Chapter 10. BJT Fundamentals
Chapter 10.
BJT Fundamentals
Sung June [email protected]
http://helios.snu.ac.krp
Chapter 10. BJT Fundamentals
Contents
Terminology
Electrostatics
Introductory Operational ConsiderationsIntroductory Operational Considerations
Performance Parameters
2
Chapter 10. BJT Fundamentals
Terminology
The BJT is a device containing three adjoining, alternately doped regions, with the middle region being very narrow compared to the diffusion length
heavy doping heavy doping
SMDLSemiconductor Device Fundamentals BJT Fundamentals
Chapter 10. BJT Fundamentals
All t i l t iti h th t i t i t d i thAll terminal currents are positive when the transistor is operated in the standard amplifying mode
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
Chapter 10. BJT Fundamentals
E B CI I I= +
0 ( )V V V V V+ + = = −EB BC CE CE EC0 ( )V V V V V+ + = =
The basic circuit configurations in which the device is connected
The most widely employed configuration
Seldom used
co gu at o
Chapter 10. BJT Fundamentals
Chapter 10. BJT Fundamentals
Biasing Mode
Biasing Polarity E-B Junction
Biasing Polarity C-B Junction
Saturation Forward ForwardSaturation Forward Forward
ActiveInverted
ForwardReverse Forward
Reverse
Cutoff Reverse Reverse
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 conceptsfor establishing operational principles and concepts
Chapter 10. BJT Fundamentals
BJT fabrication
Chapter 10. BJT Fundamentals
BJT fabrication
Chapter 10. BJT Fundamentals
El t t tiElectrostaticsTwo independent pn junctions Assuming the pnp transistor regions to be uniformly doped and taking g p p g y p g
NAE (E) >> NDB (B) > NAC (C)
W=quasineutral base width
Chapter 10. BJT Fundamentals
Chapter 10. BJT Fundamentals
Introductory Operational Considerations
Carrier activity in a pnp BJT under active mode biasing
The primary carrier activity in the vicinity of the forward-biased E-B junction is majority carrier injection across the junction
The p+-n nature of the junction leads to many more holes being injected than electrons being injected
Chapter 10. BJT Fundamentals
The vast majority of holes diffuse completely through the quasineutral base and enter the C-B depletion regionp g
The accelerating electric field in the C-B depletion region rapidly sweeps these carriers into the collector
Chapter 10. BJT Fundamentals
I th h l t i j t d i t th b I th l t tIEp: 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 j y Cn
the minority carrier electrons in the collector that wander into the C-B depletion region and are swept into the base
Very few of the injected holes are lost by recombination in the baseVery few of the injected holes are lost by recombination in the base ICp ≈ IEp
I I I+E Ep EnI I I= +
C Cp CnI I I= +p
I I=EnE II
P>>
B1 EnI I=
B3 CnI I=EC
CnC
II
IIP
≅∴
>>
B2 recombination current in BI =EC
Chapter 10. BJT Fundamentals
d t i I /I h I i l t t i BJTd.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
Control of the larger IC by the smaller IB is made possible
Chapter 10. BJT Fundamentals
Bipolar Junction Transistor (BJT)p ( )
• Current components• Current components
Chapter 10. BJT Fundamentals
P f P tPerformance Parameters
• Emitter Efficiency
0 1γ≤ ≤PP
EE
E
E
E
II
I
I
I
+==γ
Current gain is maximized by making γ as close as possible to unitynP EEE
• Base Transport FactorThe fraction of the minority carriers injected into the base thatThe fraction of the minority carriers injected into the base that
successfully diffuse across the quasineutral width of the base and enter the collector
T0 1α≤ ≤p
p
E
CT I
I=α
Maximum amplification occurs when αT is as close as possible to unity
Chapter 10. BJT Fundamentals
• Common Base d.c. Current GainWhen connected in the common base configuration,
III +αwhere is αdc the common base d.c. current gain and ICB0 is the collector current that flows when IE=0
0CBEdcC III += α
Cp T Ep T EI I Iα γα= =
I I I I Iγα+ +
E
C Cp Cn T E CnI I I I Iγα= + = +
dc Tα γα= dc0 1α≤ ≤
CBO CnI I=
• Common Emitter d.c. Current GainWhen connected in the common emitter configuration,
III β 0CEBdcC III += β
Chapter 10. BJT Fundamentals
where is βdc the common emitter d.c. current gain and ICE0 is the collector current that flows when IB=0
)( IIII ++ CBEdcC III += ,0αQ
Rearranging and solving for IC,
0)( CBBCdcC IIII ++= αBCE
CBEdcC
III +=,0
dc
CBB
dc
dcC
IIIαα
α−
+−
=11
0
dcdc
dc1αβ
α=
−
dcdc
1>>dcβdc
CBCE
IIα−
=1
00
Cd
Iβ = If I is negligible compared to I
dcα−1
dcBI
β If ICE0 is negligible compared to IC
Chapter 10. BJT Fundamentals
(ref)Evaluation of the terminal currents(ref)Evaluation of the terminal currents