bjt band diagram analysis تجزيه وتحليل دياگرام باند انرژي
TRANSCRIPT
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BJT Band diagram
Analysisتجزيه وتحليل دياگرام
باند انرژي
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W
PN
The “typical” electron travels into the p-type region a distancegiven by DL
D is the diffusion coefficient (technically, the minority carrier diffusion coefficient)t is the lifetimetime (technically, the minority carrier lifetime)
W
The diffusion length L can easily by ~100 x times larger than the depletion width W
Diffusion (injection)
Recombination(excess e- combinewith holes)
How far does an electron go into the p-type region before it finds a “hole” to recombine with?
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Transistors (Transfer Resistor)
Transistors
Junction-FETs (JFETS)
Field Effect TransistorsBipolar transistors
Insulated Gate FET’s
MOSFETs
NPN,PNP
N-channel, P-channel
Enhancement, DepletionN-channel, P-channel
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The bipolar junction transistor (BJT)
NP
P
B
C
E
C
B
E
PN
N
B
C
E
B
C
E
B
C
E
C
B
ENPN
PNP
Arrow always points away from base and toward emitterMy pneumonic: No Point iN
Arrow always points away from emitter towards baseMy pneumonic: Points IN
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Diffusion
Drift
Diffusion
Drift
At equilibrium:
BaseCollector Emitter
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CB
E
+
-
+
-
PN N
+
-+
-} }
This junction isreverse-biased
This junction isforward-biased
BC E
“Quasi”-Fermi level
Since we are not at thermodynamic equilibrium, we cannotdefine a single chemical potential (Fermi level) is everywhere
A Quasi – fermi-level can be used to describe the local equilibrium ofelectrons and holes
In Use: Forward bias one p-n junction, and reverse-bias the other
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PN N
+
-
+ -
BC E--
++
W (Width of depletion region)
LDiffusion
Physical thickness of base
There are three important length scales that are relevant to understandinghow a transistor operates:
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DiffusionDrift
PN N
+
-
+
-
} }
This junction isreverse-biased
This junction isforward-biased
BC E
Basis of bipolar transistor operation: 1) The Base-emitter junction is forward-biased: Electrons flow from the emitter
to the base, just like in a normal forward-biased diode
2) Because the base is very thin, electrons continue to move through the baseand find themselves at the collector-base junction. Once they ‘feel’ the large electricfield at this junction, they are pushed downhill to the collector. Only a very smallfraction (typically ~ 1% - 3%) of the electrons come out through the base; theremaining 97%-99% come out through the collector.
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When base is made very thin, IC>>IB--
++
VC
VB
VEIBIB
IE
VBEVCB
PN N
+
-
+ -
BC E
VC
IB
VE
IE
IB
When base is made very thin, IC>>IBand IC~IE
Bipolar transistor can be considered a current amplifier: If one can control thebase current, then this will induce a much larger change in the current in the collector and in the emitter.
a=IC/IE 1-a=IB/IE
B
C
I
I
1is the current gain of a transistor. b is commonly ~30-100
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--
++
VC
VB
VEIBIB
IE
VBEVCB
If VCB constant, then as VBE is increased, current IC and IB increase exponentially
VBE
IC~IE
B
+-
+
-DVB
Small wiggle in VB, DVB, induces large change in IC. By Ohm’s Law, the voltage across RC shows a big change. So,Small DVB Big DVRE
Bipolar transistor as a voltage amplifier = Transistor + resistor(s)
VRE
Collector resistor
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Field-effect Transistors
Main differences from bipolar transistors:
1) Use an electric field, established by applying a voltage to a “gate” electrode, to control current flow (voltage in Voltage out)
2) Ideally, no current flow at all into the “gate” electrode. Important: No current implies no power dissipation, at least under
certain conditions Two fundamentally different types:
1) Junction FET (J-FET)
2) IGFET (insulated-gate FET)
The MOSFET (Metal-oxide-semiconductor FET) is the most common type
Relies on a reverse-biased PN junction to prevent current flow in the gate
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n
p
p
Depletion region
Depletion region
e- e- e- e- e-
+-
Source (S) Drain (D)
Gate (G)
Gate (G)
Gate forms a diode (p-n) junction with source and drain
JFET is always operated under conditions where this diode junctionis reverse-biased, so that only very little current flows from the gateto the source or the drain
N-channel JFET
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Depletion region is larger on the right-hand side because thegreen region is more positive on the right than on the left (due to VSD),so the Gate-Drain junction is reverse-biased more strongly than thegate-source junction is.
np
pDepletion region
Depletion region
e- e- e- e-
+-
S D
G
G
+
VSD
VGS
connection so both gate electrodes have the same voltage-
np
pe- e- e- e-
+-
S D
G
G
+
VSD
VGS
-
“Pinch-off”Larger (more negative) VGS
Small, negative VGS
n-channel
SYMBOL:
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V”pinch-off”
IDS
V”pinch-off”
Larger (more positive) VDS
VDS
IDS
Purely resistive here (silicon actslike a resistor)
Current goes up less quicklyas depetion region narrows
Once pinch-off occurs, nofurther increase i n current
pinch-off
VDS
IDS
pinch-off
VGS~0
VGS~ -1.0 V
VGS~ -2.0 V
VGSn-channel
SYMBOL:
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Everywhere switch N, PSwitch signs of all voltage sources and currents
p
n
n
Depletion region
Depletion region
+ -
Source (S) Drain (D)
Gate (G)
Gate (G)
h+ h+ h+ h+ h+
pn
nh+
+-
S D
G
G
+
VSD
VGS
- “Pinch-off”
VDS
-IDS
pinch-off
VGS~0
VGS~ +1.0 V
VGS~ +2.0 V
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IGFET (Insulated-gate FET)
InsulatorMetal (G)
SiO2
Metal
S D
Semiconductor
CB
VB
Gate (G)
Body
p-Silicon
S D
n-Si n-Si
4 terminals: Source, Drain, Gate, and “Body” (sometimes called “Substrate”)
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SiO2
Metal
Gate (G)
Body
p-Silicon
S D
n-Si n-Si
diode-like junctionhere (and similarly at drain)
For VG<0, the p-type silicon is in depletion or possibly accumulation. It formsresistive p-n junctions with the source and drain.
For VG>0, the p-type silicon goes into depletion.
When VG is large and positive, enough electrons are attracted to the near-surfaceregion that the region right under the SiO2 becomes inverted, and electrons canfrom from the source to the drain.
SiO2
Metal
Gate (G)
Body
p-Silicon
S D
n-Si n-Si
Inverted region
With VG=0 With VG>0
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CB
VB
CB
VB
Depletion
CB
VB
Inversion
CB
Accumulation
VB
Small negative gate(for n-type sample)Surface becomes resistive, but electronsstill majority carrier
Large negative gate(for n-type sample)Surface becomesp-type, as holes become majority carrierat surface
positive gate(for n-type sample)Surface remains n-type, but becomes more conductive
“flat-band” condition
As a function of gate voltage, three different characteristic behaviors:
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CB
VBe-
CB
VB
If metal has smaller work function, then when connected by a wire,Electrons move from metal to semiconductor, making semiconductor Negaitve and metal positive until their Fermi levels line up
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CB
VB
CB
VB
Depletion
CB
VB
Inversion
CB
Accumulation
VB
Small positive gate(for p-type sample)Surface becomes resistive, but holesstill majority carrier
Large positive gate (for p-type sample) surface becomes n-type, as electrons become majority carrier at surface
negative gate(for p-type sample)Surface remains p-type, but becomes more conductive
“flat-band” condition
As a function of gate voltage, three different characteristic behaviors:
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CB
VB
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SiO2
Metal
Gate (G)
Body
p-Silicon
S D
n-Si n-Si
diode-like junctionhere (and similarly at drain)
Body is always held at potential of drain or possibly biased more negatively(to reverse bias the S and D junctions to the p-type Body)
Applying a voltage to the gate controls whether the near-surface regionis in accumulation, depletion, or inversion
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SiO2
Metal
Gate (G)
Body
p-Silicon
S D
n-Si n-Si
diode-like junctionhere (and similarly at drain)
For VG<0, the p-type silicon is in depletion or possibly accumulation. It formsresistive p-n junctions with the source and drain.
For VG>0, the p-type silicon goes into depletion.
When VG is large and positive, enough electrons are attracted to the near-surfaceregion that the region right under the SiO2 becomes inverted, and electrons canfrom from the source to the drain.
SiO2
Metal
Gate (G)
Body
p-Silicon
S D
n-Si n-Si
Inverted region
With VG=0 With VG>0
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