물리 전자/김삼동 10-1

Bipolar Transistor Action

FET devices use reverse biased PN junctions and minority carriers for transport. The bipolar transistor involves forward biased junctions and both minority & majority carriers.

• Operation - Structures of Bipolar Junction Transistors (BJTs)

n+ n+ p

n+ buried layer

p+ p+ n epitaxy

C

p substrate

(G) (+ 0.7 V) (+ 5 V)

E B

물리 전자/김삼동 10-2

Bipolar Transistor Action

BJTs have the following characteristics - Higher gm than in FET devices

- Lower input impedance (current control)

- Faster switching

- More complex device physics

- More complex fabrication than FET’s, i.e. More

size/cell required: Larger physical size than FET’s

Parasitic exists in the structure →

RB: base resistance RC: collector resistance

(predominantly through n- layer) Isolation must be provided between the adjacent

devices. Typically reverse biased PN junctions are

used.

n+ p

n ~1015/cm3

n+ ~1019/cm3

n+ SiO2

Metal E B C

Log conc. (cm-3)

20

18

17

16

19

N+ emitter

P base

N collector

N+ buried layer

물리 전자/김삼동 10-3

Principle

Water flowing over ground

Bipolar devices: potential “bump” is controlled by base-emitter voltage: BJT

Field-effect devices: channel constriction is controlled by gate bias: FET

Water flowing in a pipe

“bump” in the ground to shut off the flow

Water flows when“bump” is reduced

A facet to shut off the channel flow

Water flows when the constriction is reduced

물리 전자/김삼동 10-4

Basic Operation

• Charge Flow

n

n

p

C

B

E

Ic

IE

IB

-

+

- - - - - - - - -

- - - - -

+ + +

- - - - -

n+

p

n

+

- - - - - - - -

- - - - -

- -

+ + +

-

-

-

-

-

n+

p

n

xB << Ln

InE

InC

IpB

E C BI I I= +

물리 전자/김삼동 10-5

Basic Operation

(I) Recombination current in the base region : IBrec (hole)

(II) Base current injected (hole) into the emitter : IBp (III) Recombination current in the base region : IBrec (electron) = (I) (IV) Electron current coming from the emitter to collector: Icn ~ IC

(V) Electron current injected across reverse-biased B-C junction (VI) Hole current injected across reverse-biased B-C junction

+ - + -

IE IC

IB

n n p

(I) (II)

(III) (IV)

(V) (VI)

+

-

n- n+ p C

B

E n+

-

+

~ 0.7 V

Most of e-’s

Holes: recombined and injected into E

-

물리 전자/김삼동 10-6

Basic Operation

4) Since most of injected e-’s reach collector and only a few holes are injected into the emitter,

IB << IC

: the device has substantial current gain !!

- The BJT operates basically as follows:

1) An external voltage applied across the E-B junction to forward bias (ex: ~0.7 volts) 2) e-’s are injected into the base by the emitter (holes are also injected into the emitter by the base, but their numbers are much smaller because of relative value Na << Nd) If xB << Ln in the base, most of injected e-’s get to the collector without recombination. A few do recombine; the holes are necessary for this are supplied as base current. 3) The e-’s reaching the collector are collected across the C-B depletion region.

0 V

-

+

- - -

-

-

-

-

-

0.7 V+3 V+

물리 전자/김삼동 10-7

Basic Operation

1) Collector current; IC : Most portion is produced by e- diffusion in the base 2) Base current: IB - Base current injected (hole) into the emitter : (II) : Small due to low doping in base region - Recombination current in the base region : (III) 3) Emitter current: IE - Electron current coming from the emitter to collector: (IV) - Current by injected holes into the emitter : (II) : Small due to low doping in base region

E C BI I I= +

Emitter (n+)

Base (p)

Collector (n)

pn0

pn(x)

np(x)

np0

pn0

pn(x)

- +

+

Ε Ε

++ + 0

Ideal electron profile in base

Actual electron profile (recombination) in base

IC

IB

IE

물리 전자/김삼동 10-8

The e- flow between E and C is given by

I-V Characteristics

• Electron Current : IC ~ IE

First, to derive the basic relationship for e- current flow between the E and C, we proceed as follows: We start by assuming the device current gain is high enough

n n x n

n n n

nn

nn

dnJ ne eDdx

n dp dnJ kTμ eD p dx dx

eD dp dn J n p p dx dx

or eD d(pn) J

p dx

= µ +

= +

∴ = +

=

E

n

n

D kT = μ e

xB

xD xD

E B C

N+ P N-

x=xB x=0

++ + 0

-

drift diffusion

B

p

I 0 J hole current in the base 0

⇒ ≈∴ ≈ =

p p x p

px

p

dp J 0 peμ - eDdx

D 1 dp kT 1 dp μp dx e p dx

≅ =

→ = =

E

E

물리 전자/김삼동 10-9

I-V Characteristics

Note that VBE > 0 and VBC < 0 represent xB is the width of quasi neutral base region.

Here, we assume Jn = constant; and the (pn) product

can be given by from our diode analysis

[ ] [ ]B

B

x pn@ x

n x x x 0n0 pn@0

p J dx d(pn) pn - pn eD

B

= == =∫ ∫

i FpFn ii i

E - EE - En = n exp and p = n expkT kT

Fn Fp2 2 ai i

E - E eV np = n exp = n exp kT kT

→

BC BE

B

B

BC BE

eV eV2 kT kTi

n x

n0x

B0

eV eVkT kT

n n o

qn J

p dxD

Since eA pdx Q : total charge in undepleted base

I AJ I

e e

e e

−

∴ =

=

= = −

∫

∫

2 2 2i n

oB

e A n D I Q

=

BCBE eVeV2 2kT kTi B ipn(0) n e & pn(X ) n e = =

E(n) B(p) C(n)B(p)

forward reverse

물리 전자/김삼동 10-10

I-V Characteristics

2) The quantity

: It is the total integrated base charge (atoms/cm2).

Since I ∝1/QB, it is important to minimize QB

⇒ IC transistors use low doping in the base.

3) In the case of forward-active mode

: VBE > 0 (forward) and VBC < 0 (reverse)

This is an important result. Note that

1) Usually only one of the two exponential terms is

more important because of the fact that one junction

is typically reverse biased.

⇒ “exponent of the exponential term ~ 0” :

if VBC < 0

(When the device is in saturation, both junctions are

forward biased and both terms must be included)

BC BEeV eVkT kT

n oI I e e

= −

2B po po ipn(X ) p n n ≈ =

BxB

A0

Q = N (x) dx base "Gummel number"eA ∫

BEeVkT

n oI I 1 e

= −

물리 전자/김삼동 10-11

Modes of Operation

NdE

NdC

np0 ~ 0

np

NdC

np0 ~ 0

np

NdE

NdC

np0 =ni2/Na

N P N

Equilibrium mode

No bias

- -

+

- -

+ +

- - - - - -

N P N

Active mode

VBE > 0: forward

VBC < 0: reverse

(-) (+) (++)

- - -

-

-

-

-

- -

IE

IC

N P N Saturation mode

VBE > 0: forward

VBC > 0: forward

(-) (+) (-)

- - - -

- -

- - - -

IE ~0 IC ~0

2innp ≅

2i BCnp n exp(V /kT)≈

물리 전자/김삼동 10-12

Modes of Operation

The derived equation predicts an exponential relationship between IC & VBE, and this relationship holds extremely well for IC transistors over many decades of current. In general, QB is obtained by integrating over the base. QB is typically well controlled to ~1012/cm2 to give high IC for a given VBE.

• Forward Active mode : VBE > 0 (forward) and VBC < 0 (or VCB > 0 : reverse)

If base doping (Na) is constant, QB = eANaxB

or

BEeVkT

n oI - I 1e

= −

2 2 2i n

oB

e A n D I Q

← =

BEqV2 2 2i n kT

nB

e A n DI - e 1Q

= −

BEeV2i n kT

na B

eAn DI - e 1N x

= −

VBE (volt) 0 0.2 0.6 0.4

10-16

10-12

10-8

10-4

100

0.8

I C (A

)

물리 전자/김삼동 10-13

Modes of Operation

Under normal operating conditions, the base-emitter junction is forwarded & collector-base junction is reverse biased.

- VCE > VBE, C-B is reverse biased (active mode): IC ≠ f (VCE), IC ∝ exp (eVBE/kT) - VCE ≤ VBE, C-B is forwarded (saturation mode): IC(saturation) < IC (active)

In the saturation region, substantial minority carrier charge storage (in the base) occurs because both junctions are injecting.

n- n+ p

C

B

E n+

~ 0.7 volts

+5 volts -

+5 volts

VCE

IC

Saturation

Active

Cut-off

VBE

VCE = VBE , i.e. VBC = 0

물리 전자/김삼동 10-14

Modes of Operation

VCE

Cut-off

VBE

( I )

( II ) ( III )

( IV )

(V )

IC ( I ) : VCE ~ 0,

VBE = +0.7 V

- - - -

- -

- - - -

IC ~ 0

( II ) : VCE ~ 0.4 V,

VBE = +0.7 V

- - - -

- -

- - - -

IC ≥ 0

( III ) : VCE ~ +1.5 V,

VBE = +0.7 V

- - - -

- -

- - - -

IC > 0

≠ f (VCE)

( IV ) : VCE ~ +1.5 V,

VBE = +1.0 V

- - - - - -

- - - - IC >> 0

= f (VBE)

( V ) : VCE ~ +1.5 V,

VBE ~ 0 V

- - - -

- -

- - - -

IC ~ 0

물리 전자/김삼동 10-15

Modes of Operation

• In npn BJT's

VBE VBC Mode

+ - Active

+ + Saturation

- - Cut-off

- + Inverted

Active

Saturation

Cut-off

Inverted

많이 보내 줄게 !

보내는 데로 받을게 !

-

-

-

나도 보내는데

!

기다리는데 !

나도 !

많이 보내 줄게 !

이제 내가 받는 입장 !

이젠 내가 보낼게 !

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-

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