Bipolar Junction Transistors

Microelectronic Circuits, Kyung Hee Univ. Spring, 2016

1

Bipolar Junction Transistors


2

Introduction

• physical structure of the bipolar transistor and how it works

• How the voltage between two terminals of the transistor controls the current that flows through the third terminal

• The equations that describe these current-voltage characteristics

• How to analyze and design circuits that contain bipolar transistors, resistors, and dc sources


3

Introduction

• Three-terminal device

• Multitude of applications• Signal amplification/Digital logic/Memory circuit/Switch

• Voltage between two terminals to control the current flowing in third terminal

• Bipolar junction transistor (BJT)

• Metal-oxide-semiconductor field-effect transistor (MOSFET)

• BJT was invented in 1948 at Bell Telephone Laboratories

• Ushered in a new era of solid-state circuits

• It was replaced by MOSFET as predominant transistor used in modern electronics.

• BiCMOS


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4.1 Device Structure and Physical Operation

• Simplified structure of BJT

• Consists of three semiconductor regions:• Emitter region (n-type)

• Base region (p-type)

• Collector region (n-type)

• Type described above is referred to as npn

• Dual of npn is pnp transistor

• Three terminals: emitter(E), base(B), collector(C)


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4.1.1 Simplified Structure/Operation Modes

• Transistor consists of two pn-junctions:• Emitter-base junction (EBJ)

• Collector-base junction (CBJ)

• Operating mode depends on bias condition• Active mode – used for amplification

• Cutoff and saturation modes – used for switching application (logic circuits)

• Bipolar(electron and hole) participate in conduction


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4.1.2 npn-Transistor in the Active Mode

• Active mode is “most important”

• Two external voltage sources are required for biasing to achieve it

• Refer to Figure 4.3

Figure 4.3: Current flow in an npn transistor biased to operate in the active mode. (Reverse current components due to drift of thermally generated minority carriers

are not shown.)


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Current Flow

• Forward bias on emitter-base junction will cause current to flow

• This current has two components:• Electrons injected from emitter into base

• Holes injected from base into emitter

• It will be shown that first (of the two above) is desirable

• This is achieved with heavy doping of emitter, light doping of base


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Current Flow

• Emitter current (𝑖𝐸) : current which flows across EBJ • Flows “out” of emitter lead

• Dominate by electron components

• 𝑖𝐸 ∝ 𝑒 𝑣𝐵𝐸 𝑉𝑇

• Minority carriers – in p-type region• These electrons will be injected from emitter into base

• Small proportion of recombination process

• Reach most of diffusing electrons to the boundary of the collector-base depletion region

• Because base is thin, concentration of excess minority carriers within it will exhibit constant gradient


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Collector Current

• Reach most of diffusing electrons to the boundary of the collector-base depletion region

• Opposite direction to that of the flow of electrons

• 𝑖𝐶 = 𝐼𝑆 𝑒 𝑣𝐵𝐸 𝑉𝑇

• 𝐼𝑆: constant of proportionality (saturation current)• Inversely proportional to W and directly proportional to area of EBJ

• Typically between 10-12 and 10-18A

• Also referred to as scale current

• 𝑖𝐶 is independent of the value of 𝑣𝐶𝐵• As long as collector is positive, with respect to base


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Base Current

• Base current consists of 𝑖𝐵 = 𝑖𝐵1 + 𝑖𝐵2• 𝑖𝐵1: due to holes injected from base region into emitter

• 𝑖𝐵2: due to holes that have to be supplied by external circuit to replace those recombined

• Each current will be proportional to 𝑒 𝑣𝐵𝐸 𝑉𝑇


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Base Current

• Common-emitter current gain (𝛽)

• Is influenced by two factors:

• Width of base region (W)

• Relative doping of base / emitter regions (NA/ND)

• High Value of 𝛽 (50~200, >1000)

• Thin base (small W in nano-meters)

• Lightly doped base / heavily doped emitter (small NA/ND)

transistor parameter

/

(eq6.5)

(eq6.6) BE T

CB

v VSB

ii

Ii

e

(eq4.2)

(eq4.3)


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Emitter Current

• All current which enters transistor must leave

iE = iC + iB• Equations (4.7) through

(4.13) expand upon this idea

• α : common-base current gain (less than but very close to unity)

this expression is generated through combination of (6.5) and (6.7)

/(eq6.8/6.9)

(eq6.10)

(eq6.11

1

)

1

BE T

C

v VE C S

C E

i

i i I

i i

e

this parameter is reffered

/

toas

(eq6.13)

(eq6.1

1 1

2)

,

BE Tv VSE

Ii

common-base current gain

e

(eq4.8/4.9)

(eq4.10)

(eq4.11)

(eq4.12)

(4.5) and (4.7)

(eq4.13)


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The Emitter Current

• All current which enters transistor must leave

• iE = iC + iB• Equations (4.7)

through (4.10) expand upon this idea

• α : common-base current gain (less than but very close to unity)

• Small change in 𝛼correspond to very large changes in 𝛽

this expression is generated through combination of (6.5) and (6.7)

/(eq6.8/6.9)

(eq6.10)

(eq6.11

1

)

1

BE T

C

v VE C S

C E

i

i i I

i i

e

this parameter is reffered

/

toas

(eq6.13)

(eq6.1

1 1

2)

,

BE Tv VSE

Ii

common-base current gain

e

(eq4.5)

(eq4.7)

(eq4.8)

(eq4.9)

(4.2) and (4.4)

(eq4.10)


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0

0

( ) concentration of minority carriers a position x (where 0 represents EBJ boundary) thermal-equilibrium value of minority carrier (elect

/

ron) concentration in base

0

reg

(eq6.1) 0 BE

p

p

Tv

nn

Vp

x

n e

pn

pn

0

0

0

ionvoltage applied across base-emitter junction

thermal voltage (constant)

p

pBE

pT

nn

nv

V

Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

Figure 4.4


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Minority-Carrier Distribution

• Concentration of minority carrier np at boundary EBJ is defined by (4.11)

• Concentration of minority carriers np at boundary of CBJ is zero• Positive vCB causes these electrons to be swept across junction

0

0

( ) concentration of minority carriers a position x (where 0 represents EBJ boundary) thermal-equilibrium value of minority carrier

/

(electron) concentration in ase

0

b

(eq6 0.1)

p

p

BE T

x

n

v V

n

pn e

pn

pn

0

0

0

regionvoltage applied across base-emitter junction


p

pE

p

B

T

nn

nv

V

(eq4.11)


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Minority-Carrier Distribution

• Tapered minority-carrierconcentration profile exists

• It causes electrons injected into base to diffusethrough base toward collector

• As such, electron diffusion current (In) exists

cross-sectiona area of the base-emitter junction magnitude of the electr

this simplificationmay be made if

gradient assumedto be straight line

(e

(eq6.2)

q6.2)

0

E

p

n E n

p

E n

Aq

n

dn xI A qD

dx

dnAI qD

W

on charge electron diffusivity in base

width of basenD

W

(eq4.12)


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Current Flow

• Some “diffusing” electrons will combine with holes(majority carriers in base)

• Since base is very thin and lightly doped, recombination is minimal

• Recombination does, however, cause gradient to take slightly curved shape

• The straight line is assumed


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0

0

( ) concentration of minority carriers a position x (where 0 represents EBJ boundary) thermal-equilibrium value of minority carrier (elect

/

ron) concentration in base

0

reg

(eq6.1) 0 BE

p

p

Tv

nn

Vp

x

n e

pn

pn

0

0

0

ionvoltage applied across base-emitter junction


p

pBE

pT

nn

nv

V

Oxford University PublishingMicroelectronic Circuits by Adel S. Sedra and Kenneth C. Smith (0195323033)

Recombination causes actual gradient to be curved, not straight.

Figure 4.4

(eq4.11)


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• It is observed that most diffusing electrons will reach boundary of collector-base depletion region

• Because collector is more positive than base, these electrons are swept into collector

• Collector current (iC) is approximately equal to In

• iC = In

Collector Current

intrinsic carrier density doping concentration of base

/

0

2

(eq6.3)

saturation current:

(eq6.4)

BE Tv VC S

E

n

n p

S

E n iS

A

iNA

i I

A qD nI

W

A qD nI

W N

e

(eq4.13)


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Recapitulation / Equivalent-Circuit Models

• Present first-order BJT model

• Assumes npn transistor in active mode

• Basic relationship is collector current (iC) is related exponentially to forward-bias voltage (vBE)

• It remains independent of vCB as long as this junction remains reverse biased• vCB > 0

• iB is much smaller than iC

• Nonlinear voltage-controlled current source


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Figure 4.5: Large-signal equivalent-circuit models of the npn BJT operating in the forward active mode.

Common base model

Common emitter model

Bipolar Junction Transistors

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