low hole mobility making p-channel devices slower.

  • JFET

    2 ( )bin

    D

    V VxeN

    Pinch-off at h = xn2

    2D

    peN hV

    Vp = pinch-off voltage

    n-channel JFET

    G

    D

    Sn-channel JFET

    G

    D

    Sp-channel JFET

    At Pinch-off, Vp = Vbi - V.

    n

  • JFET

    The drain is the side of the transistor that gets pinched off.


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