#23
QUIZ Lecture #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. 4. Recombination of injected minority carriers in the base Injection of carriers into the emitter Reverse saturation current in collector junction
Reduces | IB |
Recombination in the base-emitter depletion region
EMITTER BASE COLLECTOR
p-type
n-type
p-type
Spring 2007
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 CUTOFF Forward ACTIVE Reverse ACTIVE SATURATION
Spring 2007
Emitter Junction reverse bias forward bias reverse bias* forward bias
EE130 Lecture 23, Slide 4
Collector Junction reverse bias reverse bias* forward bias forward bias *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)
Electric field,
(x)
Charge density, (x)
Spring 2007
EE130 Lecture 23, Slide 6
BJT Performance Parameters (PNP)
Emitter Efficiency: I Ep I Ep + I En
Decrease (5) relative to (1+2) to increase efficiency
Base Transport Factor: I Cp T I Ep
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 = dc I E + I CB 0
where ICB0 is the collector current I = when I = whichCflows dc I C +E I B + 0 I CB 0
(
)
dc I CB 0 IC = IB + 1 - dc 1 - dc = I 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:
pnp BJT
IE = IB + IC
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
I Ep I Ep + I En
Base transport factor
T
I Cp I Ep
Common base d.c. current gain
dc T =
I Cp IE
Common emitter d.c. current gain
IC dc dc I B 1 - dc
Spring 2007
EE130 Lecture 23, Slide 10
Notation (PNP BJT)
NE = NAE DE = DN E= n LE = LN nE0 = np0 = ni2/NE
NB = NDB DB = DP B= p LB = LP pB0 = pn0 = ni2/NB
NC = NAC DC = DN C= n LC = LN nC0 = np0 = ni2/NC
Spring 2007
EE130 Lecture 23, Slide 11
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
I En = - qADE ddxn"E x "= 0
I Ep = - qADB dpB dx I Cp = - qADB dpB dx
x =0
I Cn = qAD
dnC C dx ' x '= 0
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 n E dx "2
- nEE
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 p B dx 2
- pBB
Boundary Conditions:
p B (0) = p B 0 (e qVEB / kT - 1) p B (W ) = p B 0 (e qVCB / kT - 1)
Spring 2007
EE130 Lecture 23, Slide 14
Collector Region Formulation
Diffusion equation:
0 = DC
d 2 nC dx '2
- nCC
Boundary Conditions:
nC ( x' ) = 0 nC ( x' = 0) = nC 0 (e qVCB / kT - 1)
Spring 2007
EE130 Lecture 23, Slide 15