Lundstrom ECE 305 S15

ECE-305: Spring 2015

Introduction to BJTs: active region

Professor Mark Lundstrom

Electrical and Computer Engineering Purdue University, West Lafayette, IN USA

lundstro@purdue.edu

4/20/15

Pierret, Semiconductor Device Fundamentals (SDF) pp. 371-385

1

SiGe HBTs

Martin Claus TU-Dresden Lundstrom ECE 305 S15

2

MOS transistors

Lundstrom ECE 305 S15

VDSn-channel enhancement mode MOSFET

ID

VGS1

S

D

G

ID

ISIG ≈ 0

saturation

linear region

3

bipolar transistors

Lundstrom ECE 305 S15

VCE

E: emitter

C: collector

B: base

IC

NPN BJT

IC

VBE1, IB1

IE

IB

(forward) active region

saturation region

4

bipolar transistors

Lundstrom ECE 305 S15

VCE

E: emitter

C: collector

B: base

IC

NPN BJT

IC

VBE1, IB1

IE

IB

(forward) active region EB: FB, BC: RB

saturation region EB: FB, BC: FB

cut-off region EB: RB, BC: RB

5

NPN bipolar transistor

IC

IB VCE

VBE

VCB

IE

IB + IC = IE

VBE +VCB = VCE

KCL:

KVL:

Lundstrom ECE 305 S15 6

common base (active region)

IC

IB VCE

VBE

VCB

IE

N

N P

IC =α dcIE

VCBVEB IB

IE = IC + IB

BE: FB BC: RB

VEB < 0VCB ≥ 0

N N

P

Lundstrom ECE 305 S15 7

common emitter (active region)

IC

IB VCE

VBE

VCB

IE

N

N P

BE: FB BC: RB

VBE > 0VCB = VCE −VBE > 0

IB VCE

IEVBE

N

N P

VCB

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IC = βdcIB

8

alpha and beta

Lundstrom ECE 305 S15

α dc ≡ICIE

= ICIC + IB

<1

IC

IB VCE

VBE

VCB

IE = IC + IB

N

N P

α dc =IC

IC + IC βdc

= βdc

βdc +1<1

IC = βdcIBIC =α dcIE

α dc =βdc

βdc +1

βdc =α dc

1−α dc9

outline

Lundstrom ECE 305 S15

1)  Introduction 2)  E-Band view of BJT active region

10

understanding MOSFETs

VGS >VT VD 0

p-Si

n-Si n-Si

y

To understand this device, we should first draw an Energy Band Diagram.

Lundstrom ECE 305 S15

x

11

how MOS transistors work

2007 N-MOSFET

(Courtesy, Shuji Ikeda, ATDF, Dec. 2007)

Lundstrom ECE 305 S15

VGS

EC

VBE VCE = 0.05 V

EC

VGS VBE

VCE = 0.6 V

12

BJT operation: active region

13 13

n+ emitter

p base

n collector

n+

FB RB

y

To understand this device, we should first draw an Energy Band Diagram.

Lundstrom ECE 305 S15

equilibrium

y

EF

EC

EV

E

emitter base collector

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qVbi

14

VBE = 0, VCE > 0

y

EC

EV

E

collector

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qVbi

15 emitter base

Fn

Fn

EC

EV“off” “cut-off”

VBE > 0, VCE > 0

y

EC

EV

E

collector

Lundstrom ECE 305 S15

q Vbi −VBE( )

16 emitter base

Fn

Fn

EC

EV“active”

IEn

ICn =αT IEn = IC

IC = I0eqVBE kBT

IEp

IB = IEp

IB = IC βdc << IC