I-V Characteristics of BJT
Common-Emitter Output Characteristics
C
B
iB
Lecture 26
i
C
C
v CE
E
B
iB
i
C
v EC
E
26 - 1
To illustrate the IC-VCE characteristics, we use an enlarged βR
IB = 100 µA
Collector Current (mA)
Saturation
Region
2
Forward Active
Region
IB = 80 µA
v CE ≤ v BE
v CE ≥ v BE
iC = βF iB
IB = 40 µA
1
IB = 20 µA
v CE ≤ v BE ≤ 0
iC = –( βR + 1 ) iB
0
IB = 0 µA
Cutoff
Reverse-Active
Region
-1
-5
Lecture 26
IB = 60 µA
Saturation
Region
0
VCE (V)
βF = 25; βR = 5
v CE ≤ v BE
5
10
26 - 2
Common Base Output Characteristics
E
iE
v
B
Lecture 26
iC
C
CB
iE
iC
C
E
v
B
BC
26 - 3
1.0
Collector Current (mA)
IE = 1.0 mA
IE = 0.8 mA
IE = 0.6 mA
βF = 25; βR = 5
0.5
IE = 0.4 mA
IE = 0.2 mA
0
IE = 0 mA
Forward-Active
Region
-2
0
2
4
6
8
10
vCB or vBC (V)
Lecture 26
26 - 4
Common-Emitter Transfer Characteristic iC - vBE
. BE voltage changes as -1.8 mV/oC - this is its temperature coefficient (recall from diodes).
10
10-2
8
10-4
6
10-6
60 mV/decade
4
10-8
Log(IC)
Collector Current IC (mA)
vBC = 0
2
10-9
0
-2
0.0
0.2
0.4
0.6
0.8
10-11
1.0
Base-Emitter Voltage (V)
Lecture 26
26 - 5
Common-Emitter Transfer Characteristic iC - vBE (p. 180)
 v BE


I C = I S  exp  --------- – 1  . BE voltage changes as -1.8 mV/oC - this is its temperature coef VT 

 ficient (recall from diodes).
10
10-2
8
10-4
6
10-6
60 mV/decade
4
10-8
Log(IC)
Collector Current IC (mA)
vBC = 0
2
10-9
0
-2
0.0
0.2
0.4
0.6
0.8
10-11
1.0
Base-Emitter Voltage (V)
Lecture 26
26 - 6
Junction Breakdown - BJT has two diodes back-to-back. Each diode has a
breakdown. The diode (BE) with higher doping concentrations has the lower
breakdown 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.
Lecture 26
26 - 7
Junction Breakdown - BJT has two diodes back-to-back. Each diode has a
breakdown. The diode (BE) with higher doping concentrations has the lower
breakdown 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.
Lecture 26
26 - 8
Junction Breakdown - BJT has two diodes back-to-back. Each diode has a
breakdown. The diode (BE) with higher doping concentrations has the lower
breakdown 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.
Lecture 26
26 - 9
Minority Carrier Transport in Base Region
- +
iB
vBE
Inj.
Holes
IF/βF
iT
Inj.
iE Elec.
- +
IREC
(pno, npo)
vBC
IR/βR
recombined electrons
P
IREC
N
Emitter
Base
Coll.
Elec.
iC
N
Collector
Space Charge regions
n(x)
n(0)
 v BE
n ( 0 ) = n bo exp  ---------
 VT 
dn
i T = qAD n -----dx
(pno, npo)
0
Lecture 26
Electron conc.
in base (neglects
recombination)
n(WB)
x
WB
26 - 10
Transport current iT results from diffusion of minority carriers (holes in npn)
across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B.
The minority carrier concentrations at two ends of base are
and
where n bo is the equilib-
rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at
ends of base region. For a narrow base, we get
W B is the B width; A is the cross-sectional area of B region.
The saturation current is
Lecture 26
26 - 11
Transport current iT results from diffusion of minority carriers (electrons in
npn) across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B.
The minority carrier concentrations at two ends of base are
 v BE
n ( 0 ) = n bo exp  ---------
 VT 
 v BC
and n ( W B ) = n bo exp  --------- where n bo is the equilib VT 
rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at
ends of base region. For a narrow base, we get
W B is the B width; A is the cross-sectional area of B region.
The saturation current is
Lecture 26
26 - 12
Transport current iT results from diffusion of minority carriers (holes in npn)
across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B.
The minority carrier concentrations at two ends of base are
 v BE
n ( 0 ) = n bo exp  ---------
 VT 
 v BC
and n ( W B ) = n bo exp  --------- where n bo is the equilib VT 
rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at
ends of base region. For a narrow base, we get
n bo 
 v BE
 v BC 
dn
i T = qAD n ------ = – qAD n --------  exp  --------- – exp  ---------  .
dx
WB 
 VT 
 VT  
W B is the B width; A is the cross-sectional area of B region.
The saturation current is
Lecture 26
26 - 13
Transport current iT results from diffusion of minority carriers (holes in npn)
across base region.
Base current iB is composed of holes injected back into E and C and IREC
needed to replenish holes lost to recombination with electrons in B.
The minority carrier concentrations at two ends of base are
 v BE
n ( 0 ) = n bo exp  ---------
 VT 
 v BC
and n ( W B ) = n bo exp  --------- where n bo is the equilib VT 
rium electron density in the base region.
The junction voltages establish a minority carrier concentration gradient at
ends of base region. For a narrow base, we get
n bo 
 v BE
 v BC 
dn
i T = qAD n ------ = – qAD n --------  exp  --------- – exp  ---------  .
dx
WB 
 VT 
 VT  
W B is the B width; A is the cross-sectional area of B region.
2
n bo
ni
--------------------------The saturation current is I S = qAD n
.
= qAD n
WB
N AB W
B
Lecture 26
26 - 14
Base Transit Time
Forward transit time is time associated with storing charge Q in Base region
and it is
WB
Q
τ F = ---- with Q = qA [ n ( 0 ) – n bo ] -------- .
2
iT
 v BE

WB
--------Using Q = qAn bo  exp 
 – 1  -------- we get
 VT 

 2
Lecture 26
26 - 15
n(x)
Q = excess minority
charge in Base
n(0)
Q
nbo
n(WB) = nbo
WB
0
x
 v BE

WB
Using Q = qAn bo  exp  --------- – 1  -------- we get
 VT 

 2
2
2
qAD n
WB
WB
 v BE


---------------------------------------------.
n bo  exp 
iT =
=
 – 1  and τ F =
WB
2D n
2V T µ
 VT 


n
Lecture 26
26 - 16
This defines an upper limit on frequency
1
f ≤ ------------- .
2πτ F
n(x)
Q = excess minority
charge in Base
n(0)
Q
nbo
n(WB) = nbo
0
Lecture 26
WB
x
26 - 17
PSPICE EXAMPLE
Q1
V1
0Vdc
I1
0Adc
IS=1.0e-15
BF=100
VAF=80
*Libraries:
* Local Libraries :
.LIB ".\example10.lib"
* From [PSPICE NETLIST] section of C:\Program Files\OrcadLite\PSpice\PSpice.ini file:
.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
Lecture 26
26 - 18
PSPICE EXAMPLE (Cont’d)
Q_Q1
N00060 N00159 0 Qbreakn
V_V1
N00060 0 0Vdc
I_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
Lecture 26
.21
26 - 19
PSPICE EXAMPLE (Cont’d)
12mA
8mA
4mA
0A
-4mA
0V
0.5V
1.0V
1.5V
2.0V
2.5V
3.0V
3.5V
4.0V
4.5V
5.0V
-I(V1)
V_V1
Lecture 26
26 - 20