UNIT I
BJT AND SEMICONDUCTORS
PART A
1.What is barrier potential ?
When a PN junction is formed , carries from p type and n type materials diffuses across the
junction. This produces a electric field of strength 0.7 Volts. It is called as barrier potential.
2. What is Thermal runaway?
When the collector current increases, the temperature of the BJT also increases. The increase
in temperature once again increases the collector current. This scenario is called as Thermal
runaway.
3.What is avalanche breakdown?
During the reverse biased condition in PN junction diode, the minority carries will bombard with
the atoms and produces free electrons. This free electron in turn bombards with another atom and
produces huge amount of free electrons. It is called as avalanche breakdown.
4.What is zener Breakdown?
In zener diode during reverse biased condition, The heavy doping produces a heavy electric field,
which is capable of pulling electrons near by the junction. It is called as zener breakdown.
5.What is tunnelling?
The electrons with low energy will penetrate the potential barrier to reach the other side .This
process is called as tunnelling,
6.What are the salient features of BJT ?
It can be operated as a switch and as an amplifier.
BJT is current controlled device.
7.What are the configurations available in BJT?
Common base
Common collector
Common emitter
8. Why biasing is necessary in BJT ?
The transistor, without biasing might oscillate between saturation and cut off. In order to keep
the transistor in active region we go for biasing.
9. What is doping?
The addition of impurities to a intrinsic semiconductor to make it conduct is called as doping.
10.What are the current components in PN junction diode ?
P side- majority carriers- holes; minority carriers- electrons.
N side- majority carriers- electrons; minority carriers- holes.
PART B
1. With a neat graph, explain VI characteristics of PN junction diode
The Junction Diode
A diode is one of the simplest semiconductor devices, which has the characteristic of passing
current in one direction only. However, unlike a resistor, a diode does not behave linearly with
respect to the applied voltage as the diode has an exponential I-V relationship and therefore we
can not described its operation by simply using an equation such as Ohm's law.
If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it
can supply free electrons and holes with the extra energy they require to cross the junction as the
width of the depletion layer around the PN junction is decreased. By applying a negative voltage
(reverse bias) results in the free charges being pulled away from the junction resulting in the
depletion layer width being increased. This has the effect of increasing or decreasing the
effective resistance of the junction itself allowing or blocking current flow through the diode.
Then the depletion layer widens with an increase in the application of a reverse voltage and
narrows with an increase in the application of a forward voltage. This is due to the differences in
the electrical properties on the two sides of the PN junction resulting in physical changes taking
place. One of the results produces rectification as seen in the PN junction diodes static I-V
(current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when
the polarity of bias voltage is altered as shown below.
Junction Diode Symbol and Static I-V Characteristics.
But before we can use the PN junction as a practical device or as a rectifying device we need to
firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above,
"Reverse Bias" refers to an external voltage potential which increases the potential barrier. An
external voltage which decreases the potential barrier is said to act in the "Forward Bias"
direction.
There are two operating regions and three possible "biasing" conditions for the standard
Junction Diode and these are:
1. Zero Bias - No external voltage potential is applied to the PN-junction.
2. Reverse Bias - The voltage potential is connected negative, (-ve) to the P-type material
and positive, (+ve) to the N-type material across the diode which has the effect of
Increasing the PN-junction width.
3. Forward Bias - The voltage potential is connected positive, (+ve) to the P-type material
and
negative, (-ve) to the N-type material across the diode which has the effect of
Decreasing
the
PN-junction width.
Zero Biased Junction Diode
When a diode is connected in a Zero Bias condition, no external potential energy is applied to
the PN junction. However if the diodes terminals are shorted together, a few holes (majority
carriers) in the P-type material with enough energy to overcome the potential barrier will move
across the junction against this barrier potential. This is known as the "Forward Current" and is
referenced as IF
Likewise, holes generated in the N-type material (minority carriers), find this situation
favourable and move across the junction in the opposite direction. This is known as the "Reverse
Current" and is referenced as IR. This transfer of electrons and holes back and forth across the
PN junction is known as diffusion, as shown below.
Zero Biased Junction Diode
The potential barrier that now exists discourages the diffusion of any more majority carriers
across the junction. However, the potential barrier helps minority carriers (few free electrons in
the P-region and few holes in the N-region) to drift across the junction. Then an "Equilibrium" or
balance will be established when the majority carriers are equal and both moving in opposite
directions, so that the net result is zero current flowing in the circuit. When this occurs the
junction is said to be in a state of "Dynamic Equilibrium".
The minority carriers are constantly generated due to thermal energy so this state of equilibrium
can be broken by raising the temperature of the PN junction causing an increase in the generation
of minority carriers, thereby resulting in an increase in leakage current but an electric current
cannot flow since no circuit has been connected to the PN junction.
Reverse Biased Junction Diode
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the Ntype material and a negative voltage is applied to the P-type material. The positive voltage
applied to the N-type material attracts electrons towards the positive electrode and away from the
junction, while the holes in the P-type end are also attracted away from the junction towards the
negative electrode.
The net result is that the depletion layer grows wider due to a lack of electrons and holes and
presents a high impedance path, almost an insulator. The result is that a high potential barrier is
created thus preventing current from flowing through the semiconductor material.
Reverse Biased Junction Diode showing an Increase in the Depletion Layer
This condition represents a high resistance value to the PN junction and practically zero current
flows through the junction diode with an increase in bias voltage. However, a very small leakage
current does flow through the junction which can be measured in microamperes, (μA). One final
point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough
value, it will cause the PN junction to overheat and fail due to the avalanche effect around the
junction. This may cause the diode to become shorted and will result in the flow of maximum
circuit current, and this shown as a step downward slope in the reverse static characteristics
curve below.
Reverse Characteristics Curve for a Junction Diode
Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a
series limiting resistor is used with the diode to limit this reverse breakdown current to a preset
maximum value thereby producing a fixed voltage output across the diode. These types of diodes
are commonly known as Zener Diodes and are discussed in a later tutorial.
Forward Biased Junction Diode
When a diode is connected in a Forward Bias condition, a negative voltage is applied to the Ntype material and a positive voltage is applied to the P-type material. If this external voltage
becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts
for germanium, the potential barriers opposition will be overcome and current will start to flow.
This is because the negative voltage pushes or repels electrons towards the junction giving them
the energy to cross over and combine with the holes being pushed in the opposite direction
towards the junction by the positive voltage. This results in a characteristics curve of zero current
flowing up to this voltage point, called the "knee" on the static curves and then a high current
flow through the diode with little increase in the external voltage as shown below.
Forward Characteristics Curve for a Junction Diode
The application of a forward biasing voltage on the junction diode results in the depletion layer
becoming very thin and narrow which represents a low impedance path through the junction
thereby allowing high currents to flow. The point at which this sudden increase in current takes
place is represented on the static I-V characteristics curve above as the "knee" point.
Forward Biased Junction Diode showing a Reduction in the Depletion Layer
This condition represents the low resistance path through the PN junction allowing very large
currents to flow through the diode with only a small increase in bias voltage. The actual potential
difference across the junction or diode is kept constant by the action of the depletion layer at
approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.
Since the diode can conduct "infinite" current above this knee point as it effectively becomes a
short circuit, therefore resistors are used in series with the diode to limit its current flow.
Exceeding its maximum forward current specification causes the device to dissipate more power
in the form of heat than it was designed for resulting in a very quick failure of the device.
2. Explain the characteristics of Tunnel diode with a neat sketch
A Tunnel Diode is s pn junction that exhibits negative resistance between two values of
forward voltage
Theory
The tunnel diode s basically a pn junction with heavy doping of p type and n type
semiconductor materials .tunnel diode is doped 1000 times as heavily as a conventional diode
Heavy doping results in large no of majority carriers. Because this large no of carriers, most
are not used during initial recombination that produces depletion layer. It is very narrow.
Depletion layer of tunnel diode is 100 times narrower. Operation of tunnel diode depend son
the tunneling effect
TUNNELING
The movement of valence electrons from the valence energy band to the conduction band
with little or no applied forward voltage is called tunneling.
VI CHARACTERISTICS
As the forward voltage is first increased, the tunnel diode is increased from zero, electrons
from the n region tunnel through the potential barrier to the potential barrier to the p region.
As the forward voltage increases the diode current also increases until the peak to peak is
reached. Ip = 2.2 mA. Peak point voltage =0.07V
As the voltage is increased beyond Vp the tunneling action starts decreasing and the diode
current decreases as the forward voltage is increased until valley point V is reached at valley
point voltage Vv= 0.7V between Vand P the diode exhibits negative resistance i.e., as the
forward bias is increased , the current decreases. When operated in the negative region used
as oscillator.
3. Explain the transistor operation and the collector characteristics?
Input characteristics
IB (Base Current) is the input current, VBE (Base – Emitter Voltage) is the input voltage for CE
(Common Emitter) mode. So, the input characteristics for CE mode will be the relation between
IB and VBE with VCE as parameter. The characteristics are shown below
Common Emitter (CE) mode input characteristics of BJT
The typical CE input characteristics are similar to that of a forward biased of p – n diode. But as
VCB increases the base width decreases
.
Output characteristics
Output characteristics for CE mode is the curve or graph between collector current (I C) and
collector – emitter voltage (VCE) when the base current IB is the parameter. The characteristics is
shown below in the figure.
Common Emitter Output Characteristics of p – n – p transistor
Like the output characteristics of common – base transistor CE mode has also three regions
named (i) Active region, (ii) cut-off regions, (iii) saturation region. The active region has
collector region reverse biased and the emitter junction forward biased. For cut-off region the
emitter junction is slightly reverse biased and the collector current is not totally cut-off. And
finally for saturation region both the collector and the emitter junction are forward biased.
Application of BJT
BJT’s are used in discrete circuit designed due to availability of many types, and obviously
because of its high transconductane and output resistance which is better than MOSFET. BJT’s
are suitable for high frequency application also. That’s why they are used in radio frequency for
wireless systems. Another application of BJT can be stated as small signal amplifier, metal
proximity photocell, etc.
4. Explain the operation of BJT as switch and Amplifier
The basis for the amplifier application is the fact that when the BJT is operated in the active
mode, it acts as a voltage-controlled current source: Changes in the base-emitter voltage
give rise to changes in the collector current
used
to
implement
. Thus, in the active mode, the BJT can be
a
transconductance
amplifier.
Voltage amplification can be obtained by simply by passing the collector current through a
resistance
Large-Signal Operation-The Transfer Characteristic
Fig-5.26 shows the basic structure of the most commonly used BJT amplifier, the commonemitter (CE) circuit.
The total input voltage is applied between base and emitter:
The total output voltage is taken between collector and emitter:
Resistor
has two functions: to establish a DC bias voltage at the collector and to convert the
collector signal current
The supply voltage
to an output voltage
or
is needed to bias the BJT and supply the power for the operation of the
amplifier.
To understand the characteristic shown in Fig-5.26b, we can express
as:
Since
, the BJT will be cutoff for
For
, the BJT begins to conduct and
However, since initially
. Thus:
increases and from *5.50*,
decreases.
will be large, the BJT will be operating in the active mode (segment
YZ of the voltage transfer curve).
The equation for this segment is obtained by substituting in *5.50* the active-mode expression
for
, and neglecting the Early effect:
Active-mode operation ends when the collector voltage (
the base (
or
or
) falls by ~0.4 V below that of
.
At this point, the CBJ turns on and the BJT enters the saturation region.
In the saturation region:
It is the almost constant
that gives this region of BJT operation the name saturation.
The collector current will remain nearly constant:
Amplifier Gain
To operate the BJT as a linear amplifier, it must be biased at a point in the active region. Fig5.26b shows such a bias point, labeled Q (quiescent point)
This point is characterized by DC voltages
The signal to be amplified,
and by collector current:
, is superimposed on
and kept sufficiently small, the
instantaneous operating point will be constrained to a short, almost-linear segment of the transfer
curve around the bias point Q.
The slope of this segment is equal to the slope of the tangent to the transfer curve at Q. This
slope is the voltage gain
Note
of the amplifier for small-input signals around Q.
that
the
CE
amplifier
is
inverting.
*5.56* shows that the voltage gain of the CE amplifier is the ratio of the DC voltage drop across
to the thermal voltage
(25 mV at room temperature). Thus, to maximize the voltage gain,
we should use as large a voltage drop across
For a given value of
However, a lower
as possible.
, *5.57* shows that to increase
we have to operate at a lower
.
brings the bias point Q close to the end of the active-region segment,
which might not leave sufficient room for the negative-output signal swing without the amplifier
entering the saturation region. If this happens, the negative peaks of the waveform
will be
flattened.
Placing Q too high on the segment not only reduces the gain, since
is lower, but could limit
the range of positive signal swing as the BJT will be cutting off which results in the positiveoutput peaks being clipped off at a level equal to
Note that the theoretical max gain
.
is obtained by biasing the BJT at the edge of saturation,
which would not leave any room for negative signal swing.
Although the gain can be increased by using a larger supply voltage, other considerations come
into play when determining an appropriate value for
The trend has been toward using values for
.
close to 1 V, and replacing
with a constant-
current source.
Graphical Analysis
Although formal graphical methods are of little practical value in the analysis and design of
transistor circuits, it is illustrative to portray graphically the operation of a simple transistor
circuit.
5. Explain Voltage divider bias circuit of BJT in detail
The most effective method to bias the base of a transistor amplifier is using a voltage divider. In
the next chapter, we will analyze each transistor connection in detail and we will be using always
this biasing method. Therefore, let's take some time to explain this method thoroughly.
The idea is that the voltage divider maintains a very stable voltage at the base of the transistor,
and since the base current is many times smaller than the current through the divider, the base
voltage remains practically unchanged. The resistor Re provides the negative feedback as
explained before (Emitter Feedback Bias). Due to the fact that the base voltage remains
unchanged, the negative feedback works very effectively and any unwanted increment of the
current gain produces an -almost- equal negative feedback. The collector and emitter currents
change just a few, and the Q point remains practically stable. Now, let's see in detail how this
works...
Voltage Divider Bias Equations
We start with the assumption that the base current (I B) is many times smaller than the current
through the voltage divider (IVD). A ratio of 20 is a good approach. This means that the base
current must be at least 20 times smaller than the voltage divider current. This condition allows
us to exclude IB from our calculations, with an error less than 5%. Now we can safely calculate
the base voltage as follows:
VB = IVD x R2
Or using the classic voltage divider equation:
VB = (VCC x R2) / (R1 + R2)
The current that flows through the voltage divider is:
IVD = VCC / (R1 + R2)
From the base voltage we can calculate the emitter voltage and the collector-emitter voltage drop
as follows:
VE
=
VB
VCE = VC - VE
The emitter current is calculated using the Ohm's law:
IE = VE / RE
-
VBE
And since the collector current is practically equal to the emitter current, we can calculate all the
transistor currents and voltages:
VRC
VC
=
=
VCC
IC
-
VRC
=
x
VCC
-
RC
IC
x
RC
VCE = VCC - IC x RC - IE x RE = VCC - IC (RC + RE)
As you see, we can calculate everything we need without using any hybrid parameter. This
is an amazing and unexpected result. Two transistors with different current gains can operate as
amplifiers with exactly the same amplification, only because they are biased with a voltage
divider.
Moreover, since VBE is many times smaller than VB and VB remains unchanged all the time, the
emitter voltage VE remains also unchanged, hence maintaining a stable emitter current.
Firm and Stiff Voltage Divider
Previously, we made the assumption that the voltage divider current I VD is many times bigger
than the base current IB, about 20 times as big. This is a good approach for an error less than 5%.
This is not always possible thought. If the base current is high, the resistor values for the voltage
divider must become very small, and this leads to numerous problems.
In such cases, we design the voltage divider with a ratio of 10 instead of 20. This approach has
an error less than 10% when the IB is excluded from the calculations, which is still acceptable.
The voltage divider that satisfies this condition is named Firm Voltage divider:
IVD > 10 x IB => RVD < 0.1 x Î²dc x RE (do not forget that Î² = hfe)
On the other hand, the application may require a very good Q stability with an error less than
1%. A ratio of 100 can be used to calculate the resistors, if this is possible:
IVD > 100 x IB => RVD < 0.01 x Î²dc x RE
The voltage divider that satisfies this condition is named Stiff Voltage divider, and has an error
of less than 1%.
Condition Confirmation
Suppose that the designer wants to design a transistor amplifier with stiff voltage divider bias. He
designed a circuit that has (along others) emitter current I E=1mA. The voltage divider he
designed is calculated according to the stiff VDB condition, which means that the base current
must be 100 times smaller than the voltage divider current. According to this calculation, the
maximum base current cannot be greater than 40uA. The question now is: Does this circuit
works efficiently for the whole hfe range?
The fact that IB and hfe are excluded from the calculations, does not mean that these values do not
affect the operation. They still have a small affect, but this is very small (1 to 10%). What we
have to confirm now is that this affect will always remain small, even at the worst case scenario.
But which is the "worst case scenario"? Well, simply: The worst case scenario is when the
transistor operates with minimum current amplification. When this happens, the base current
becomes maximum to supply the required emitter current. Suppose that the transistor that our
designer used, has an hfe with range from 30 to 300. We have to confirm that the base current
will remain under the calculated value (40uA) and still it will be able to provide full emitter
current (1mA), even at the lowest hfe (30):
IE = Î² x IB => IB = IE / Î² = 1mA / 30 => IB = 33uA
So, the base current for the worst case scenario (33uA) is still less than the calculated base
current (40uA), therefore we can say that this voltage divider remains stiff.
What each part does
Designing a transistor amplifier with VDB (Voltage Divider Bias) is not that hard, but
sometimes it takes time to select the proper part values to begin with. And many times the
designer has to change some parts to change the amplifier parameters. Here is a quick reference
for the designer to know what each part controls:
R1 - This resistor should be used to control the current through the voltage divider
R2 - This resistor controls the base voltage VB
RE - This resistor controls the emitter current I E
RC - The collector resistor can control the VCE voltage
6. Explain various biasing techniques used in BJT
After selecting the proper connection, the one that is most suitable for your application, you must
select a biasing method. Biasing in general means to establish predetermined voltages and
currents at specific points of a circuit, so that the circuit components will operate normally. For
transistors, biasing means to set the proper voltage and current of the transistor base, thus setting
the operating point, also known as quiescence point (Q). We will discuss in details the
quiescence point within the next chapters. For now, you need to know that this point will
determine how the transistor will operate (amplifier or switch). A correctly placed Q offers
maximum amplification without signal distortion or clipping.
The most efficient and commonly used biasing method for transistor amplifiers, it the voltage
divider bias (VDB). We will analyze this method in detail, but first we need to discuss the other
biasing methods. In this chapter, we will use a Common Emitter NPN transistor amplifier to
analyze the various biasing methods, but each method can be used for other connections as well.
Fixed bias
This is the most rarely used biasing method with transistor amplifiers, but it is widely used when
the transistor operates as a switch. The base current IB is controlled by the base resistor RB. From
the second law of Kirchhoff, we have:
VCC = VB + VBE
VB is calculated using the Ohm's law:
VB = IB x RB
So, by selecting the proper base resistor RB, we can define the required base voltage VB and base
current IB. Now we can calculate the collector current using the appropriate hybrid parameter.
Since
this
is
a
common
emitter
circuit,
we
use
the
h fe:
IC = IB x hfe
The problem with this method is that the collector current is very sensitive is slight current gain
changes. Suppose for example that this is a silicon transistor and operates as a B-class amplifier
with
current
gain
300,
RB=80
Kohms,
RC=200
Ohms
and
VCC
=
10
volts:
VCC = VB + VBE => VCC = IB x RB + VBE => IB = (VCC - VBE) / RB = (10-0.7) / 80000 = 112.25
uA
IC = IB x 300 = 33.67mA
The output of this circuit is taken from the collector resistor RC:
VRC = IC x RC = 6.7 Volts
Now suppose that the temperature rises. As we've discussed in earlier pages, this will increase
the current gain. An increment by 15% is a realistic and rather small value. From 300 it will
climb up to 345. This means that the collector current will become 38.7mA, and the output
voltage will also become 7.7 Volts! A whole volt higher than before. That is why this biasing
method is not used for transistor amplifiers.
On the other hand, due to the fact that this method is very simple and cost-effective, it is widely
used when the transistor operates as a load switch, for example as a relay or LED driver. That is
because the Q point operates from cut-off to hard saturation, and even large current gain changes
have
little
or
no
effect
at
the
output.
Emitter feedback bias (Fixed bias with emitter resistor)
This is the first method that was historically used to fix the problem of the unstable current gain
discussed previously. In a transistor circuit with fixed bias, a resistor was added at the emitter.
This method never worked as it should, so it is not used anymore. This is how it was supposed to
work. If the collector current is increased due to a temperature increment, the emitter current is
also increased, thus the current through RE is also increased. The voltage drop across RE is
increased (emitter voltage) which eventually increases the base voltage. Finally, this base voltage
increment has as a result the decrement of the voltage across the base resistor R B, which
eventually decreases the current of the base I B. The idea is that this base current decrement
decreases also the collector current!
This sounds amazing since a change of the output of the circuit has an effect on the input. This
effect is called "feedback" and more specifically it is a "negative feedback", since an output
increment causes an input decrement. Here is how the new collector current is calculated:
IC = (VCC - VBE) / (RE + (RB / hfe))
Let's see how the previous circuit (Fixed bias) would react if we add a 100 Ohms R E feedback
resistor.
IC = (10-0.7) / (100 + (80000 / 300)) = 9.3 / 366.6 = 25.3 mA
We
assume
again
that
the
current
gain
is
increased
by
15%:
IC = (10 - 0.7) / (100 + (80000 / 345)) = 9,3 / 331,88 = 28mA
So, a 15% current gain increment caused a 15.1% output current increment. By adding a 100
Ohms feedback resistor at the emitter, a 15% current gain increment caused a 10.6% output
current increment. The increment is 4.5% less which means that this method works somehow,
but
still
the
shifting
of
the
Q-point
Collector feedback bias (Collector to base bias)
is
too
large
to
be
acceptable.
The next method that the researchers used to stabilize the Q point is the collector feedback bias.
According to this method, the base resistor is not connected at the power supply, instead it is
connected at the collector of the transistor. If the current gain is increased due to temperature
increment, the current through the collector is increased as well, and this decreases the voltage
on the collector VC. But the base resistor is connected at this point, so less current will go
through the resistor in the base. Less current through the base means less current through the
collector.
Again, there is a negative feedback in this circuit. But how much is it? Lets do some math. The
collector current is now calculated by the following formula:
IC = (VCC - VBE) / (RC + (RB / hfe))
To see the change, we will apply this formula in our first example (fixed bias):
IC = (10 - 0.7) / (100 + (80000 / 300)) = 9.3 / 366.6 = 25.3 mA
When
the
current
gain
is
increased
by
15%:
IC = (10 - 0.7) / (100 + (80000 / 345)) = 9.3 / 331.8 = 28 mA
The effectiveness of this method compared to the emitter resistor feedback bias shown before is
exactly the same. The difference is that, RC is usually much larger than RE, which results in
higher
stability.
Nevertheless,
quiescence
point
Q
cannot
be
considered
stable.
Collector feedback bias (Collector to base bias)
It did not take long before someone tried to utilize both the previous methods to work together to
achieve better results. And indeed, the stabilization is much better than each one separately. The
formula to calculate the collector current is the following:
IC = (VCC - VBE) / (RC + RE + (RB / hfe))
Let's apply this formula to our previous examples
IC = 9.3 / (100 + 100 + (80000 / 300)) => IC = 9.3 / 466.6 = 19.9 mA
With a 15% current gain increment:
IC = (VCC - VBE) / (RC + RE + (RB / hfe)) = 9.3 / (100 + 100 + (80000 / 345)) = 21.3 mA
So, a 15% current gain increment causes a 7% output current increment. Although it is better
than the previous circuits, still the Q point is not stable enough. Add to this that h fe is extremely
sensitive to temperature changes and the transistor generates a lot of heat when it operates as a
power amplifier. So we need a much better stabilization technique.