26 - 1

I-V Characteristics of BJT

ECv

Lecture 26

Common-Emitter Output Characteristics

i B

B

C

E

Ci

CEv B

C

Ei B

Ci

26 - 2

To illustrate the IC-VCE characteristics, we use an enlarged βR

10

A

= 5

Lecture 26

0 5-5-1

0

1

2

VCE (V)

Reverse-ActiveRegion

SaturationRegion

Cutoff

IB = 100 µ

IB = 80 µA

IB = 60 µA

IB = 40 µA

IB = 20 µA

IB = 0 µA

Forward ActiveRegion

SaturationRegion

βF = 25; βR

Col

lect

orC

urre

nt(m

A)

vCE vBE≥

iC βFiB=

vCE vBE≤

vCE vBE≤

iC βR 1+( )– iB=

vCE vBE 0≤ ≤

26 - 3

Common Base Output Characteristics

BC

Ci

Lecture 26

iEB

CE

vCB

Ci

B

CE

viE

26 - 4

A

A

A

A

A

10

Lecture 26

Forward-ActiveRegion

IE = 0 mA

IE = 0.2 m

IE = 0.4 m

IE = 0.6 m

IE = 0.8 m

IE = 1.0 m

βF = 25; βR = 5

vCB or vBC (V)-2 0 2 4 6 8

0

0.5

1.0

Col

lect

orC

urre

nt(m

A)

26 - 5

Common-Emitter Transfer Characteristic iC - vBE

its temperature coef-

Log

(IC

)

Lecture 26

. BE voltage changes as -1.8 mV/oC - this isficient (recall from diodes).

vBC = 0C

olle

ctor

Cur

rent

I C(m

A)

-2

0

4

6

8

10

2

60 mV/decade

Base-Emitter Voltage (V)0.0 0.2 0.4 0.6 0.8 1.0

10-11

10-9

10-8

10-6

10-4

10-2

26 - 6

Common-Emitter Transfer Characteristic iC - vBE (p. 180)

its temperature coef-

Log

(IC

)

Lecture 26

. BE voltage changes as -1.8 mV/oC - this isficient (recall from diodes).

IC IS

vBE

VT---------

1–exp

=

vBC = 0C

olle

ctor

Cur

rent

I C(m

A)

-2

0

4

6

8

10

2

60 mV/decade

Base-Emitter Voltage (V)0.0 0.2 0.4 0.6 0.8 1.0

10-11

10-9

10-8

10-6

10-4

10-2

26 - 7

Junction Breakdown - BJT has two diodes back-to-back. Each diode has ans has the lower

Lecture 26

breakdown. The diode (BE) with higher doping concentratiobreakdown voltage (5 to 10 V).

In forward active region, BC junction is reverse biased.

In cut-off region, BE and BC are both reverse biased.

The transistor must withstand these reverse bias voltages.

26 - 8

Junction Breakdown - BJT has two diodes back-to-back. Each diode has ans has the lower

Lecture 26

breakdown. The diode (BE) with higher doping concentratiobreakdown voltage (5 to 10 V).

In forward active region, BC junction is reverse biased.

In cut-off region, BE and BC are both reverse biased.

The transistor must withstand these reverse bias voltages.

26 - 9

Junction Breakdown - BJT has two diodes back-to-back. Each diode has ans has the lower

Lecture 26

breakdown. The diode (BE) with higher doping concentratiobreakdown voltage (5 to 10 V).

In forward active region, BC junction is reverse biased.

In cut-off region, BE and BC are both reverse biased.

The transistor must withstand these reverse bias voltages.

26 - 10

Minority Carrier Transport in Base Region

iC

conc.eglectsation)

Lecture 26

Inj.Elec.

recombined electrons

Coll.Elec.

iT

IF/βF IR/βRIREC

N PN

Emitter Base Collector

Space Charge regions

(pno, npo)

+- +-

iE

iBvBE vBC

n(x)

xn(WB)

WB

iT qADndndx------=

n(0)

0

(pno, npo)

Electronin base (nrecombin

Inj.Holes

IREC

n 0( ) nbo

vBE

VT---------

exp=

26 - 11

Transport current iT results from diffusion of minority carriers (holes in npn)

and C and IRECns in B.

is the equilib-

ation gradient at

.

bo

Lecture 26

across base region.

Base current iB is composed of holes injected back into Eneeded to replenish holes lost to recombination with electro

The minority carrier concentrations at two ends of base are

and where

rium electron density in the base region.

The junction voltages establish a minority carrier concentrends of base region. For a narrow base, we get

is the B width; is the cross-sectional area of B region

The saturation current is

n

WB A

26 - 12

Transport current iT results from diffusion of minority carriers (electrons in

and C and IRECns in B.

is the equilib-

ation gradient at

.

bo

Lecture 26

npn) across base region.

Base current iB is composed of holes injected back into Eneeded to replenish holes lost to recombination with electro

The minority carrier concentrations at two ends of base are

and where

rium electron density in the base region.

The junction voltages establish a minority carrier concentrends of base region. For a narrow base, we get

is the B width; is the cross-sectional area of B region

The saturation current is

n 0( ) nbo

vBE

VT---------

exp= n WB( ) nbo

vBC

VT---------

exp= n

WB A

26 - 13

Transport current iT results from diffusion of minority carriers (holes in npn)

and C and IRECns in B.

is the equilib-

ation gradient at

.

bo

Lecture 26

across base region.

Base current iB is composed of holes injected back into Eneeded to replenish holes lost to recombination with electro

The minority carrier concentrations at two ends of base are

and where

rium electron density in the base region.

The junction voltages establish a minority carrier concentrends of base region. For a narrow base, we get

.

is the B width; is the cross-sectional area of B region

The saturation current is

n 0( ) nbo

vBE

VT---------

exp= n WB( ) nbo

vBC

VT---------

exp= n

iT qADndndx------ qADn

nboWB--------

vBEVT--------- vBC

VT---------

exp–exp

–= =

WB A

26 - 14

Transport current iT results from diffusion of minority carriers (holes in npn)

and C and IRECns in B.

is the equilib-

ation gradient at

.

bo

Lecture 26

across base region.

Base current iB is composed of holes injected back into Eneeded to replenish holes lost to recombination with electro

The minority carrier concentrations at two ends of base are

and where

rium electron density in the base region.

The junction voltages establish a minority carrier concentrends of base region. For a narrow base, we get

.

is the B width; is the cross-sectional area of B region

The saturation current is .

n 0( ) nbo

vBE

VT---------

exp= n WB( ) nbo

vBC

VT---------

exp= n

iT qADndndx------ qADn

nboWB--------

vBEVT--------- vBC

VT---------

exp–exp

–= =

WB A

IS qADn

nboWB-------- qADn

ni2

NABWB

--------------------= =

26 - 15

Base Transit Time Q in Base region

Lecture 26

Forward transit time is time associated with storing chargeand it is

with .

Using we get

τFQiT----= Q qA n 0( ) nbo–[ ]

WB

2--------=

Q qAnbo

vBEVT---------

1–exp WB

2--------=

26 - 16

o

Lecture 26

Using we get

and .

Q

n(x)

x

nbo

n(0)

n(WB) = nb

0 WB

Q = excess minority charge in Base

Q qAnbo

vBEVT---------

1–exp WB

2--------=

iTqADn

WB---------------nbo

vBE

VT---------

1–exp

= τF

WB2

2Dn----------

WB2

2VTµn

-----------------= =

26 - 17

o

Lecture 26

This defines an upper limit on frequency . f1

2πτF-------------≤

Q

n(x)

x

nbo

n(0)

n(WB) = nb

0 WB

Q = excess minority charge in Base

26 - 18

PSPICE EXAMPLE

\PSpice.ini file:

Lecture 26

*Libraries: * Local Libraries :.LIB ".\example10.lib" * From [PSPICE NETLIST] section of C:\Program Files\OrcadLite\PSpice.lib "nom.lib" *Analysis directives: .DC LIN V_V1 0 5 0.05 + LIN I_I1 10u 100u 10u .PROBE V(*) I(*) W(*) D(*) NOISE(*) .INC ".\example10-SCHEMATIC1.net" **** INCLUDING example10-SCHEMATIC1.net ***** source EXAMPLE10

Q1 V10Vdc

BF=100

I10Adc

IS=1.0e-15

VAF=80

26 - 19

PSPICE EXAMPLE (Cont’d)

*

**********

Lecture 26

Q_Q1 N00060 N00159 0 Qbreakn V_V1 N00060 0 0VdcI_I1 0 N00159 DC 0Adc **** RESUMING example10-SCHEMATIC1-Example10Profile.sim.cir ***.END **** BJT MODEL PARAMETERS******************************************************************** Qbreakn NPN IS 1.000000E-15 BF 100 NF 1 VAF 80 BR 3 NR 1 VAR 30 CN 2.42 D .87 JOB CONCLUDED TOTAL JOB TIME .21

26 - 20

PSPICE EXAMPLE (Cont’d)

4.5V 5.0V

Lecture 26

V_V1

0V 0.5V 1.0V 1.5V 2.0V 2.5V 3.0V 3.5V 4.0V-I(V1)

-4mA

0A

4mA

8mA

12mA