Download Bipolar Junction Transistors: Basics

Transistor/switch/amplifier – a 3 terminal device
Source
Gate
Incoherent
Light
Coherent
Light
Vein
Artery
Valve
Gain medium
Drain
Laser
Dam
Emitter
Collector
Heart
Ion Channel
Base
BJT
MOSFET
Axonal conduction
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All of these share a feature with…
• Output current can toggle between large and small
(Switching  Digital logic; create 0s and 1s)
• Small change in ‘valve’ (3rd terminal) creates Large
change in output between 1st and 2nd terminal
(Amplification  Analog applications; Turn 0.5  50)
Example: BJT common emitter characteristics
Gain = 300
http://www.computerhistory.org/semiconductor/timeline.html#1940s
Aim of this chapter
• How can we get ‘Gain’?
• What is the structure of the device to get gain?
• What is the equation for gain?
• How can we use this equation to maximize gain?
• How can we model this device as a circuit element?
• What are its AC characteristics and speed?
Recall p-n junction
W
+
P
N
N
P
W +
-
-
Vappl < 0
Vappl > 0
Forward bias, + on P, - on N
(Shrink W, Vbi)
Reverse bias, + on N, - on P
(Expand W, Vbi)
Allow holes to jump over barrier
into N region as minority carriers
Remove holes and electrons away
from depletion region
I
I
V
V
So if we combine these by fusing their terminals…
N
P
W
+
-
Vappl > 0
P
N
W +
-
Vappl < 0
Holes from P region (“Emitter”) of 1st PN junction
driven by FB of 1st PN junction into central N region (“Base”)
Driven by RB of 2nd PN junction from Base into P region of
2nd junction (“Collector”)
• 1st region FB, 2nd RB
• If we want to worry about holes alone, need P+ on 1st region
• For holes to be removed by collector, base region must be thin
Bipolar Junction Transistors: Basics
+
-
IE
IC
-
+ IB
IE = I B + IC
………(KCL)
VEC = VEB + VBC ……… (KVL)
BJT configurations
GAIN
CONFIG
ECE 663
Bipolar Junction Transistors: Basics
+
-
IE
IC
-
+ IB
VEB >-VBC > 0  VEC > 0 but small
IE > -IC > 0  IB > 0
VEB, VBC > 0  VEC >> 0
IE, IC > 0  IB > 0
VEB < 0, VBC > 0  VEC > 0
IE < 0, IC > 0  IB > 0 but small
ECE 663
Bipolar Junction Transistors: Basics
Bias Mode
E-B Junction
C-B Junction
Saturation
Forward
Forward
Active
Forward
Reverse
Inverted
Reverse
Forward
Cutoff
Reverse
Reverse
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BJT Fabrication
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PNP BJT Electrostatics
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PNP BJT Electrostatics
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NPN Transistor Band Diagram: Equilibrium
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PNP Transistor Active Bias Mode
VEB > 0
VCB > 0
Few recombine
in the base
Collector Fields drive holes
far away where they can’t
return thermionically
Large injection
of Holes
Most holes
diffuse to
collector
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Forward Active minority carrier distribution
P+
N
P
pB(x)
nE(x’)
nC0
nE0
pB0
nC(x’’)
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PNP Physical Currents
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PNP transistor amplifier action
IN (small)
OUT (large)
Clearly this works in common emitter
configuration
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Emitter Injection Efficiency - PNP
IE
E
ICp
IEp
IEn
ICn
IC
C
IB
IEp
IEp


IE IEp  IEn
Can we make the emitter
see holes alone?
0   1
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Base Transport Factor
IE
E
ICp
IEp
IEn
ICn
IC
C
IB
ICp
T 
I Ep
0  T  1
Can all injected holes
make it to the collector?
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Common Base DC current gain - PNP
Common Base – Active Bias mode:
IC = DCIE + ICB0
ICp = TIEp
= TIE
DC = T
IC = TIE + ICn
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Common Emitter DC current gain - PNP
Common Emitter – Active Bias mode:
IE = bDCIB + ICE0
bDC =
DC /(1-DC)
IC = DCIE + ICB0
= DC(IC + IB) + ICB0
IC = DCIB + ICB0
1-DC
GAIN !!
IC
IB
IE
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Common Emitter DC current gain - PNP
b dc
T

1  T
Thin base will make T  1
Highly doped P region will make   1
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PNP BJT Common Emitter Characteristic
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