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ECE 342 – Jose Schutt‐Aine 1 ECE 442 Solid‐State Devices & Circuits 6. Bipolar Transistors Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois [email protected] 1
ECE 342 – Jose Schutt‐Aine 1
ECE 442Solid‐State Devices & Circuits
6. Bipolar Transistors
Jose E. Schutt-AineElectrical & Computer Engineering
University of [email protected]
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• Bipolar Junction Transistor (BJT)– First Introduced in 1948 (Bell labs)– Consists of 2 pn junctions– Has three terminals: emitter, base, collector
Bipolar Junction Transistor
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BJT – Modes of Operation
Mode EBJ CBJ
Cutoff Reverse Reverse
Forw. Active Forward Reverse
Rev. Active Reverse Forward
Saturation Forward Forward
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BJT in Forward Active Mode (NPN)
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Electrons are minority carriers in the base (p-type)/(0) BE TV V
p pon n e
( ) (0)p pn E n E n
dn x nI A qD A qD
dx W
iC is independent of vCB
Collector current:
Minority electrons will diffuse in the p-type base
Longitudinal Current Flow
/BE Tv VC n Si I I e
2E n i
SA
A qD nIN W
AE: cross section area of BEJW: Effective width of baseNA: doping concentration baseDn: electron diffusivityq: electron charge
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Base Current
/2
1
BE Tv VE p i
BD p
A qD n ei
N L
Dp: hole diffusivity in emitterLp: hole diffusion length in emitterND: doping concentration of emitter
• Base current: Two components– Hole injection into emitter iB1– Electron recombination in base iB2
2n
Bb
Qi
Qn: minority carrier charge in baseb: minority carrier lifetime
2/1 (0)
2 2BE Tv VE i
n E pA
A qWnQ A q n W eN
Basepnp(0)
np(ideal)
effectivebase width
From area under triangle
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• Base current has two functionsBJT Operation: Longitudinal and Base Currents
• Base current is small because
• Longitudinal current
Support reverse injectionFeed recombination that occur in the base
Has large lifetimeBase is thin
Emitter is much more heavily doped than base
Depends (exponentially) on emitter junction voltage Is independent of collector junction voltageField due to collector‐base voltage attracts carriers but has no effect on rate of attraction
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BJT Operation: Current Gain
1 2B B Bi i i • Total Base current:
Define a current gain such that
2/1
2BE Tp v VA
B Sn D p n b
D N W Wi I eD N L D
C
B
ii
2
112
p A
n D p n b
D N W WD N L D
Using previous relation for iC
is the common-emitter current gain
In order to achieve a high gain we need
Dn: largeLp: largeND: largeNA: smallW: small
Typically 50 < < 200
In special transistors, can be as high as 1000
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Current Gain Temperature Dependence
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BJT Operation: Emitter Current
E C Bi i i • Emitter current:
Define such that
/1 1BE Tv V
E C Si i I e
C Ei i
Using previous relation for iC
is the common-base current gain
1
1
0.99
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B
C
E
Structure of BJT’s
Collector surrounds emitter region electrons will be collected
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PNP
NPN
BJT Transistor Polarities
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Ebers-Moll Model
NPN Transistor
// 1 1BC TBE T v Vv V SC S
R
Ii I e e
// 1 1BC TBE T v Vv VSE S
F
Ii e I e
// 1 1BC TBE T v Vv VS SB
F R
I Ii e e
1F
FF
1
RR
R
Describes BJT operation in all of its possible modes
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Common-Emitter Large-Signal Model
Common terminal is common to input and outputCommon terminal is used as reference or ground
