L07 February 05 1

EE 4345 - SemiconductorElectronics Design ProjectSpring 2002 - Lecture 07

Professor Ronald L. Carterronc@uta.edu

http://www.uta.edu/ronc/

L07 February 05 2

Lab schedule

• As announced last week, plan toattend your lab session (Tu or Th at230 to 500 PM) this week foradditional Cadence instruction.

• Topics to be covered– schematic capture of a circuit– making a symbol– simulating a circuit

L07 February 05 3

Bipolar junctiontransistor (BJT)• The BJT is a

‘silicon sandwich’p+np- or n+pn-

• npn For. ActiveBJT action when

VBE > 0 andVBC < 0

p n p

E B C

VEB VCB

Charge neutralRegion

Depletion Region

General bias ona pnp transistor

L07 February 05 4

npn BJT currents(F A region, ©RLC)

IC =JCAC

IB=-(IE+IC )

JnE JnC

IE =-JEAE

JRB=JnE-JnC

JpE

JGC

JRE JpC

L07 February 05 5

Ebers-Moll(npn injection model)

C

E

B

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expt

BC

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ECV

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BES

ECCCCT

VVV

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III

exp

exp

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BE

F

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F

CCV

VII(common-emitter)

L07 February 05 6

E-M modelequations

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1exp1exp

1exp1exp

1

and ,1

defining ,0

1exp1exp

1exp1exp

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BC

R

S

t

BE

F

S

t

BCS

R

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t

BES

F

SB

R

RR

F

FFECB

t

BCCSR

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BEESE

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BEESF

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BCCSC

VVI

VVI

VVII

VVIII

III

VVI

VVII

VVI

VVII

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L07 February 05 7

Ebers-Moll Model(npn bipolar jctn. tran. w/o inj.)

CSRESFS

CSRESF

III

1*NR

expII , 1*NF

expII

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L07 February 05 8

IC npn BJT(*Fig 9.2a)

L07 February 05 9

npn BJT regionsof operation

VBE

VBC

ForwardActive

ReverseActive Saturation

Cutoff

L07 February 05 10

npn FA BJT minoritycarrier distribution (Fig 9.4*)

L07 February 05 11

npn RA BJT minoritycarrier distribution (Fig 9.11a*)

L07 February 05 12

npn cutoff BJT mincarrier distribution (Fig 9.10a*)

L07 February 05 13

npn sat BJT minoritycarrier distribution (Fig 9.10b*)

L07 February 05 14

Defining currents inFA mode npn BJT (Fig 9.13*)

L07 February 05 15

Non-ideal effectsin BJTsº Base-width modulation (FA: xB

changes with changes in VBC)º Current crowding in 2-dim base�High-level injection (minority carriers

g.t. dopant - especially in the base).�Emitter Bandgap narrowing for NE -->

density of states at cond. band. edge�Junction breakdown at BC junction

L07 February 05 16

npn Base-width modulation(Early Effect) Fig 9.15*

xnqDJ nn

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B

BBC

BB

BC

BBjC

BC

j

Vx

xJ

VJ

xJ

xJ

VxAqNC

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L07 February 05 17

Base-width modulation(Early Effect, cont.)

Fig 9.16*

ACEB

jC

CE

B

jC

B

BC

B

BCB

VVI

QC

VI

AqNC

xJ

VxA

xJ

VI

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L07 February 05 18

Emitter currentcrowding in baseFig 9.21*

L07 February 05 19

Interdigitated basefixes emitter crowdingFig 9.23*

L07 February 05 20

Effect of HLI innpn base region Figs 9.18 and 9.19*

L07 February 05 21

Ebers-Moll Model(Neglecting G-R curr)

(Fig. 9.30*) N

nqD

,aB

2inB

CSRESFeffBW

II ����

L07 February 05 22

Hybrid-piCircuit model

Fig 9.33 Semiconductor Physicsand Devices, 2nd ed., by Neamen,Irwin, Boston, 1997.

L07 February 05 23

Hybrid-picircuit model• Adapted from inj. model version of E-M

model with parasitic rb, rc, re, and CSubst

• C-E branch is linking current• B-E branch is the reduced B-E diode

with diffusion (for and rev) resistanceand capacitance and junction cap.

• B-C branch is the reduced B-C diodewith diffusion (for and rev) resistanceand capacitance and junction cap.

L07 February 05 24

References

* Semiconductor Physics & Devices, byDonald A. Neamen, Irwin, Chicago,1997.