A Bipolar Transistor essentially consists of a pair of PN Junction
Diodes that are joined back-to-back. This forms a sort of a sandwich
where one kind of semiconductor is placed in between two others. There
are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties.
The three layers of the sandwich are conventionally called the
Collector, Base, and Emitter. The reasons for these names will become
clear later once we see how the transistor works.
Some of the basic properties exhibited by a Bipolar
Transistor are immediately recognisable as being diode-like. However,
when the 'filling' of the sandwich is fairly thin some interesting
effects become possible that allow us to use the Transistor as an
amplifier or a switch. To see how the Bipolar Transistor works we can
concentrate on the NPN variety.
Figure 1 shows the energy levels in an NPN transistor when we aren't
externally applying any voltages. We can see that the arrangement looks
like a back-to-back pair of PN Diode junctions with a thin P-type
filling between two N-type slices of 'bread'. In each of the N-type
layers conduction can take place by the free movement of electrons in
the conduction band. In the P-type (filling) layer conduction can take
place by the movement of the free holes in the valence band. However, in
the absence of any expernally applied electric field, we find that
depletion zones form at both PN-Junctions, so no charge wants to move
from one layer to another.
Consider now what happens when we apply a moderate voltage between
the Collector and Base parts of the transistor. The polarity of the
applied voltage is chosen to increase the force pulling the N-type
electrons and P-type holes apart. (i.e. we make the Collector positive
with respect to the Base.) This widens the depletion zone between the
Collector and base and so no current will flow. In effect we have
reverse-biassed the Base-Collector diode junction. The precise value of
the Base-Collector voltage we choose doesn't really matter to what
happens provided we don't make it too big and blow up the transistor! So
for the sake of example we can imagine applying a 10 Volt Base-Collector
Now consider what happens when we apply a relatively small
Emitter-Base voltage whose polarity is designed to forward-bias the
Emitter-Base junction. This 'pushes' electrons from the Emitter into the
Base region and sets up a current flow across the Emitter-Base boundary.
Once the electrons have managed to get into the Base region they can
respond to the attractive force from the positively-biassed Collector
region. As a result the electrons which get into the Base move swiftly
towards the Collector and cross into the Collector region. Hence we see
a Emitter-Collector current whose magnitude is set by the chosen
Emitter-Base voltage we have applied. To maintain the flow through the
transistor we have to keep on putting 'fresh' electrons into the emitter
and removing the new arrivals from the Collector. Hence we see an
external current flowing in the circuit.
The precise value of the chosen Emitter-Base voltage isn't important to
our argument here, but it does determine the amount of current we'll
see. For the sake of example we've chosen a half a volt. Since the
Emitter-Base junction is a PN diode we can expect to see a current when
we apply forward voltages of this sort of size. In practice with a
Bipolar transistor made using Silicon we can expect to have to use an
Emitter-Base voltage in the range from around a half volt up to almost
one volt. Higher voltages tend to produce so much current that they can
destroy the transistor!
It is worth noting that the magnitude of the current we see isn't really
affected by the chosen Base-Collector voltage. This is because the
current is mainly set by how easy it is for electrons to get from the
Emitter into the Base region. Most (but not all!) the electrons that get
into the Base move straight on into the Collector provided the Collector
voltage is positive enough to draw them out of the Base region. That
said, some of the electrons get 'lost' on the way across the Base. This
process is illustrated in Figure 4
Some of the free electrons crossing the Base encounter a hole and
'drop into it'. As a result, the Base region loses one of its positive
charges (holes) each time this happens. If we didn't do anything about
this we'd find that the Base potential would become more negative (i.e.
'less positive' becuase of the removal of the holes) until it was
negative enough to repel any more electrons from crossing the
Emitter-Base junction. The current flow would then stop.
To prevent this happening we use the applied Emitter-Base voltage to
remove the captured electrons from the Base and maintain the number of
holes it contains. This have the overall effect that we see some of the
electrons which enter the transistor via the Emitter emerging again from
the Base rather than the Collector. For most practical Bipolar
Transistors only about 1% of the free electrons which try to cross Base
region get caught in this way. Hence we see a Base Current, IB, which is
typically around one hundred times smaller than the Emitter Current, IE
Bipolar transistors, having 2 junctions, are 3 terminal semiconductor
devices. The three terminals are emitter, collector, and base. A
transistor can be either NPN or PNP. See the schematic
Note that the direction of the emitter arrow defines the type
transistor. Biasing and power supply polarity are positive for NPN and
negative for PNP transistors. The transistor is primarily used as an
current amplifier. When a small current signal is applied to the base
terminal, it is amplified in the collector circuit. This current
amplification is referred to as HFE or beta and equals Ic/Ib.
As with all semiconductors, breakdown voltage is a design
limitation. There are breakdown voltages that must be taken into account
for each combination of terminals. i.e. Vce, Vbe,and Vcb. However,
Vce(collector-emitter voltage) with open base, designated as Vceo, is
usually of most concern and defines the maximum circuit voltage.
Also as with all semiconductors there are undesireable leakage
currents, notably Icbo ,collector junction leakage; and Iebo,
emitter junction leakage. A typical collector characteristic curve
is shown below:
Their work led them first to the point-contact transistor and then to
the bipolar junction transistor. Since then, the technology has
progressed rapidly. The development of a planar process yielded the
first circuits on a chip and for a decade, bipolar transistor
operational amplifiers and digital TTL
circuits were the workhorses of any circuit designer.
The spectacular rise of the MOSFET market share during the last
decade has completely removed the bipolar transistor from center stage.
Almost all logic circuits, microprocessor and memory chips contain
Nevertheless, bipolar transistors remain important devices for
ultra-high-speed discrete logic circuits such as emitter coupled logic (ECL),
power-switching applications and in microwave power amplifiers.
In this chapter we first present the structure of the bipolar
transistor and show how a three-layer structure with alternating n-type
and p-type regions can provide current and voltage amplification. We
then present the ideal transistor model and derive an expression for the
current gain in the forward active mode of operation. Next, we discuss
the non-ideal effects, the modulation of the base width and
recombination in the depletion region of the base-emitter junction.
Structure and principle of operation
A bipolar junction transistor consists of two back-to-back p-n
junctions, who share a thin common region with width, wB.
Contacts are made to all three regions, the two outer regions called the
emitter and collector and the middle region called the base. The
structure of an NPN bipolar transistor is shown in Figure 1 (a). The
device is called "bipolar" since its operation involves both types of
mobile carriers, electrons and holes.
||(a) Structure and sign convention of a NPN bipolar
junction transistor. (b) Electron and hole flow under forward active
bias, VBE > 0 and VBC = 0.
|Since the device consists of two back-to-back
diodes, there are depletion regions between the quasi-neutral
|The sign convention of the currents and voltage is
indicated on Fig 1(a). The base and collector current are positive
if a positive current goes into the base or collector contact. The
emitter current is positive for a current coming out of the emitter
contact. This also implies the emitter current, IE,
equals the sum of the base current, IB, and the
collector current, IC:
|The base-emitter voltage and the base-collector
voltage are positive if a positive voltage is applied to the base
contact relative to the emitter and collector respectively.
|The operation of the device is illustrated with Fig
1 (b). We consider here only the forward active bias mode of
operation, obtained by forward biasing the base-emitter junction and
reverse biasing the base-collector junction. To simplify the
discussion further, we also set VCE = 0. The
corresponding energy band diagram is shown in Fig 2. Electrons
diffuse from the emitter into the base and holes diffuse from the
base into the emitter. This carrier diffusion is identical to that
in a p-n junction. However, what is different is that the electrons
can diffuse as minority carriers through the quasi-neutral region in
the base. Once the electrons arrive at the base-collector depletion
region, they are swept through the depletion layer due to the
electric field. These electrons contribute to the collector current.
In addition, there are two more currents, the base recombination
current, indicated on Fig 2 by the vertical arrow, and the
base-emitter depletion layer recombination current (not shown).
|Figure 2. :
||Energy band diagram of a bipolar transistor biased
in the forward active mode.
|The total emitter current is the sum of the
electron diffusion current, IE,n, the hole
diffusion current, IE,p and the base-emitter
depletion layer recombination current, Ir,d.
|The total collector current is the electron
diffusion current, IE,n, minus the base
recombination current, Ir,B.
|The base current is the sum of the hole diffusion
current, IE,p, the base recombination current,
Ir,B and the base-emitter depletion layer
recombination current, Ir,d.
|The transport factor, a,
is defined as the ratio of the collector and emitter current:
|Using Kirchoff's current law and the sign
convention shown in Figure 1(a), we find that the base current
equals the difference between the emitter and collector current. The
current gain, b, is defined as the ratio
of the collector and base current and equals:
|This explains how a bipolar junction transistor can
provide current amplification. If the collector current is almost
equal to the emitter current, the transport factor,
a, approaches one. The current gain,
b, can therefore become much larger than
|To facilitate further analysis, we now rewrite the
transport factor, a, as the product of
the emitter efficiency, gE,
the base transport factor, aT,
and the depletion layer recombination factor, dr.
|The emitter efficiency, gE,
is defined as the ratio of the electron current in the emitter, IE,n,
to the sum of the electron and hole current diffusing across the
base-emitter junction, IE,n + IE,p.
|The base transport factor, aT,
equals the ratio of the current due to electrons injected in the
collector, to the current due to electrons injected in the base.
|Recombination in the depletion-region of the
base-emitter junction further reduces the current gain, as it
increases the emitter current without increasing the collector
current. The depletion layer recombination factor,
dr, equals the ratio of the
current due to electron and hole diffusion across the base-emitter
junction to the total emitter current:
|The forward active mode is obtained by
forward-biasing the base-emitter junction. In addition we eliminate
the base-collector junction current by setting VBC
= 0. The minority-carrier distribution in the quasi-neutral regions
of the bipolar transistor, as shown in Figure 3, is used to analyze
this situation in more detail.
|Figure 3. :
||Minority-carrier distribution in the quasi-neutral
regions of a bipolar transistor (a) Forward active bias mode. (b)
|The values of the minority carrier densities at the
edges of the depletion regions are indicated on the Fig 3. The
carrier densities vary linearly between the boundary values as
expected when using the assumption that no significant recombination
takes place in the quasi-neutral regions. The minority carrier
densities on both sides of the base-collector depletion region equal
the thermal equilibrium values since VBC was set
to zero. While this boundary condition is mathematically equivalent
to that of an ideal contact, there is an important difference. The
minority carriers arriving at x = wB - xp,C
do not recombine. Instead, they drift through the base-collector
depletion region and end up as majority carriers in the collector
|The emitter current due to electrons and holes are
obtained using the "short" diode expressions yielding:
|It is convenient to rewrite the emitter current due
to electrons, IE,n, as a function of the total
excess minority charge in the base, DQn,B.
This charge is proportional to the triangular area in the
quasi-neutral base as shown in Fig 3 a) and is calculated from:
|which for a "short" diode becomes:
|And the emitter current due to electrons, IE,n,
|where tr is the average time the
minority carriers spend in the base layer, i.e. the transit time.
The emitter current therefore equals the excess minority carrier
charge present in the base region, divided by the time this charge
spends in the base.
|A combination of equations (11), (14) and (15)
yields the transit time as a function of the quasi-neutral layer
width, wB', and the electron diffusion
constant in the base, Dn,B.
|We now turn our attention to the recombination
current in the quasi-neutral base and obtain it from the continuity
|In steady state and applied to the quasi-neutral
region in the base, the continuity equation yields the base
recombination current, Ir,B:
|which in turn can be written as a function of the
excess minority carrier charge, DQn,B,
using equation (13).
|The long minority-carrier lifetime and the long
diffusion lengths in those materials justify the exclusion of
recombination in the base or the depletion layer. The resulting
current gain, under such conditions, is:
|From this equation, we conclude that the current
gain can be larger than one if the emitter doping is much larger
than the base doping. A typical current gain for a silicon bipolar
transistor is 50 - 150.
|The base transport factor, as defined in equation
|This expression is only valid if the base transport
factor is very close to one, since it was derived using the
"short-diode" carrier distribution. This base transport factor can
also be expressed in function of the diffusion length in the base:
|As the voltages applied to the base-emitter and
base-collector junctions are changed, the depletion layer widths and
the quasi-neutral regions vary as well. This causes the collector
current to vary with the collector-emitter voltage as illustrated in
|Figure 4. :
||Variation of the minority-carrier distribution in
the base quasi-neutral region due to a variation of the
|A variation of the base-collector voltage results
in a variation of the quasi-neutral width in the base. The gradient
of the minority-carrier density in the base therefore changes,
yielding an increased collector current as the collector-base
current is increased. This effect is referred to as the Early
effect. The Early effect is observed as an increase in the collector
current with increasing collector-emitter voltage as illustrated
with Figure 5. The Early voltage, VA, is obtained
by drawing a line tangential to the transistor I-V
characteristic at the point of interest. The Early voltage equals
the horizontal distance between the point chosen on the I-V
characteristics and the intersection between the tangential line and
the horizontal axis. It is indicated on the figure by the horizontal
|Figure 5. :
||Collector current increase with an increase of the
collector-emitter voltage due to the Early effect. The Early
voltage, VA, is also indicated on the figure.
||Now, to the heart of the matter!
We have an operating curve consisting of a fairly linear segment
bounded by two nonlinear ends: cutoff and saturation.
Operating in the Middle
The transistor will operate very nicely if one could insure that
no input voltage, i.e., signal voltage--would cause the
collector current to ever operate beyond either end of the
linear portion of the operating curve.
|To further beat a point into the ground: if one
increased the input signal beyond this level, the output signal
would now start to "clip" and cause distortion (sine wave gets flat
on top and bottom). If the bias point were set either too low or too
high, then the sine wave would start to clip on the top before the
bottom, or visa versa (asymmetric clipping).
Effects of different bias settings
Effects of different bias settings
||When (negative) feedback is introduced, most of
these problems diminish or disappear, resulting in improved
performance and reliability. There are several ways to introduce
feedback to this simple amplifier, the easiest and most reliable of
which is accomplished by introducing a small value resistor in the
emitter circuit. The amount of feedback is dependent on the relative
signal level dropped across this resistor, e.g., if the resistor
value approached that of the collector load resistor, the gain would
approach unity (Gv ~ 1).
From the explanation of how a Bipolar Transistor works, we can expect
the main characteristic of a Bipolar Transistor to be its Current Gain
value. In practice this value isn't a 'universal constant' but depends
on various factors: e.g. the transistor's temperature, the size and
shape of its Base region, the way it's various parts were doped to make
them into semiconductors, etc.
The above illustration shows how, for a 'typical'
transistor, the Current Gain varies with the Collector Current level,
IC. from this graph we can see that the proportion of electrons 'caught'
by a hole whilst trying to cross the Base region does vary a bit
depending on the current level. Note that the graph doesn't show the
transistor's beta value, it shows a related figure called the
transistor's Small Signal current gain, hfe. This is similar to the beta
value, but is defined in terms of small changes in the current levels.
This parameter is more useful than the beta value when considering the
transistor's use in signal amplifiers where we're interested in how the
device responds to changes in the applied voltages and currents.
The second way we can characterise the behavior of a Bipolar
Transistor is by relating the Base-Emitter voltage, VBE, we apply to the
Base current, IB, it produces. As can expect from the diode-like nature
of the Base-Emitter junction this voltage/current characteristic curve
has an exponential-like shape similar to that of a normal PN Junction
As with the previous curve, the graph shown here should only be regarded
as a 'typical' example as the precise result will vary a bit from device
to device and with the temperature, etc.
In most practical situations we can expect the Collector current to be
set almost entirely by the chosen Base-Emitter voltage. However, this is
only true when the the Base-Collector voltage we are applying is 'big
enough' to quickly draw over to the Collector any free electrons which
enter the Base region from the Emitter.
The above plot of characteristic curves gives a more complete picture
of what we can expect from a working Bipolar Transistor. Each curve
shows how the colletor current, IC, varies with the Collector-Emitter
voltage, VCE, for a specific fixed value of the Base current, IB. This
kind of characteristic curve 'family' is one of the most useful ones
when it comes to building amplifiers, etc, using Bipolar Transistors as
it contains quite a lot of detailed information.
When the applied VCE level is 'large enough' (typically above two or
three volts, shown as the region in blue) the Collector is able to to
remove free electrons from the Base almost as quickly as they Emitter
injects them. Hence we get a current which is set by the Base-Emitter
voltage and see a current gain value which doesn't alter very much if we
change either the base current or the applied Collector potential.
However, when we reduce the Collector potential so that VCE is less than
a couple of volts, we find that it is no longer able to efficiently
remove electrons from the Base. This produces a sort of partial
'roadblock' effect where free electrons tend to hang about in the Base
region. (cream colored region) These makes the Base region seem 'more
negative' to any electrons in the Emitter and tends to reduce the
overall flow of current through the device. As we lower the Collector
potential to become almost the same as that of the Base and Emitter it
eventually stops drawing any electrons out of the device and the
Collector current falls towards zero.
The precise voltage at which the Collector ceases to be an effective
'collector of electrons' depends on the temperature and the
manufacturing details of the transistor. In general we can expect most
Bipolar Transistors to work efficiently provided that we arrange for a
VCE value of at least two or three volts - and preferrably five volts or
more. Such a device can be used as an effective amplifier. Lower
voltages may prevent it from working correctly.
Note that the graphs shown on this page are only meant as a general
guide. Some transistors can work with much higher currents, or have much
higher current gains, etc. However, the general pattern of behavior of
all Bipolars is essentially the same as described in these pages.
The unijunction transistor (UJT) is a three terminal device with
characteristics very different from the conventional 2 junction, bipolar
transistor. It is a pulse generator with the trigger or control signal
applied at the emitter . This trigger voltage is a fraction (n) of
interbase voltage, Vbb.The UJT circuit symbol, junction schematic, and
characteristic curve are shown below.
The emitter terminal does not inject current into the base
region until its voltage reaches Vp. Once Vp is reached the base circuit
conducts and a positive pulse appears at the B1 terminal and a negative
pulse at B2. The UJT incorporates a negative resistance region, a low
emitter current, and a high output pulse current at terminals B1 and B2,
making it an ideal pulse trigger. A simple RC timer circuit using a UJT
is shown below.