Microelectronic Circuit Design McGraw-Hill Chapter 5 Bipolar Junction Transistors Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock.

Microelectronic Circuit DesignMcGraw-Hill

Chapter 5Bipolar Junction Transistors

Microelectronic Circuit Design

Richard C. Jaeger

Travis N. Blalock

Microelectronic Circuit DesignMcGraw-Hill

Circuit Representations for the Transport Models

In npn transistor (expressions analogous for pnp transistors), total current traversing base is modeled by a current source given by:

iTi

F i

RI

Sexp v

BEV

T

exp v

BCV

T

iB

IS

F

exp vBE

VT

1

IS

R

exp vBC

VT

1

Diode currents correspond directly to the two components of base current.

Microelectronic Circuit DesignMcGraw-Hill

Operation Regions of Bipolar Transistors

Base-Emitter Junction

Base-Collector Junction

Reverse Bias Forward Bias

Forward BiasForward-Active

Region

(Good Amplifier)

Saturation Region

(Closed Switch)

Reverse BiasCutoff Region

(Open Switch)

Reverse-Active Region

(Poor Amplifier)

Binary Logic States

Microelectronic Circuit DesignMcGraw-Hill

i-v Characteristics of Bipolar Transistor: Common-Emitter Output Characteristics

For iB = 0, transistor is cutoff. If iB > 0, iC also increases.

For vCE > vBE, npn transistor is in forward-active region, iC = F iB is independent of vCE.

For vCE < vBE, transistor is in saturation.

For vCE < 0, roles of collector and emitter reverse.

Microelectronic Circuit DesignMcGraw-Hill

i-v Characteristics of Bipolar Transistor: Common-Emitter Transfer Characteristic

Defines relation between collector current and base-emitter voltage of transistor.

Almost identical to transfer characteristic of pn junction diode

Setting vBC = 0 in the collector-current expression yields

iCI

Sexp v

BEV

T

1

Collector current expression has the same form as that of the diode equation

Microelectronic Circuit DesignMcGraw-Hill

Simplified Forward-Active Region Model

In forward-active region, emitter-base junction is forward-biased and collector-base junction is reverse-biased. vBE > 0, vBC < 0If we assume that

then the transport model terminal current equations simplify to

vBE

4kTq

0.1V and vBC

4kTq

0.1V

iCI

Sexp v

BEV

T

IS

R

ISexp v

BEV

T

iE

IS

F

exp vBE

VT

IS

F

I

S

F

exp vBE

VT

iB

IS

F

exp vBE

VT

IS

F

I

S

R

I

S

F

exp vBE

VT

iC

FiE

iC

FiB

iE(

F1)i

B

BJT is often considered a current-controlled device, though fundamental forward-active behavior suggests a voltage- controlled current source.

Microelectronic Circuit DesignMcGraw-Hill

Simplified Forward-Active Region Model(Example 1)

• Problem: Estimate terminal currents and base-emitter voltage• Given data: IS =10-16 A, F = 0.95, VBC = VB - VC = -5 V, IE = 100 A• Assumptions: Simplified transport model assumptions, room temperature operation, VT = 25.0 mV• Analysis: Current source forward-biases base-emitter diode, VBE > 0, VBC < 0, we know that transistor is in forward-active operation region.

IC

FI

E0.95100A95A

F

F

1 F

0.951 0.95

19

IB

IE

F1

100A20

5A

VBE

VTln

FI

E

IS

0.69V

Microelectronic Circuit DesignMcGraw-Hill

Simplified Forward-Active Region Model (Example 2)

• Problem: Estimate terminal currents, base-emitter and base-collector voltages.• Given data: IS = 10-16 A, F = 0.95, VC = +5 V, IB = 100 A• Assumptions: Simplified transport model assumptions, room temperature operation, VT = 25.0 mV• Analysis: Current source causes base current to forward-bias base-emitter diode, VBE > 0, VBC <0, we know that transistor is in forward-active operation region.

IC

FI

B19100A1.90mA

IE(

F1)I

B20100A2.00mA

VBE

VTln

IC

IS

0.764V

VBC

VB V

CV

BE V

C 4.24V

Jaeger/Blalock4/26/07

Microelectronic Circuit DesignMcGraw-Hill

Simplified Circuit Model for Forward-Active Region

Jaeger/Blalock4/26/07

Microelectronic Circuit DesignMcGraw-Hill

VBE

8200IE V

EE0

IE 8.3V

82001.01 mA

IB

IE

F1

1.02mA51

19.8 A

IC

FI

B0.990 mA

VCE

VCC

IC

RC

( VBE

)

9 0.99mA(4.3K)0.75.44 V

Jaeger/Blalock4/26/07

Microelectronic Circuit DesignMcGraw-Hill

iC

IS

R

exp vBC

VT

iE I

Sexp v

BCV

T

iB

IS

R

exp vBC

VT

iE

RiC

iE

RiB

Jaeger/Blalock4/26/07

Microelectronic Circuit DesignMcGraw-Hill

IC 0.7V-(-9V)

82001.01 mA

IB

IC

R1

1.01mA2

0.505 mA

IEI

B0.505 mA

Jaeger/Blalock4/26/07

Microelectronic Circuit DesignMcGraw-Hill