ch10 bjt fundamentals.ppt [호환 모드] · 2018. 1. 30. · chapter 10. bjt fundamentals...
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![Page 1: CH10 BJT Fundamentals.ppt [호환 모드] · 2018. 1. 30. · Chapter 10. BJT Fundamentals qPerformance Parameters •Emitter Efficiency 0 1£ £g üCurrent gain is maximized by making](https://reader036.vdocuments.pub/reader036/viewer/2022071515/613794ed0ad5d2067648b69e/html5/thumbnails/1.jpg)
Chapter 10. BJT Fundamentals
BJT Fundamentals
Sung June [email protected]
http://helios.snu.ac.kr
Chapter 10.
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Chapter 10. BJT Fundamentals
Contents
q Drift
q Diffusion
q Generation-Recombination
q Equations of State
2
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Chapter 10. BJT Fundamentals
q Terminology
ü The BJT is a device containing three adjoining, alternately doped regions, with the middle region being very narrow compared to the diffusion length
SMDLSemiconductor Device Fundamentals BJT Fundamentals
heavy doping heavy doping
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Chapter 10. BJT Fundamentals
ü All terminal currents are positive when the transistor is operated in the standard amplifying mode
ü The current flowing into a device must be equal to the current flowing out, and voltage drop around a closed loop must be equal to zero
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Chapter 10. BJT Fundamentals
E B CI I I= +
EB BC CE CE EC0 ( )V V V V V+ + = = -
ü The basic circuit configuration in which the device is connected and the biasing mode
The most widely employed configuration
Seldom used
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Chapter 10. BJT Fundamentals
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Chapter 10. BJT Fundamentals
Biasing Mode
Biasing Polarity E-B Junction
Biasing Polarity C-B Junction
Saturation Forward Forward
ActiveInverted
Cutoff
ForwardReverse
Reverse
Forward
Reverse
Reverse
ü Although the npn BJT is used in a far greater number of circuit applications and IC designs, the pnp BJT is a more convenient vehicle for establishing operational principles and concepts
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Chapter 10. BJT Fundamentals
§ ElectrostaticsüTwo independent pn junctions üAssuming the pnp transistor regions to be uniformly doped and taking NAE (E) >> NDB (B) > NAC (C)
W=quasineutral base width
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Chapter 10. BJT Fundamentals
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Chapter 10. BJT Fundamentals
q Introductory Operational Considerations
ü The primary carrier activity in the vicinity of the forward-biased E-B junction is majority carrier injection across the junction üThe p+-n nature of the junction leads to many more holes being injected than electrons being injected
Carrier activity in a pnp BJT under active mode biasing
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Chapter 10. BJT Fundamentals
ü The vast majority of holes diffuse completely through the quasineutral base and enter the C-B depletion regionüThe accelerating electric field in the C-B depletion region rapidly sweeps these carriers into the collector
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Chapter 10. BJT Fundamentals
ü IEp: the hole current injected into the base, IEn: the electron current injected into the emitter, ICp: a current almost exclusively resulting from the injected holes that successfully cross the base, ICn: a current from the minority carrier electrons in the collector that wander into the C-B depletion region and are swept into the baseü Very few of the injected holes are lost by recombination in the base àICp » IEp
E Ep EnI I I= +
C Cp CnI I I= +
B1 EnI I=
B3 CnI I=
B2 recombination current in BI =EC
CnC
EnE
II
II
II
P
P
@\
>>
>>
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Chapter 10. BJT Fundamentals
ü d.c. current gain: IC/IB, where IB is an electron current in a pnp BJT and IC is predominantly a hole current
Schematic visualization of amplification in a pnp BJT under active mode biasing
ü Control of the larger IC by the smaller IB is made possible
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Chapter 10. BJT Fundamentals
q Performance Parameters
• Emitter Efficiency
0 1g£ £
ü Current gain is maximized by making g as close as possible to unity
• Base Transport Factorü The fraction of the minority carriers injected into the base that successfully diffuse across the quasineutral width of the base and enter the collector
T0 1a£ £
ü Maximum amplification occurs when aT is as close as possible to unity
nP
PP
EE
E
E
E
II
I
I
I
+==g
p
p
E
CT I
I=a
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Chapter 10. BJT Fundamentals
• Common Base d.c. Current Gainü When connected in the common base configuration,
Cp T Ep T EI I Ia ga= =
C Cp Cn T E CnI I I I Iga= + = +
dc Ta ga=
CBO CnI I=
where is adc the common base d.c. current gain and ICB0 is the collector current that flows when IE=0
dc0 1a£ £
• Common Emitter d.c. Current Gainü When connected in the common emitter configuration,
0CBEdcC III += a
0CEBdcC III += b
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Chapter 10. BJT Fundamentals
dcdc
dc1ab
a=
-
Cdc
B
II
b =
where is bdc the common emitter d.c. current gain and ICE0 is the collector current that flows when IB=0
ü Rearranging and solving for IC,
If ICE0 is negligible compared to IC
0)( CEECdcC IIII ++= aBCE
CBEdcC
IIIIII
+=+= ,0aQ
dc
CBB
dc
dcC
IIIaa
a-
+-
=11
0
1>>dcb
dc
CBCE
IIa-
=1
00
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Chapter 10. BJT Fundamentals
Summary
17
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Chapter 10. BJT Fundamentals
Summary
18
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