4-双极晶体管

3. There is low-level injection.
4. There are no generation-combination currents in the depletion region.
5. There are no series resistance in the devices.
Carrier Profile in Active Mode
Carrier distribution in this region
To derive the current-voltage expression for an ideal transistor, we assume the following:
Field Effect Transistor (FET) is unipolar, in which only one type of carrier is important
Bipolar Transistor
In VLSI era, BJTs starts to lose their show stage due to the emergence of MOSFETs, which possess advantage of simplicity in term of process and circuit design
region are:
n x x n eqVEB kT
E
E
E0
n x x n e 0 q VCB
C
C
C0
When nE0 and nC0 are the equilibrium electron concentrations in the emitter and collector, respectively. Substituting these boundary conditions into expressions similar to Eq.1 yields
current
from
the
emitter
Bion
PNP
VEB
NPN
VBE
Forward active
Cutoff
Saturation
VCB Inverted active
Forward active
Cutoff
Saturation
Bipolar operation
Operation depends on the bias condition
IC IB
IE
§3-2 Carrier distribution
Current Flow
I
EB n
emitter
current
injected
into
the
base
I
BE p
base
Carrier Distribution in each Region
Base region
Steady-state continuity equation
D
p
d2p dx 2
n
pn pn0
p
0
where Dp and pare the diffusion constant and the
life time of minority carriers, respectively.
Inverse-active Mode Junction between E and B is in forward bias and junction between B and C is in reverse bias.
Current Gain
IE=IEp+IEn IC=ICp+ICn IB=IE-IC=IEn+(IEp-ICp)-ICn
The general solution is
pn x pn C1e x Lp C2ex Lp
Where Lp Dp p is the diffusion length of holes.
pn W 0
By the boundary conditions for the active mode:
current
injected
into
the
emitter
I
BE R
recombination
in
the
base
current
region
I
CB p
reverse
biased
current
across
the
BCJ
I
CB n
reverse
biased
current
across
the
BCJ
I
C n
electron
For apart enough that depletion regions don’t interact (no “punchthrough”)
Uniqueness of BJT: high current drivability per input capacitance fast excellent for analog and front-end communications applications.
Common base configuration
Minority carrier distributions in the base region of a p-n-p transistor. a) Active mode for VBC=0. b) Saturation mode with both junctions forward biased.
Bipolar Transistor
n+
p
n
p+
n
p
Bipolar Transistor
The ”Planar Process” developed by Fairchild in the late 50s shaped the basic structure of the BJT, even up to the present day.
pn x
p e qVEB n0
kT
1 x W
pn
01
x W
Carrier distribution
Carrier Distribution in each Region
Emitter and collector region
The boundary condition in the neutral region and collector
Electron Flow Hole Flow
Transistor Action
Saturation Mode
Both junction are in forward bias Cutoff Mode
Both junctions are in reverse bias and all currents in the transistor are zero.
Bipolar transistor
§3-1 Introduction
Bipolar Transistor
First bipolar transistor (BJT) was invented in 1948
The term bipolar came from the fact that both types of carriers, i.e., electron and hole play important roles in operation
1. The device has uniform doping in each region.
2. The hole drift current in the base region as well as the collector saturation current is negligible.
The impurity densities in the three doped regions, where the emitter is more heavily doped than the collector. However the base doping is less than the emitter doping, but greater than the collector doping.
I Cp I Ep
0 T
Collector current
IC 0 I E ICBO
CB: current measured between these two terminals O: refers to the state of the third terminal with respect to the second
Transistor Action
ACTIVE MODE In active mode, the emitter-base junction is forward biased and collector base-junction is reverse biased.
Current Flow
Forward bias Reverse bias
pn
0
p e qVEB kT n0
pn W 0
the solution can expressed
pn x
pn0
e qVEB
kT
1
sinh
W Lp
sinh
W Lp
x
pn0 1
sinh
sinh
x Lp
W Lp
When W/Lp<<1, the distribution equation can be simplified as
However, BJT’s refuse to step down because of their high current drive capability and superior analog performance (also useful in power applications)
x xE
n x n
n
e 1 e qVEB kT
LE
E
E0
E0
x xC
n x n n e LC
C
C0
C0
x x E xx C
Common base Active mode is BEJ forward biased while CBJ reverse biased Saturation occurs when CBJ is forward biased
Current trend is to combine the best of MOSFETs and Bipolar devices, which is known as BiCMOS process
BJT devices are also the preferred device for high speed (e.g. Emitter Couple Logic .ECL) and RF applications
Current Gain
Common base current gain
0
I Cp IE
0
I Cp I Ep I En
I
I Ep Ep I
En
I Cp I Ep
Emitter efficiency
I Ep I Ep
I E I Ep I En
Base transport fact
T
Modern BJT
BJT basically consists of two neighbouring pn junctions back to back:
Close enough that minority carriers interact (negligible recombination in base)
VBC Inverted active
E-B
forward reverse forward reverse
C-B reverse forward
forward reverse
Mode
active inverted saturation cutoff
An idealized p-n-p transistor in thermal equilibrium, that is ,where all there leads are connected together or all are ground.
When IE=0, the device is cutoff, IC is the reverse leakage current of the CBJ. Note that IC≠0 for VCB=0. The current is contributed by IE if the BEJ is forward biased.