In this article we will discuss about:- 1. Introduction to Junction Transistor 2. Operation of a Transistor 3. Current Amplification Factors 4. Transistor Leakage Currents 5. Characteristics.
Contents:
- Introduction to Junction Transistor
- Operation of a Transistor
- Current Amplification Factors of a Transistor
- Transistor Leakage Currents
- Characteristics of Transistor
1. Introduction to Junction Transistor:
The junction transistor consists of a silicon (or germanium) single crystal of two P-N junctions formed among the three layers base- (B), emitter (E) and the collector (C). Junction transistors can be classified into two main groups, viz., N-P-N and P-N-P. This classification depends on the impurity elements used for the construction of emitter, base and collector.
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Fig 1.35(a) illustrates the physical construction of an N-P-N junction transistor while Fig. 1.35(b) is for the P-N-P type. In the figure the constructions of- (a) the grown junction transistor, (b) alloy junction transistor and (c) planar-type double diffused transistor are shown. Physically the base is narrower than the emitter or the collector. The relative size of the emitter in comparison to the collector depends on the type of transistor and also on its application.
2. Operation of a Transistor
:
In order to explain the operation of a transistor, let us first consider a P-N-P type which is like two P-N junction diodes placed back-to-back. At each junction a depletion region is there which gives rise to an internal potential barrier.
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In Fig. 1.36 two batteries are connected in a manner, as shown. As the C-B junction is reversed bias, the width of the depletion region increases at the junction and the majority carriers, i.e., electrons are blocked in the base.
Let under the forward bias condition of the E-B junction, numbers of holes/second cross the junction and go to the base. At that time let y numbers of Fig. 1.36 electrons/second, which are very small in number, flow from base to emitter where they recombine with equal numbers of holes. The loss of (X + y) numbers of holes/second in the emitter is made up by the flow of equal numbers of electrons from the emitter to the positive terminal of the battery connected to it.
The flow of x holes/second from emitter to base and y electrons/second from base to emitter produces the emitter current IE, where-
Next, let us consider an N-P-N transistor where two P-N junction diodes are kept back-to-back with their P-regions as common. A depletion region is there at each of the junction which produces an internal potential barrier.
For the transistor action, the circuit arrangement is made by connecting the two batteries as shown in Fig. 1.37. In the figure, the emitter-base junction is forward biased while the collector- base junction is reverse biased.
Let under the forward bias condition of the E-B junction, X number of electrons/second cross the junction and go to the base. At that time, let a very small number, y holes/second flow from base to emitter where they recombine with an equal number of electrons. The loss of (X + y) number of electrons/second in the emitter is made up by the flow of equal number of electrons from the negative terminal of the battery to the emitter.
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The flow of X electrons/second from emitter to base and y holes/second from base to emitter produces the emitter current IE, where-
i. Collector Action:
For a P-N-P transistor, due to the recombination process in the base, (X – x) number of holes will reach to the depletion region. By the reverse bias voltage, these holes are swept out of the base into the collector where these are neutralized by an equal number of electrons flowing from the negative terminal of the battery connected to collector.
ADVERTISEMENTS:
For an N-P-N transistor, due to the recombination process in the base, (X – x) number of electrons will reach to the depletion region. By the reverse bias voltage, these electrons are swept out of the base into the collector wherefrom these are attracted by the positive terminal of the battery connected to collector.
3. Current Amplification Factors
of a Transistor:
The characteristics of a bipolar transistor are described in terms of current. At the transistor electrodes, the magnitudes of current are related by the equation-
lE = lB + lc …(1.15)
The above equation indicates that there are two current amplification factors for the static (d.c.) currents and two for small changes in the currents.
Definition of Amplification Factors:
i. Static Current Amplification Factor (αdc):
For a transistor with common base configuration it is defined as the ratio of static (d.c.) collector current lC to the static emitter current IE at a constant collector voltage with respect to base.
Mathematically, we can write-
αdc is also known as the static forward current transfer ratio or the d.c. current gain for the common base configuration.
ii. Static Current Amplification Factor (βdc):
For a transistor with common emitter configuration it is defined as the ratio of static collector current IC to the static base current IB at a constant collector voltage with respect to emitter.
Mathematically, we can write-
βdc is also known as the static forward current transfer ratio or the d.c. current gain for the common emitter configuration.
Relation Between αdc and βdc:
iii. Small-Signal Current Amplification Factor (α):
For a transistor with common base configuration, it is defined as the ratio of the small change in the collector current to the corresponding small change in the emitter current when collector voltage is kept constant with respect to the base.
Thus,
iv. Small-Signal Current Amplification Factor (β):
For a transistor with common emitter configuration it is defined as the ratio of a small change in the collector current to the corresponding small change in the base current when the collector voltage is kept constant with respect to the emitter.
Thus,
It is seen from above that as α approaches towards unity, β becomes increasingly larger. For a transistor whose α is 0.98, β becomes 49 while an α of 0.99 gives β of 99. So, during the performance of the experiment α should be measured very accurately for avoiding formula error in β.
It has been noted from experimental data that the values of αdc and a are very nearly equal.
4. Transistor Leakage Currents:
It can be classified as- (i) collector-to-base leakage current (lCBO), (ii) collector-to-emitter leakage current (ICEO) and (iii) emitter-to-base leakage current (IEBO).
i. If the emitter is open circuited and the collector-base junction is reversed biased [Fig. 1.38(a)], a small collector current called as the collector-to-base leakage current (ICBO) flows. In the symbol ICBO, the subscript CB shows a collector-base current while the subscript O indicates that the current in the third electrode (viz., the emitter, E) is zero. In transistor biasing circuits, the current ICBO has high importance.
When the emitter current is zero, the transistor is said to be off and in this condition the leakage current continues to flow. For the common base configuration of the transistor with the emitter-base junction forward biased and the collector-base junction reverse biased, the part of the emitter current which reaches the collector is IC – ICBO. Thus, when the leakage current is taken into account, we can define α as-
ii. If the base is open-circuited and the collector is reverse biased with respect to emitter [Fig. 1.38(b)], a small collector current called as the collector-to-emitter leakage current (ICEO) flows.
For the common emitter configuration of the transistor with the emitter-base junction forward biased and the collector-base junction reverse biased, the part of the emitter current which reaches the collector is IC – ICEO. Thus, when the leakage current is taken into account we can define β as-
Equation (1.30) shows that the collector-emitter leakage current in common emitter configuration is (β + 1) times greater than that in common base configuration.
iii. If the collector is open-circuited and the emitter-base junction is reverse biased [Fig. 1.38(c)], a small emitter current called as the emitter-to-base leakage current (IEBO) flows.
5. Characteristics
of Transistor:
Characteristics with Common Emitter Configuration:
The circuit diagram for determining the static characteristics under common emitter configuration of a P-N-P junction transistor is shown in Fig. 1.39. It is seen that a forward bias is applied to the emitter junction while a reverse bias to the collector junction.
(a) Input Characteristics:
Values of base current (IB) are plotted against base to emitter voltage (VBE) for a fixed VCE. The nature of the curves obtained for different sets of VCE is shown in Fig. 1.40.
(b) Output Characteristics:
Values of collector current (IC) are plotted against collector to emitter voltage (VCE) for a fixed lB. The nature of the curves obtained for different sets of IB is shown in Fig. 1.41.
(c) Transfer Characteristics:
Values of collector current (IC) are plotted against the base current (IB) for fixed VCE. The characteristic is shown in Fig. 1.42.
Characteristics with Common Base Configuration:
The circuit diagram for studying the characteristics under common-base configurations of a P-N-P junction transistor is shown in Fig. 1.43.
(a) Input Characteristics:
The curve obtained by plotting emitter current (IB) against the emitter voltage (VEB) keeping the collector voltage (VCB) as parameter is called the input characteristic of a transistor. Typical input characteristic of a transistor is shown in Fig. 1.44.
(b) Output Characteristics:
The curve obtained on plotting collector current (IC) against the collector voltage (VCB) keeping the emitter current (IE) as parameter is the output characteristics of a transistor. Typical characteristic curves such obtained are shown in Fig. 1.45.
(c) Transfer Characteristics:
Values of collector current (IC) are plotted against the emitter current (IE) keeping the collector voltage (VCB) constant. Typical transfer characteristic of a transistor is shown in Fig. 1.46.
