Chapter #6: Bipolar Junction Transistorstioh.weebly.com/uploads/2/5/4/3/25433593/12week__chapter_4__-_ti.pdf · Chapter #6: Bipolar Junction ... •The three basic ways for connecting

Microelectronic Circuits, Kyung Hee Univ. Fall, 2015

1

Chapter #6: Bipolar Junction Transistors


2

Introduction

• IN THIS CHAPTER YOU WILL LEARN• The 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, and the equations that describe these current-voltage relationships.

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

• How the transistor can be used to make an amplifier.

• How to obtain linear amplification from the fundamentally nonlinear BJT.

• The three basic ways for connecting a BJT to be able to construct amplifiers with different properties.

• Practical circuits for bipolar-transistor amplifiers that can be constructed by using discrete components.


3

Introduction

• This chapter examines another three-terminal device.

• bipolar junction transistor

• Presentation of this material mirrors chapter 5.

• 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

• 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.


4

4.1. Device Structure and Physical Operation

• Figure 4.1. shows 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

• However, pnp types do exist


5

4.1.1. Simplified Structure and Modes of Operation

• Transistor consists of two pn-junctions:

• emitter-base junction (EBJ)

• collector-base junction (CBJ)

• Operating mode depends on biasing

• active mode – used for amplification

• cutoff and saturation modes – used for switching

• Bipolar(electron and hole) participate in conduction


6

4.1.2. Operation of the 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.)


7

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


8

Current Flow

• emitter current (iE) – is current which flows across EBJ

• Flows “out” of emitter lead

• minority carriers – in p-type region

• These electrons will be injected from emitter into base.

• Opposite direction

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


9

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)

Straight line represents constant gradient

Figure 4.4

(eq4.1)


10

Current Flow

• Concentration of minority carrier np at boundary EBJ is defined by (4.1)

• 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

(eq4.1)


11

Current Flow

• 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

(eq4.2)

(eq4.2)


12

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


13

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

(eq4.1)


14

The Collector Current

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

• Because collector is more positive than base, these electrons are swept into collector• collector current (iC) is

approximately equal to In

• iC = In 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(eq4.3)

(eq4.4)


15

The Collector Current

• Magnitude of iC is independent of vCB

• As long as collector is positive, with respect to base

• saturation current (IS) – is inversely proportional to W and directly proportional to area of EBJ

• Typically between 10-12 and 10-18A

• Also referred to as scale current


16

The Base Current

• base current (iB) – composed of two components:

• 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


17

The Base Current

• common-emitter current gain(b.) – is influenced by two factors:

• width of base region (W)

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

• High Value of b 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

b

b

b

e

(eq4.5)

(eq4.6)


18

The 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

b b

b b

b

b

e

this parameter is reffered

/

toas

(eq6.13)

(eq6.1

1 1

2)

,

BE Tv VSE

Ii

b

common-base current gain

e

(eq4.8/4.9)

(eq4.10)

(eq4.11)

(eq4.12)

(4.5) and (4.7)

(eq4.13)


19

Recapitulation and Equivalent-Circuit Models

• Previous slides 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


20

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

Common base model

Common emitter model

Documents

Chapter #6: Bipolar Junction Transistorstioh.weebly.com/uploads/2/5/4/3/25433593/12week__chapter_4__-_ti.pdf · Chapter #6: Bipolar Junction ... •The three basic ways for connecting