JUNCTION TRANSISTOR
The credit of inventing the transistor in the year 1947 goes to J. Bardeen and W.H. Brattain of Bell Telephone Laboratories, U.S.A. That transistor was a point-contact transistor. The first junction transistor consisting of two back-to-back p-n junctions was invented by William Schockley in 1951. As long as only the junction transistor was known, it was known simply as transistor. But over the years new types of transistors were invented and to differentiate it from the new ones it is now called the Bipolar Junction Transistor (BJT). Even now, often the word transistoris used to mean BJT when there is no confusion. Since our study is limited to only BJT, we shall use the word transistor for BJT without any ambiguity.
`text(Transistor: structure and action :)`
A transistor has three doped regions forming two p-n junctions between them. Obviously, there are two types of transistors, as shownin Fig..
(i) n-p-n transistor: Here two segments of n-type semiconductor (emitter and collector) are separated by a segment of p-type
semiconductor (base).
(ii) p-n-p transistor: Here two segments of p-type semiconductor (termed as emitter and collector) are separated by a segment of n-type semiconductor (termed as base). The schematic representations of an n-p-n and a p-n-p configuration are shown in Fig. (a). All the three segments of a transistor have different thickness and their doping levels are also different. In the schematic symbols used for representing p-n-p and n-p-n transistors [Fig. (b)] the arrowhead shows the direction of conventional current in the transistor. A brief description of the three segments of a transistor is given below:
- Emitter: This is the segment on one side of the transistor shown in Fig. (a). It is of moderate size and heavily doped. It supplies
a large number of majority carriers for the current flow through the transistor.
- Base: This is the central segment. It is very thin and lightly doped.
- Collector: This segment collects a major portion of the majority carriers supplied by the emitter. The collector side is moderately
doped and larger in size as compared to the emitter.
We have seen earlier in the case of a p-n junction, that there is a formation of depletion region acors the junction. In case of a transistor depletion regions are formed at the emitter base-junction and the base collector junction. For understanding the action of a transistor, we have to consider the nature of depletion regions formed at these junctions. The charge carriers move across different regions of the transistor when proper voltages are applied across its terminals.
The biasing of the transistor is done differently for different uses. The transistor can be used in two distinct ways. Basically, it was
invented to function as an amplifier, a device which produces a enlarged copy of a signal. But later its use as a switch acquired equal
importance. We shall study both these functions and the ways the transistor is biased to achieve these mutually exclusive functions. First we shall see what gives the transistor its amplifying capabilities. The transistor works as an amplifier, with its emitter-base junction
forward biased and the base-collector junction reverse biased. This situation is shown in Fig, where `V_(C C)` and `V_(E E)` are used for creating the respective biasing. When the transistor is biased in this way it is said to be in active state.We represent the voltage between emitter and base as `V_(EB)` and that between the collector and the base as `V_(CB)`. InFig, base is a common terminal for the two
power supplies whose other terminals are connected to emitter and collector, respectively. So the two power supplies are represented as `V_(E E),` and `V_(C C)`, respectively. In circuits, where emitter is the common terminal, the power supply between the base and the emitter is represented as `V_(BB)` and that between collector and emitter as `V_(C C).`
Let us see now the paths of current carriers in the transistor with emitter-base junction forward biased and base-collector junction reverse biased. The heavily doped emitter has a high concentration of majority carriers, which will be holes in a p-n-p transistor and electrons in an n-p-n transistor. These majority carriers enter the base region in large numbers. The base is thin and lightly doped.
So the majority carriers there would be few. In a p-n-p transistor the majority carriers in the base are electrons since base is of n-type semiconductor. The large number of holes entering the base from the emitter swamps the small number of electrons there. As the base collector-junction is reverse biased, these holes, which appear as minority carriers at the junction, can easily cross the junction and enter the collector. The holes in the base could move either towards the base terminal to combine with the electrons entering from outside or cross the junction to enter into the collector and reach the collector terminal. The base is made thin so that most of the holes find themselves near the reverse-biased base-collector junction and so cross the junction instead of moving to the base
terminal. It is interesting to note that due to forward bias a large current enters the emitter-base junction, but most of it is diverted to adjacent reverse-biased base-collector junction and the current coming out of the base becomes a very small fraction of the current that entered the junction. If we represent the hole current and the electron current crossing the forward biased junction by
`I_h` and Ie respectively then the total current in a forward biased diode is the sum `I_h + I_e`. We see that the emitter current `I_E = I_h + I_e` but the base current `I_B < < I_h + I_e`, because a major part of `I_E` goes to collector instead of coming out of the base terminal. The base current is thus a small fraction of the emitter current. The current entering into the emitter from outside is equal to the emitter current `I_E`. Similarly the current emerging from the base terminal is `I_B` and that from collector terminal is `I_C`. It is obvious from the above description and also from a straight forward application of Kirchhoff-s law to Fig.(a) that the emitter current is the sum of collector current `I_E = I_C + I_B` We also see that `I_C ≈ I_E.` Our description of the direction of motion of the holes is identical
with the direction of the conventional current. But the direction of motion of electrons is just opposite to that of the current. Thus in a p-n-p transistor the current enters from emitter into base whereas in a n-p-n transistor it enters from the base into the emitter. The arrowhead in the emitter shows the direction of the conventional current. The description about the paths followed by the majority and minority carriers in a n-p-n is exactly the same as that for the p-n-p transistor. But the current paths are exactly opposite, as shown in Fig. In Fig. (b) the electrons are the majority carriers supplied by the n-type emitter region. They cross the thin p-base region and are able to reach the collector to give the collector current, `I_C` . From the above description we can conclude that in the active state of the transistor the emitter-base junction acts as a low resistance while the base collector acts as a high resistance.
`text(Transistor: structure and action :)`
A transistor has three doped regions forming two p-n junctions between them. Obviously, there are two types of transistors, as shownin Fig..
(i) n-p-n transistor: Here two segments of n-type semiconductor (emitter and collector) are separated by a segment of p-type
semiconductor (base).
(ii) p-n-p transistor: Here two segments of p-type semiconductor (termed as emitter and collector) are separated by a segment of n-type semiconductor (termed as base). The schematic representations of an n-p-n and a p-n-p configuration are shown in Fig. (a). All the three segments of a transistor have different thickness and their doping levels are also different. In the schematic symbols used for representing p-n-p and n-p-n transistors [Fig. (b)] the arrowhead shows the direction of conventional current in the transistor. A brief description of the three segments of a transistor is given below:
- Emitter: This is the segment on one side of the transistor shown in Fig. (a). It is of moderate size and heavily doped. It supplies
a large number of majority carriers for the current flow through the transistor.
- Base: This is the central segment. It is very thin and lightly doped.
- Collector: This segment collects a major portion of the majority carriers supplied by the emitter. The collector side is moderately
doped and larger in size as compared to the emitter.
We have seen earlier in the case of a p-n junction, that there is a formation of depletion region acors the junction. In case of a transistor depletion regions are formed at the emitter base-junction and the base collector junction. For understanding the action of a transistor, we have to consider the nature of depletion regions formed at these junctions. The charge carriers move across different regions of the transistor when proper voltages are applied across its terminals.
The biasing of the transistor is done differently for different uses. The transistor can be used in two distinct ways. Basically, it was
invented to function as an amplifier, a device which produces a enlarged copy of a signal. But later its use as a switch acquired equal
importance. We shall study both these functions and the ways the transistor is biased to achieve these mutually exclusive functions. First we shall see what gives the transistor its amplifying capabilities. The transistor works as an amplifier, with its emitter-base junction
forward biased and the base-collector junction reverse biased. This situation is shown in Fig, where `V_(C C)` and `V_(E E)` are used for creating the respective biasing. When the transistor is biased in this way it is said to be in active state.We represent the voltage between emitter and base as `V_(EB)` and that between the collector and the base as `V_(CB)`. InFig, base is a common terminal for the two
power supplies whose other terminals are connected to emitter and collector, respectively. So the two power supplies are represented as `V_(E E),` and `V_(C C)`, respectively. In circuits, where emitter is the common terminal, the power supply between the base and the emitter is represented as `V_(BB)` and that between collector and emitter as `V_(C C).`
Let us see now the paths of current carriers in the transistor with emitter-base junction forward biased and base-collector junction reverse biased. The heavily doped emitter has a high concentration of majority carriers, which will be holes in a p-n-p transistor and electrons in an n-p-n transistor. These majority carriers enter the base region in large numbers. The base is thin and lightly doped.
So the majority carriers there would be few. In a p-n-p transistor the majority carriers in the base are electrons since base is of n-type semiconductor. The large number of holes entering the base from the emitter swamps the small number of electrons there. As the base collector-junction is reverse biased, these holes, which appear as minority carriers at the junction, can easily cross the junction and enter the collector. The holes in the base could move either towards the base terminal to combine with the electrons entering from outside or cross the junction to enter into the collector and reach the collector terminal. The base is made thin so that most of the holes find themselves near the reverse-biased base-collector junction and so cross the junction instead of moving to the base
terminal. It is interesting to note that due to forward bias a large current enters the emitter-base junction, but most of it is diverted to adjacent reverse-biased base-collector junction and the current coming out of the base becomes a very small fraction of the current that entered the junction. If we represent the hole current and the electron current crossing the forward biased junction by
`I_h` and Ie respectively then the total current in a forward biased diode is the sum `I_h + I_e`. We see that the emitter current `I_E = I_h + I_e` but the base current `I_B < < I_h + I_e`, because a major part of `I_E` goes to collector instead of coming out of the base terminal. The base current is thus a small fraction of the emitter current. The current entering into the emitter from outside is equal to the emitter current `I_E`. Similarly the current emerging from the base terminal is `I_B` and that from collector terminal is `I_C`. It is obvious from the above description and also from a straight forward application of Kirchhoff-s law to Fig.(a) that the emitter current is the sum of collector current `I_E = I_C + I_B` We also see that `I_C ≈ I_E.` Our description of the direction of motion of the holes is identical
with the direction of the conventional current. But the direction of motion of electrons is just opposite to that of the current. Thus in a p-n-p transistor the current enters from emitter into base whereas in a n-p-n transistor it enters from the base into the emitter. The arrowhead in the emitter shows the direction of the conventional current. The description about the paths followed by the majority and minority carriers in a n-p-n is exactly the same as that for the p-n-p transistor. But the current paths are exactly opposite, as shown in Fig. In Fig. (b) the electrons are the majority carriers supplied by the n-type emitter region. They cross the thin p-base region and are able to reach the collector to give the collector current, `I_C` . From the above description we can conclude that in the active state of the transistor the emitter-base junction acts as a low resistance while the base collector acts as a high resistance.