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  • J. B. Gunn, "Microwave Oscillation of Current in III-V Semiconductors",

    Solid State Commun., 1 88 (1963)

    Gunn Diodes

    n-type GaAsMetal

    Metal

    In 1960s GaAs was a new emerging semiconductor materialJohn Gunn research objective was to study the ohmic contacts to GaAs

  • VI

    GaAs sample I-V characteristic in Gunn experiments

    n-type GaAsMetal

    Metal

    5V

  • VI

    GaAs sample I-V characteristic in Gunn experiments

    n-type GaAsMetal

    Metal

    15V

  • VI

    GaAs sample I-V characteristic in Gunn experiments

    30V

    n-type GaAsMetal

    Metal

  • .0

    4

    8

    12

    16

    20

    0 20 40 60 80 100 120 140 160

    C

    u

    r

    r

    e

    n

    t

    (

    m

    A

    )

    Time (ps)

    js = qnovs

    jp = qnovp

    Short-pulse current waveform in Gunn experiment

  • Electron drift velocity Electric field dependence in GaAs

    2 4 6 8 10 12 14

    0.5

    1

    1.5

    2

    Electric field (kV/cm)

    = 0.85 m2/Vs

    = 0.5 m2/Vs

    Physical mechanism of the Gunn effect

    Si

    GaAs

  • Such an assumption is wrong.

    2 4 6 8 10 12 14

    0.5

    1

    1.5

    2

    Electric field (kV/cm)

    = 0.85 m2/Vs

    = 0.5 m2/Vs

    Current voltage characteristic of GaAs samplein strong electric fields

    I = q n v(F) AreaSince F = V/L, one can expect that I-V characteristic would besimilar in shape to the v(F) curve

    2 4 6 8 10 12 14

    0.5

    1

    1.5

    2

    Electric field (kV/cm)

    = 0.85 m2/Vs

    = 0.5 m2/VsCu

    r

    r

    e

    n

    t

    Voltage

  • 2 4 6 8 10 12 14

    0.5

    1

    1.5

    2

    Electric field (kV/cm)

    = 0.85 m2/Vs

    = 0.5 m2/Vs

    Space charge instability in semiconductors with negative differential mobility (NDM)

    FC

    In GaAs, at electric fields exceeding the critical value of FC 3.2 kV/cmthe differential mobility is negative.When the field exceeds FC and further increases, the electron drift velocity decreases.

  • xx

    F0 Fc

    v0 = vm

    x

    n0 = ND

    F

    v

    Fc

    vm

    - +F

    v

    n

    Space charge instability in semiconductors with NDM

    Initially uniform electric field and concentrationdistribution in the sample.

  • xF0 FcF

    v

    Fc

    vm

    - +F

    x

    v0 = vm

    v

    x

    n0 = NDn

    0 0

    DF n Nqx

    = =

  • xF

    x

    v

    F0 Fc

    v0 = vm

    x

    nn0 = ND

    F

    v

    Fc

    vm

    - +

  • xF

    x

    v

    F0 Fc

    v0 = vm

    x

    nn0 = ND

    F

    v

    Fc

    vm

    - +

  • xF

    x

    v

    F0 Fc

    v0 = vm

    x

    nn0 = ND

    F

    v

    Fc

    vm

    - +vs

    vsHigh-field, orGunn domain

  • xF

    x

    v

    F0 Fc

    v0 = vm

    x

    nn0 = ND

    F

    v

    Fc

    vm

    - +vs

    vs

  • xF

    x

    v

    F0 Fc

    v0 = vm

    x

    nn0 = ND

    F

    v

    Fc

    vm

    - +vs

    vs

  • xF

    v

    F0 FcF

    v

    Fc

    vm

    - +vs

    Current time dependence in the sample with high-filed domain

    Current at the device electrodes:IV= q n vs

    When the domain is moving between the cathode and anode:

  • Fv

    Fc

    vm

    - +vs

    Current time dependence in the sample with high-filed domain

    Current at the device electrodes:Im = q n vm

    When the domain dissipates in the anode and new domain did not form yet:

    x

    x

    F0 Fc

    v0 = vm

    F

    v

  • vFc

    vmvs

    Current time dependence in the sample with high-filed domain

    Im = q n vm

    .

    0

    4

    8

    12

    16

    20

    0 20 40 60 80 100 120 140 160

    C

    u

    r

    r

    e

    n

    t

    (

    m

    A

    )

    Time (ps)

    js = qnovs

    jp = qnovp

    IV = q n vs

  • Transit-time oscillations in Gunn diodes

    .

    0

    4

    8

    12

    16

    20

    0 20 40 60 80 100 120 140 160

    C

    u

    r

    r

    e

    n

    t

    (

    m

    A

    )

    Time (ps)

    js = qnovs

    jp = qnovp

    GD

    L

    RL

    Domain transit time: ttr = sample length /domain velocity ttr = L/vs

    In GaAs, vs 107 cm/sFor the sample with the length L = 100 m,

    ttr = 100 10-4 cm / 107 cm/s = 10-9 sThe frequency of transit time oscillations:

    ftr = 1/ttr = 109 1/s = 1 GHzFor L=10 m, ftr = 10 GHz

  • .0

    4

    8

    12

    16

    20

    0 20 40 60 80 100 120 140 160

    C

    u

    r

    r

    e

    n

    t

    (

    m

    A

    )

    Time (ps)

    js = qnovs

    jp = qnovp

    GD

    L

    RL

    1. Operating frequency controlled by the sample length:

    no tuning, varies from sample to sample, sensitive to sample non-uniformities.

    2. Current waveform consist of short pulses with the width

  • 1. Resonator voltage controls the

    domain nucleation and dissipation.

    2. Current waveform pulses are wider

    as compared to transit-time mode:

    higher efficiency

    Resonator-controlled oscillations in Gunn diodes

    Gunn diode in the LC-resonator

  • Highly-efficient Limited Space charge- Accumulation mode

    Approach:

    Domain formation requires certain time td.

    If the resonator frequency fr >> (1/td), the domain cannot completely develop

    The filed and concentration in the sample remain nearly uniform.

    The dynamic I-V curve of the Gunn diode reproduces the v(F) dependence

  • Highly-efficient Limited Space charge- Accumulation mode

    Achieved frequencies: up to 100 GHz

  • Kroemer criterion in the Gunn effectC

    o

    n

    c

    e

    n

    t

    r

    a

    t

    i

    o

    n

    DistanceCathode Anode

    F

    i

    e

    l

    d

    Characteristic time of the domain formation can be evaluated by effective RC- circuit charging time:

    0

    0 | |d d d

    d

    t R Cq n

    =

    Domain formation time is equal to td (so-called Maxwell relaxation time);n0 is the equilibrium electron concentration,d is the differential electron mobility.In GaAs, typically, |d| 2000 m2/(Vs)

    Cd = SL

    Rd = Lqd noS

    0d

    d

    SCL

    =

    0

    dd

    d

    LRq n S=

  • Kroemer criterion in the Gunn effectCharacteristic domain transit time in the sample of the length L:

    trs

    Ltv

    If domain formation time td is greaterthan the domain transit time ttr, the domain does not have enough time to develop the diode is stable. Gunn diode is stable if td > ttr;Gunn diode may oscillate in one of the Gunn-domain modes if td < ttr

    C

    o

    n

    c

    e

    n

    t

    r

    a

    t

    i

    o

    n DepletionLayer

    AccumulationLayer

    Distance

    F

    i

    e

    l

    d

    AnodeCathode

    L( )( ) 0

    ,

    | |

    o o CR

    so CR

    d

    n L n Lvwhere n L

    q

    >=

    0

    0d d d

    d

    t R Cq n

    =

    Kroemer criterion for domain formation:

  • Stable Gunn diodes - amplifiers

    Field/concentration distributions and impedance frequency dependence in stable Gunn diode

    If the Kroemer criterion is not met: 0| |s

    od

    vn Lq= Gunn diode works as an oscillator

    f0 < 1/td Gunn diode operates in the Gunn domain mode.

    f0 > 1/td Gunn diode operates in the limited space charge accumulation (LSA) mode no domains are formed. For the LSA mode, f0 > 3 1/tdif f0 >1/td but f0 < 3 1/td, Gunn diode operates in a mixed Gunn domain/LSA mode

    0

    | |s

    od

    vn Lq< Gunn diode works as a stable amplifier. No Gunn domain or LSA oscillations

    0

    0d

    d

    tq n

    =

    The mode of operation depends on the relationship between the resonant frequency of the attached resonant circuit f0and the domain formation time:

    I.

    II.

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