Lecture #23
QUIZ #3 Results
(undergraduate scores only, N = 39)
Mean = 22.1; Median = 22; Std. Dev. = 1.995
High = 25; Low = 18
OUTLINE
The Bipolar Junction Transistor
– Fundamentals
– Ideal Transistor Analysis
Reading: Chapter 10, 11.1
Spring 2007
EE130 Lecture 23, Slide 1
Base Current Components (Active Bias)
The base current consists of majority carriers supplied for
1.
2.
3.
Recombination of injected minority carriers in the base
Injection of carriers into the emitter
Reverse saturation current in collector junction
•
4.
Reduces | IB |
Recombination in the base-emitter depletion region
Spring 2007
EMITTER
BASE
COLLECTOR
p-type
n-type
p-type
EE130 Lecture 23, Slide 2
Circuit Configurations
Output Characteristics for Common-Emitter Configuration
Spring 2007
EE130 Lecture 23, Slide 3
Modes of Operation
Common-emitter output characteristics
(IC vs. VCE)
Mode
Emitter Junction
Collector Junction
CUTOFF
reverse bias
reverse bias
Forward ACTIVE
forward bias
reverse bias*
Reverse ACTIVE
reverse bias*
forward bias
SATURATION
forward bias
forward bias
Spring 2007
EE130 Lecture 23, Slide 4
*or not strongly forward biased
BJT Electrostatics
• Under normal operating conditions, the BJT may be
viewed electrostatically as two independent pn junctions
Spring 2007
EE130 Lecture 23, Slide 5
Electrostatic potential, V(x)
e
Electric field, (x)
Charge density, r(x)
Spring 2007
EE130 Lecture 23, Slide 6
BJT Performance Parameters (PNP)
• Emitter Efficiency:
I Ep

I Ep  I En
• Base Transport Factor:
I Cp
T 
I Ep
– Decrease (5) relative to (1+2)
to increase efficiency
– Decrease (1) relative to (2)
to increase transport factor
• Common-Base d.c. Current Gain:
Spring 2007
EE130 Lecture 23, Slide 7
 dc  T
Collector Current (PNP)
•
The collector current is comprised of
•
Holes injected from emitter,
which do not recombine in the base  (2)
•
Reverse saturation current of collector junction  (3)
I C  α dc I E  I CB 0
where ICB0 is the collector current
which flows when IE = 0
I C  α dc I C  I B   I CB 0
α dc
I CB 0
IC 
IB 
1  α dc
1  α dc
 βI B  I CE 0
Spring 2007
• Common-Emitter d.c.
Current Gain:
EE130 Lecture 23, Slide 8
IC
 dc
 dc 

I B 1   dc
Summary: BJT Fundamentals
• Notation & conventions:
IE = IB + IC
pnp BJT
npn BJT
• Electrostatics:
– Under normal operating conditions, the BJT may
be viewed electrostatically as two independent pn
junctions
Spring 2007
EE130 Lecture 23, Slide 9
• Performance parameters:
– Emitter efficiency
– Base transport factor

I Ep
I Ep  I En
T 
I Cp
I Ep
– Common base d.c. current gain
– Common emitter d.c. current gain
Spring 2007
EE130 Lecture 23, Slide 10
 dc  T 
I Cp
IE
IC
 dc
 dc 

I B 1   dc
Notation (PNP BJT)
NE = NAE
DE = DN
tE = tn
LE = LN
nE0 = np0 = ni2/NE
Spring 2007
NB = NDB
DB = DP
tB = tp
LB = LP
pB0 = pn0 = ni2/NB
EE130 Lecture 23, Slide 11
NC = NAC
DC = DN
tC = tn
LC = LN
nC0 = np0 = ni2/NC
Ideal Transistor Analysis
• Solve the minority-carrier diffusion equation in each quasi-neutral
region to obtain excess minority-carrier profiles
– different set of boundary conditions for each region
• Evaluate minority-carrier diffusion currents at edges of depletion
regions
nE
I En  qADE ddx
"
x"0
dnC
C dx '
x ' 0
I Cn  qAD
I Ep  qADB ddxpB
x 0
I Cp  qADB ddxpB
x W
• Add hole & electron components together  terminal currents
Spring 2007
EE130 Lecture 23, Slide 12
Emitter Region Formulation
• Diffusion equation:
0  DE
d 2 nE
dx"2
 tnEE
• Boundary Conditions:
nE ( x"  )  0
nE ( x"  0)  nE 0 (e qVEB / kT  1)
Spring 2007
EE130 Lecture 23, Slide 13
Base Region Formulation
• Diffusion equation:
0  DB
d 2 pB
dx2
 tpBB
• Boundary Conditions:
pB (0)  pB 0 (e qVEB / kT  1)
pB (W )  pB 0 (e qVCB / kT  1)
Spring 2007
EE130 Lecture 23, Slide 14
Collector Region Formulation
• Diffusion equation:
0  DC
d 2 nC
dx'2
 tnCC
• Boundary Conditions:
nC ( x'  )  0
nC ( x'  0)  nC 0 (e
Spring 2007
qVCB / kT
 1)
EE130 Lecture 23, Slide 15