chapter 11 bjt static characteristics
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Chapter 11 BJT Static Characteristics. Chapter 11. BJT Static Characteristics. Ebers -Moll Model. The Ebers-Moll model is a large-signal equivalent circuit which describes both the active and saturation regions of BJT operation. - PowerPoint PPT PresentationTRANSCRIPT
President University Erwin Sitompul SDP 10/1
Dr.-Ing. Erwin SitompulPresident University
Lecture 10
Semiconductor Device Physics
http://zitompul.wordpress.com
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Chapter 11BJT Static Characteristics
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The Ebers-Moll model is a large-signal equivalent circuit which describes both the active and saturation regions of BJT operation.
This model is to be used to calculate IC, IE for given VBE, VBC.
If only VEB is applied (VCB = 0): EB
EB
EB
E F0
C F F0
B F F0
( 1)( 1)
(1 ) ( 1)
qV kT
qV kT
qV kT
I I eI I eI I e
If only VCB is applied (VEB = 0):CB
CB
CB
C R0
E R R0
B R0 R
( 1)( 1)
(1 )( 1)
qV kT
qV kT
qV kT
I I eI I eI I e
EB
CB
BE BE E0 B0
E B B
BB0
B B
cosh( 1)
sinh
1 ( 1)
sinh
qV kT
qV kT
W LD DI qA n p e
L L W L
Dp e
L W L
EB
CB
BC B0
B B
BC BC0 B0
C B B
1( 1)
sinh
cosh ( 1)
sinh
qV kT
qV kT
DI qA p e
L W L
W LD Dn p e
L L W L
IFO
IRO
Ebers-Moll ModelChapter 11 BJT Static Characteristics
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aR : reverse common base gain
aF : forward common base gain
B0BF F0 R R0
B Bsinh( )
pDI I qA
L W L
CBEBC F F0 R0( 1) ( 1)qV kTqV kTI I e I e
fraction of E-B diode C-B diode currentcurrent that makes it
to the C-B junction
CBEBE F0 R R0( 1) ( 1)qV kTqV kTI I e I e
E-B diode fraction of C-B diode current current that makes it
to the E-B junction
Ebers-Moll ModelChapter 11 BJT Static Characteristics
Reciprocity relationship:
In the general case, when VEB and VCB are non-zero:
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Deviations, due to model limitations
Deviations from the IdealChapter 11 BJT Static Characteristics
Common base
Common emitter
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dc 2
E B
B E E B
B E E
E B
1
12
D N W WD N L LD N L
D N W
Wx
D pB(x)
EBB0 ( 1)qV kTp e (VCB=0)
0
W
P+ N P
+ VEB
IE IC
Increasing –VCB
C dc B CE0I β I I
If –VCB increases→ W decreases→ bdc increases→ IC increases
Base-Width ModulationChapter 11 BJT Static Characteristics
Common-Emitter ConfigurationActive Mode Operation Recalling two formulas,
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WB = xnEB + xnCB
Punch-ThroughChapter 11 BJT Static Characteristics
Punch-Through: E-B and C-B depletion regions in the base touch each other, so that W = 0.
As –VCB increases beyond the punch-through point, the E-B potential hill decreases and therefore increases the carrier injections and IC.
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Punch-through Avalanche
Increasing reverse bias of C-B junction
EC EB CB=V V V
Breakdown MechanismsChapter 11 BJT Static Characteristics
In the common-emitter configuration, for high output voltage VCE, the output current IC will increase rapidly due to the two mechanisms: punch-through and avalanche.
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pnp BJT
dc CB0C B
dc dc
α
1 α 1 α
M II I
M M
M : multiplication factor
Avalanche MultiplicationChapter 11 BJT Static Characteristics
Holes [0] are injected into the base [1], then collected by the C-B junction.
Some holes in the C-B depletion region have enough energy do impact ionization [2].
The generated electrons are swept into the base [3], then injected into the emitter [4].
Each injected electron results in the injection of IEp/IEn holes from the emitter into the base [5].
For each pair created in the C-B depletion region by impact ionization, (IEp/IEn) + 1 > bdc additional holes flow into the collector.
This means that carrier multiplication in C-B depletion region is internally amplified.
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Geometrical EffectsChapter 11 BJT Static Characteristics
Emitter area is not equal to collector area.Current does not flow in one
direction only.Series resistance.
Voltage drop occurs not only across the junction.
Current crowding.Due to lateral flow, current is
larger around emitter periphery than the collector periphery.
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diff
kT q
xE
xdiff : exponential decay constant
Graded BaseChapter 11 BJT Static Characteristics
Dopants are injected through diffusion.More or less falling exponential
distribution with distance into beneath of the semiconductor.
The doping within the base is not constant as assumed in ideal analysis.A function of position, having
maximum at E-B junction and minimum at C-B junction.
Creating a built-in electric field.The electric field enhances the
transport of minority carrier across the quasineutral width of the base. Increase of IE and IC.
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Due to recombination in emitter depletion region
Due to high level injection in base, base series resistance,
and current crowding
Gummel Plot
Figures of MeritChapter 11 BJT Static Characteristics
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E1 E2E1 E2 d p d p
qD qDdx dx
D D
Continuity of hole current in emitter
(1 polysilicon; 2 Si) Shallower slope less JP
higher g, b
E1 E2 E2 E2 E2
E1 E1
d p D d p d p
dx D dx dx
D D D
Polysilicon Emitter BJTChapter 11 BJT Static Characteristics
bdc is larger for a poly-Si emitter BJT as compared with an all-crystalline emitter BJT.
This is due to reduced dpE(x)/dx at the edge of the emitter depletion region.
Lower mp
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Summary on BJT PerformanceChapter 11 BJT Static Characteristics
High gain (bdc >> 1)One-sided emitter junction, so that emitter efficiency g 1Emitter doped much more heavily than base (NE >> NB).
Narrow base, so base transport factor T 1.Quasi-neutral base width << minority-carrier diffusion length
(W << LB). IC determined only by IB (IC function of VCE or VCB)
One-sided collector junction, so that quasineutral base width W does not change drastically with changes in VCE or VCB.
Base doped more heavily than collector (NB > NC), W = WB – xnEB – xnCB for pnp BJT.