Bipolar Junction Transistor

A transistor is basically a Si on Ge crystal containing three separate regions.
It can be either NPN or PNP type fig.
The middle region is called the base and the outer two regions are called emitter and the collector.
In most transistors, emitter is heavily doped.
Its job is to emit or inject electrons into the base.
These bases are lightly doped and very thin, it passes most of the emitter-injected electrons on to the collector.
The doping level of collector is intermediate between the heavy doping of emitter and the light doping of the base.
The collector is so named because it collects electrons from base.
The collector is the largest of the three regions; it must dissipate more heat than the emitter or base.
The transistor has two junctions.
One between emitter and the base and other between the base and the collector.
Because of this the transistor is similar to two diodes, one emitter diode and other collector base diode.
When transistor is made, the diffusion of free electrons across the junction produces two depletion layers.
For each of these depletion layers, the barrier potential is 0.7 V for Si transistor and 0.3 V for Ge transistor.
The depletion layers do not have the same width, because different regions have different doping levels.
The more heavily doped a region is, the greater the concentration of ions near the junction.
This means the depletion layer penetrates more deeply into the base and slightly into emitter.
Similarly, it penetration more into collector.
The thickness of collector depletion layer is large while the base depletion layer is small as shown in fig.
If both the junctions are forward biased using two d.c sources, as shown in fig. free electrons (majority carriers) enter the emitter and collector of the transistor, joins at the base and come out of the base.
Because both the diodes are forward biased, the emitter and collector currents are large.
If both the junction are reverse biased as shown in fig., then small currents flows through both junctions only due to thermally produced minority carriers and surface leakage.
Thermally produced carriers are temperature dependent, it approximately doubles for every 10 degree Celsius rise in ambient temperature.
The surface leakage current increases with voltage.
When the emitter diode is forward biased and collector diode is reverse biased as shown in fig. then one expect large emitter current and small collector current but collector current is almost as large as emitter current.
When emitter diodes forward biased and the applied voltage is more than 0.7 V (barrier potential) then larger number of majority carriers (electrons in n-type) diffuse across the junction.
Once the electrons are injected by the emitter enter into the base, they become minority carriers.
These electrons do not have separate identities from those, which are thermally generated, in the base region itself.
The base is made very thin and is very lightly doped.
Because of this only few electrons traveling from the emitter to base region recombine with holes.
This gives rise to recombination current.
The rest of the electrons exist for more time.
Since the collector diode is reverse biased, (n is connected to positive supply) therefore most of the electrons are pushed into collector layer.
These collector elections can then flow into the external collector lead.
Thus, there is a steady stream of electrons leaving the negative source terminal and entering the emitter region.
The V_EB forward bias forces these emitter electrons to enter the base region.
The thin and lightly doped base gives almost all those electrons enough lifetime to diffuse into the depletion layer.
The depletion layer field pushes a steady stream of electron into the collector region.
These electrons leave the collector and flow into the positive terminal of the voltage source.
In most transistor, more than 95% of the emitter injected electrons flow to the collector, less than 5% fall into base holes and flow out the external base lead.
But the collector current is less than emitter current.

Relation between different currents in a transistor:

The total current flowing into the transistor must be equal to the total current flowing out of it. Hence, the emitter current I_E is equal to the sum of the collector (I_C) and base current (I_B). That is,

I_E = I_C + I_B

The currents directions are positive directions. The total collector current IC is made up of two components.

The fraction of emitter (electron) current which reaches the collector ( α_dc I_E )
The normal reverse leakage current I_CO

It is always positive.
Since collector current is almost equal to the I_E therefore α_dc I_E varies from 0.9 to 0.98.
Usually, the reverse leakage current is very small compared to the total collector current.
The forward bias on the emitter diode controls the number of free electrons infected into the base.
The larger (V_BE) forward voltage, the greater the number of injected electrons.
The reverse bias on the collector diode has little influence on the number of electrons that enter the collector.
Increasing V_CB does not change the number of free electrons arriving at the collector junction layer.
The symbol of npn and pnp transistors are shown in fig.

The Common Base Configuration:

If the base is common to the input and output circuits, it is know as common base configuration as shown in fig.
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For a pnp transistor the largest current components are due to holes.
Holes flow from emitter to collector and few holes flow down towards ground out of the base terminal.
The current directions are shown in fig.

(I_E = I_C + I_B ).

For a forward biased junction, V_EB is positive and for a reverse biased junction V_CB is negative.
The complete transistor can be described by the following two relations, which give the input voltage V_EB and output current I_C in terms of the output voltage (V_CB) and input current I_E.

V_EB = f1(V_CB, I_E)

I_C= f2(V_CB, I_E)

The output characteristic:

The collector current I_C is completely determined by the input current I_E and the V_CB voltage.
The relationship is given in fig.
It is a plot of I_C versus V_CB, with emitter current I_E as parameter.
The curves are known as the output or collector or static characteristics.
The transistor consists of two diodes placed in series back to back (with two cathodes connected together).
The complete characteristic can be divided in three regions.

(1)Active region:

In this region the collector diode is reverse biased and the emitter diode is forward biased.
Consider first that the emitter current is zero.
Then the collector current is small and equals the reverse saturation current I_CO of the collector junction considered as a diode.
If the forward current I_B is increased, then a fraction of I_E will reach the collector.
In the active region, the collector current is essentially independent of collector voltage and depends only upon the emitter current.
Because α_dc is, less than one but almost equal to unity, the magnitude of the collector current is slightly less that of emitter current.
The collector current is almost constant and work as a current source.
The collector current slightly increases with voltage.
This is due to early effect.
At higher voltage collector gathers in a few more electrons.
This reduces the base current.
The difference is so small, that it is usually neglected.
If the collector voltage is increased, then space charge width increases; this decreased the effective base width.
Then there is less chance for recombination within the base region.

(2)Saturation region:

The region to the left of the ordinate V_CB = 0, and above the I_E = 0, characteristic in which both emitter and collector junction are forward biased, is called saturation region.
When collector diode is forward biased, there is large change in collector current with small changes in collector voltage.
A forward bias means, that p is made positive with respect to n, there is a flow of holes from p to n.
This changes the collector current direction.
If diode is sufficiently forward biased the current changes rapidly.
It does not depend upon emitter current.

(3)Cut off region:

The region below I_E = 0 and to the right of V_CB for which emitter and collector junctions are both reversed biased is referred to cutoff region.
The characteristics I_E = 0, is similar to other characteristics but not coincident with horizontal axis.
The collector current is same as I_CO.
I_CBO is frequently used for I_CO.
It means collector to base current with emitter open.
This is also temperature dependent.

The Input Characteristic:

In the active region the input diode is forward biased, therefore, input characteristic is simply the forward biased characteristic of the emitter to base diode for various collector voltages.
Below cut in voltage (0.7 or 0.3) the emitter current is very small.
The curve with the collector open represents the forward biased emitter diode.
Because of the early effect the emitter current increases for same V_EB. (The diode becomes better diode).
When the collector is shorted to the base, the emitter current increases for a given V_EB since the collector now removes minority carriers from the base, and hence base can attract more holes from the emitter.
This mean that the curve V_CB= 0, is shifted from the character when V_CB = open.

Equivalent circuit of a transistor: (Common Base)

In an ideal transistor, α_dc= 1.
This means all emitter electrons entering the base region go on to the collector.
Therefore, collector current equals emitter current.
For transistor action, emitter diode acts like a forward bias diode and collector diode acts like a current source.
The equivalent circuits of npn and pnp transistors are shown in fig.
The current source arrow points for conventional current.
The current source is controlled by emitter current.

Common Base Amplifier

The common base amplifier circuit is shown in Fig.
The V_EE source forward biases the emitter diode and V_CC source reverse biased collector diode.
The ac source v_in is connected to emitter through a coupling capacitor so that it blocks dc.
This ac voltage produces small fluctuation in currents and voltages.
The load resistance R_L is also connected to collector through coupling capacitor so the fluctuation in collector base voltage will be observed across R_L.
The dc equivalent circuit is obtained by reducing all ac sources to zero and opening all capacitors.
The dc collector current is same as I_E and V_CB is given by
V_CB = V_CC – I_C R_C
These current and voltage fix the Q point.
The ac equivalent circuit is obtained by reducing all dc sources to zero and shorting all coupling capacitors.
r’e represents the ac resistance of the diode as shown in Fig.

If the ac signal is small, the points A and B are close to Q, and arc A B can be approximated by a straight line and diode appears to be a resistance given by:

Common Emitter Configuration

The common emitter configuration of BJT is shown in fig.

In C.E. configuration the emitter is made common to the input and output.
It is also referred to as grounded emitter configuration.
It is most commonly used configuration.
In this, base current and output voltages are taken as impendent parameters and input voltage and output current as dependent parameters

V_BE = f1 ( I_B, V_CE )

I_C = f2( I_B, V_CE )

Input Characteristic:

The curve between I_B and V_BE for different values of V_CE are shown in fig.
Since the base emitter junction of a transistor is a diode, therefore the characteristic is similar to diode one.
With higher values of V_CE collector gathers slightly more electrons and therefore base current reduces.
Normally this effect is neglected. (Early effect)
When collector is shorted with emitter then the input characteristic is the characteristic of a forward biased diode when V_BE is zero and I_B is also zero.

Output Characteristic:

The output characteristic is the curve between V_CE and I_C for various values of I_B.
For fixed value of I_B and is shown in fig.
For fixed value of I_B, I_C is not varying much dependent on V_CE but slopes are greater than CE characteristic.
The output characteristics can again be divided into three parts.

(1) Active Region:

In this region collector junction is reverse biased and emitter junction is forward biased.
It is the area to the right of V_CE = 0.5 V and above I_B= 0.
In this region transistor current responds most sensitively to I_B.
If transistor is to be used as an amplifier, it must operate in this region.
Ideally Ic should be independent of V_CE, but due to early effect it increases little bit.

(2) Cut Off:

Cut off in a transistor is given by I_B = 0, I_C= I_CO.
A transistor is not at cut off if the base current is simply reduced to zero (open circuited) under this condition,
The actual collector current with base open is designated as I_CEO.
Since even in the neighborhood of cut off, a dc may be as large as 0.9 for Ge, then I_C=10 I_CO(approximately), at zero base current.
Accordingly in order to cut off transistor it is not enough to reduce I_B to zero, but it is necessary to reverse bias the emitter junction slightly.
It is found that reverse voltage of 0.1 V is sufficient for cut off a transistor.
In Si, the a dc is very nearly equal to zero, therefore, I_C = I_CO. Hence even with I_B= 0, I_C= I_E= I_CO so that transistor is very close to cut off.
In summary, cut off means I_E = 0, I_C = I_CO, I_B = -I_C = -I_CO , and VBE is a reverse voltage whose magnitude is of the order of 0.1 V for Ge and 0 V for Si.

(3)Saturation Region:

In this region both the diodes are forward biased by at least cut in voltage.
Since the voltage V_BE and V_BC across a forward is approximately 0.7 V therefore, V_CE = V_CB + V_BE = – V_BC + V_BE is also few tenths of volts.
Hence saturation region is very close to zero voltage axis, where all the current rapidly reduces to zero.
In this region the transistor collector current is approximately given by V_CC / R_C and independent of base current.
Normal transistor action is last and it acts like a small ohmic resistance.