1
Construction and Mechanism of Operation
1.1 Construction of BJT (n-p-n)
Basically, two p-n junctions are formed back-to-back. Firstly, a lightly
doped n-region, designated n-, is diffused into the substrate to form
the collector region. This is followed by a moderately doped, very thin
p-region to form the base. Finally, this is topped with a heavily doped
n-type region, designated n+, which forms the emitter region and is
plentiful in free charge carriers (see Fig. 1.1). The base region is very
thin, of the order of 0.5 - 1.0µ
µm which makes its width much less than
the diffusion length for minority carriers in this region, i.e. Wb << Lb.
A physical model can be made of the BJT which treats it as two p-n
junctions back-to-back as shown in Fig. 1.1. At equilibrium, there are
two depletion regions formed at the junctions which are neglected in
first order approximation calculations.
Typically:
Majority carrier emitter electron concentration:
Majority carrier base hole concentration:
Majority carrier collector electron concentration:
neo ≈ Ne ≈ 1018 cm-3
pbo ≈ Nb ≈ 1016 cm-3
nco ≈ Nc ≈1015 cm-3
Minority carrier emitter hole concentration: peo ≈ ni2/Ne ≈ 2.25x102 cm-3
Minority carrier base electron concentration: nbo ≈ ni2/Nb≈2.25x104 cm-3
Minority carrier collector hole concentration: pco ≈ ni2/Nc≈2.25x105 cm-3
The electrical circuit symbol for the n-p-n bipolar junction transistor is
also shown in Fig. 1.1.
1.2 Operation of n-p-n BJT in Forward Active Mode
The bipolar junction transistor may, in fact, be operated with its baseemitter and base-collector junctions biased in either direction.
However, the most common form of bias is that of the Forward Active
Mode in which the base-emitter junction is forward biased and the
base-collector junction is reverse biased. This is the normal mode of
operation of the BJT as an amplifying device (see Fig. 1.2).
1
E
Physical
Construction
Emitter
n+
B
C
Base
p
n+
Collector
nwidth < 1µm
Substrate
Physical
Model
Depletion
regions
Collector
n+
Base
p
E
B
C
Emitter
IE
n-
WB
0
IB
C
IC
Electrical
Symbol
B
IB
IE
E
Fig.1.1
Construction of an n-p-n Bipolar Transistor
2
IC
With the base-emitter junction forward biased, an electric field is
created across this junction, which lowers its potential barrier. This
allows electrons to drift across the junction from the emitter and
diffuse into the base region. Holes also drift across the junction from
the base and diffuse into the emitter region. Since the free-electron
concentration in the emitter is far higher than that of holes in the base
by virtue of the doping (Ne >> Nb), then most of the current flow
through the base-emitter junction is due to electrons in the case of the
n-p-n transistor.
With the base-collector junction reverse biased, an electric field is
created across this junction which raises its potential barrier. Due to
the thermal generation of free carriers in the collector and the base
regions, there is a small hole current from collector to base and a small
electron current from base to collector. If the base-collector junction
was in isolation, these currents combined would form the reverse
saturation current for this junction. However, due to the very narrow
width of the base region and the high electric field intensity across the
collector-base junction, electrons “emitted” from the emitter region
diffuse right across the base region where they meet the influence of
the reverse biased collector field and are then swept across the basecollector junction to be “collected” in the collector region. A small
fraction, typically of the order of 1%, of the electrons injected from
the emitter into the base actually recombine with holes in the base and
form the primary component of base current.
In actual fact, the concentration of charge carriers in the base region
has a profound influence on the collector current and this can be
controlled by varying the external current supplied to the base. Small
changes in the base current can be used to effect large changes in the
collector current and it is this feature that gives the transistor its
amplifying property.
3
Physical
Model
V BE
IC
IE
V BC
+
+
Base-Emitter
IE Forward Biased
IB Base-Collector
IC
Reverse Biased
h+
h+
e-
e-
n+
E
Band
Diagram
p
B
E
B
E CB
E CE
E FE
n-
C
C
E FB
E VB
E CC
E FC
E VE
E VC
Minority
Carrier
Distribution
p co
nb
n bo
pe
p eo
Fig. 1.2
pc
Linear
approximation
Including
recombination
Characteristics of BJT Biased in Forward Active Mode
4
1.3 Current Components
Note that several components of current flow in each region of the
transistor as shown in Fig. 1.2. However, the principal components are
the electronic currents in the emitter and the collector regions and the
hole current in the base region which contributes to recombination.
Important points to note are:
i) The bias voltages applied to the device are almost fully developed
across the depletion regions and little across the neutral regions.
Hence, currents flow as drift currents across the junctions and
diffusion currents through the neutral regions.
ii) There is little recombination of carriers in the depletion region and
this can be neglected.
iii) Under steady-state bias conditions, the normal laws govern
diffusion in the neutral regions and, hence, the minority carrier
concentrations in these regions follow exponential profiles.
iv) The minority carrier concentration profile in the base region can be
approximated as linear due to the very narrow dimensions of this
region. Recombination of holes in the base with some of the
electrons in transit from emitter to collector accounts for
the
departure from linearity in reality. However, the approximation of a
linear profile greatly simplifies analyses of current flow.
1.4 Minority Carrier Profile
The majority carrier concentrations in each region of the transistor are
little altered by bias conditions. The minority carrier concentrations,
however, are significantly altered and it is these that control the
operation of the device. The minority carrier profiles for the bipolar
transistor operating in forward active mode are shown in Fig. 1.2.
The forward bias on the emitter-base junction leads to an increase in
the minority carriers on each side of the junction above the
equilibrium levels by a factor of eVBE/VT. It can be seen that there is an
exponential profile extending into the emitter region. The reverse bias
on the collector-base junction forces minority carriers away from the
junction so that the carrier profiles are reduced to near zero at these
boundaries. There is, again, an exponential profile extending into the
collector region. In the base region the profile is in reality non-linear
but to a good first-order approximation can be treated as linear and a
correction can be made for the non-linearity which is due to
recombination when analyzing current flow.
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Notation Applying to an n-p-n Bipolar Junction Transistor
Ne: doping concentration in emitter (donor atoms).
atoms/cm3
Nb: doping concentration in base (acceptor atoms).
atoms/cm3
Nc: doping concentration in collector (donor atoms).
atoms/cm3
peo: equilibrium minority carrier concentration emitter peo=ni2/Ne cm-3
nbo: equilibrium minority carrier concentration in base nbo=ni2/Nb cm-3
pco: equilibrium minority carrier concentration collector pco=ni2/Nc cm-3
De: diffusion coefficient for minority carriers in emitter
cm2/s
Db: diffusion coefficient for minority carriers in base
cm2/s
Dc: diffusion coefficient for minority carriers in collector
cm2/s
Le: diffusion length for minority carriers in emitter
µm or cm
Lb: diffusion length for minority carriers in base
µm or cm
Lc: diffusion length for minority carriers in collector
µm or cm
Wb: width of base region excluding depletion regions
µm or cm
τb: minority carrier lifetime in the base before recombination
ns
τF: forward transit time for minority carrier to cross the base region ns
6