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Transcript of Transistor Bipolaire eng - Polytech lorenz/Bipolar_Transistor_eng.pdf  •...

  • BIPOLARTRANSISTOR

    1

  • Bibliography:

    2

    H. Mathieu, Physique des semi-conducteurs et des composantslectroniques , 4 dition, Masson 1998.

    D.A. Neamen, semiconductor physics and devices , McGraw-Hill, Inc 2003. P. Leturcq et G.Rey, Physique des composants actifs semi-conducteurs ,

    Dunod Universit, 1985. J. Singh, semiconductors devices :an introduction , McGraw-Hill, Inc 1994. Y. Taur et T.H. Ning, Fundamentals of Modern VLSI devices , Cambridge

    University Press, 1998. K.K. Ng, complete guide to semiconductor devices , McGraw-Hill, Inc 1995. D.J. Roulston, Bipolar semiconductor devices , McGraw-Hill, Inc 1990.

  • Plan Geometry Principle of operating Static characteristics Ebers-Moll Equations Static parameters ( gain) Second order effects Switching performances Transistor in HF domain Hetero-junction Bipolar Transistor (HBT or TBH)

    3

  • Geometry

    Geometry: Lateral Vertical

    For digital circuits, vertical design

    4

    lateral

    vertical

    npn

    BC

    E

  • Geometry for IC

    5

    Muller et Kamins, device electronics for IC ,2nd Ed., Wiley, 1986

  • Geometry with insulating oxide

    6

    Muller et Kamins, device electronics for IC ,2nd Ed., Wiley, 1986

  • BJT always present: why?

    speed Low noise High gain Low output resistance

    Still present in mobile phone ( analog part) Low density, mainly in power stage BiCMOS

    7

    Analogueamplifier

  • Working principle

    2 PN Junctions with one common region (base) The first Junction (EB) will

    inject carriers The second one (CB) will

    collect carriers The base must be thin (lower than diffusion )length

    8

  • Reverse biased Junction: Low current due to empty

    tank En modulant le remplissage

    du rservoir, modulation du courant inverse collect (collecteur)

    On remplit le rservoir (la base) en polarisant en direct la jonction EB

    9

    Working principle

  • The reverve biasing CB junctioncreates a favorable electric fieldfor the collect

    Conditions: Thin base:

    Avoid recombination effects Ligthly doped base compared to

    emitter favors one type of injected

    carriers (better injection efficiency)

    10

    Working principle

  • Statics characteristics

    11

    NPNTransistor NPNTransistor

  • 12

    Minority carrier distribution in a forward biased npntransistor

    ideal

    With recombination

  • 13

    PNPTransistor

    Norecombination intheBase!( )

    1DApproximation

    HomogeneousdopingintheBase

    Low Injection

    Statics characteristics + simplifying hypotheses

  • Current components in the NPN Transistor

    In the neutral Base region: Continuity equation

    and ,

    Integrating from E-B to C-B:

    finally:

    In forward active regime, Jn is negative ( e- vers x

  • 15

    )1(

    )()1(

    )()(

    )1()1(

    '

    '

    '

    '

    '

    '

    222 kT

    eV

    C

    E

    iBnbkTeV

    C

    E

    iBnbC

    E

    kTeV

    kTeV

    iBnbn

    BCBE

    BCBE

    edxxp

    neDedxxp

    neD

    dxxp

    eeneDJ

    )1(

    )()1(

    )()(

    )1()1(

    '

    '

    '

    '

    '

    '

    222 kT

    eV

    C

    E

    iBnbkTeV

    C

    E

    iBnbC

    E

    kTeV

    kTeV

    iBnbn

    BCBE

    BCBE

    edxxp

    neDedxxp

    neD

    dxxp

    eeneDJ

    and:BeffA

    C

    E

    WNdxxpB

    '

    '

    )( BeffAC

    E

    WNdxxpB

    '

    '

    )(

    So:

    )1()1()1()1(

    22kT

    eVkT

    eV

    SnkT

    eV

    BeffA

    nbiBkTeV

    BeffA

    nbiBEn

    BCBEBC

    B

    BE

    B

    eeIeWNDene

    WNDenAI

    )1()1()1()1(

    22kT

    eVkT

    eV

    SnkT

    eV

    BeffA

    nbiBkTeV

    BeffA

    nbiBEn

    BCBEBC

    B

    BE

    B

    eeIeWNDene

    WNDenAI

    with :B

    iE

    BeffA

    nbiBESn G

    enAWNDenAI

    B

    22

    B

    iE

    BeffA

    nbiBESn G

    enAWNDenAI

    B

    22

    Saturation current of electrons in the PN narrowjunction ( or without recombination in the Base).

    BeffnbiB

    iBeff

    nb

    A

    iB

    iB WD

    pnnW

    DN

    nnG B 2

    2

    2

    2

    BeffnbiB

    iBeff

    nb

    A

    iB

    iB WD

    pnnW

    DN

    nnG B 2

    2

    2

    2

    Gummel number in the Base (s/cm4)

    Current components in the NPN Transistor

  • Emitter current Collector current

    16

    1)exp(

    kTeVJJ BEspEpE

    1)exp(

    kTeVJJ BCspCpC

    finally

    pCpEECB

    npCC

    npEE

    IIIII

    III

    III

    E B C

    JpE Jn JpCIE

    IBIC

    NPN

    Current components in the NPN Transistor

  • And (!) :

    17

    )1(exp)(

    )1](exp)(

    [ '

    '

    '

    '

    22

    kT

    eV

    dxxp

    DAenkT

    eVIdxxp

    DAenIII BCC

    E

    nbiBEspEC

    E

    nbipEnE

    )1](exp)(

    [)1(exp)(

    '

    '

    '

    '

    22

    kT

    eVIdxxp

    DAenkT

    eV

    dxxp

    DAenIII BCspCC

    E

    nbiBEC

    E

    nbipCnC

    1)exp(1)exp(

    kTeVI

    kTeVIIIIII BCspCBEspEpCpECEB

    Isn

    Current components in the NPN Transistor

  • Forward activeregion: EB(forward)andCB(reverse)

    18

    kTeVII

    kTeVI

    QQDnAeI BESpESnBESpE

    SB

    nBiE exp)(exp)(

    22

    kTeVI

    kTeV

    QQDnAeI BESnBE

    SB

    nBiC expexp)(

    22

    kTeVIIII BESpECEB exp

    *

    Statics characteristics

  • Emitter injectionefficiency:

    DCcommon basecurrent gain:

    DCcommon emitter current gain:

    19

    Ep

    nE I

    I

    SpSn

    Sn

    nBi

    SBSpE

    C

    III

    DneQQJI

    I

    22

    ).(1

    1

    1B

    C

    II NB:if we neglect recombination

    process andareequivalent E

    Statics characteristics

  • Base transport factor: Take into account recombination

    in neutral base region

    20

    n

    ppB

    n

    srB

    nnAeXQI eff

    2/))0((

    t

    sC

    QI

    12

    2

    2

    effB

    n

    t

    n

    rB

    C

    XL

    II

    Statics characteristics

  • Recombination factor: Take into account recombination in

    depleted base region

    21

    kTeVW

    AenI BET

    irD 2

    exp2

    withWT,width ofEBspace charges.

    When we add upallthecontributiontothebasecurrentwe get:

    rDrBBB IIII *

    Statics characteristics

  • The common emitter gain can be expressed as:

    intriseque base current (no recombination) recombination current in the neutral base region recombination current in the depletion zone of EB junction

    22

    C

    rD

    EC

    rDrBB

    C

    B

    II

    IIII

    II

    111 *

    *BI

    rBIrDI

    Statics characteristics

  • Other modes of operation

    Saturation mode (regime): The two junctions are forward biased.

    23

    kTeV

    BS

    nBikTeV

    spEBS

    nBiE

    BCBE

    eQQDnAe

    eIQQDnAe

    I

    2222

    kTeV

    spCBS

    nBikTeV

    BS

    nBiC

    BCBE

    eIQQDnAee

    QQDnAeI

    2222

    n(x)

    )0(exn

    )( Bex Wn

    QS1

    QS2

    Base

    0 WB

  • Low injection : (QS

  • Saturation mode

    Low injection: (QS

  • High injection level

    In this case, injected electron density reaches holes density into the base ( )

    Equivalent study leads to the following result :

    .

    26

    BkT

    eVkT

    eV

    iS WeeenQBCBE

    )(21 22 BkT

    eVkT

    eV

    iS WeeenQBCBE

    )(21 22

    kTeV

    iB

    nBkTeV

    iB

    nBn

    BCBE

    enWeDen

    WeDJ 22 22 kT

    eV

    iB

    nBkTeV

    iB

    nBn

    BCBE

    enWeDen

    WeDJ 22 22

    pn

    Saturation mode

  • Nonideal Effects

    Gummel plot : Graph of IC and IB versusVBE

    27

    1

    2

    3

  • Early effect / collector punchthrough

    Base collector junction breakdown

    Emitter and Base serie resistance

    Ic collapse fort high current

    crowding effect

    Early effect / collector punchthrough

    Base collector junction breakdown

    Emitter and Base serie resistance

    Ic collapse fort high current

    crowding effect

    28

    Nonideal Effects

  • At "first glance" Ic is independent of VCB

    In fact we have the modulation of the width of the neutral region in the base, so in the same wayQB+QS , and so Ic !

    29

    kTeV

    QQDnAeI BE

    SB

    nBiC exp)(

    22

    if VBC WB QB+QS

    Ic

    ZCE B-C

    Early effect / collector punchthrough

  • At the limit: The space charge BC

    depletes totaly the neutral base

    collector injects directlycurrent into the emitter.

    Current only limited by serieresistance Rserie form E and C

    30

    BCZCE

    SC

    BB

    dBC

    pBA

    W

    WeNCQ

    V

    C

    CBB

    DSC

    DAABpt N

    NNNeWV

    2)(2

    Early effect / collector punchthrough

  • Base - Collector junction breakdown

    Avalanche of BC: Often occurs before punchthrough

    How to prevent it? Lowering the electric field:

    Reduce the doping gradient in the collector

    Li