Transistors:
Transistors are three-terminal devices having three sections, which are the basis of semiconductor electronics. The three sections are combined such that the two at the extreme ends have the same type of majority carriers, while the section that separates them has majority carriers of opposite nature. Therefore, a transistor can be an n-p-n type or a p-n-p type. In other words, in an n-p-n transistor, the p-section is sandwiched between the 2n sections, as shown in Figure 9.2.09. On the other hand, in a p-n-p transistor, the n-section is sandwiched between the two p-sections, as shown in Figure 9..2.10.
These three sections of the transistors are called emitters E, base B, and collector C. The base of a transistor is thin. Further, the base of a transistor is lightly doped as compared to the emitter and collector, which means that the density of the majority of carriers in the base is always less than that in the emitter or collector.
The emitter supplies the majority of charge carriers for current flow, and the collector collects them. The base provides the junctions for proper interaction between the emitter and collector. The emitter can be, thus, compared to the cathode and collector with the plate in a triode valve. The base of the transistor can be compared with the grid of the triode valve, as it performs a function similar to it.
Symbol for transistors:
In symbolic form, the transistors are represented as shown in [Figures 9.2.11 and 9.2]. The arrow points in the direction of the current. Therefore, the emitter in an n-p-n transistor is represented with the help of an arrow pointed away from the base, while the emitter in a p-n-p transistor is represented with the help of an arrow pointing towards the base.
The direction of the arrows put on the emitter shows the direction of conventional current. The symbols of n-p-n and p-n-p transistors are respectively shown in Figures 9.2.11 and 9.2.12. When the transistor is used in the circuit, the base-emitter junction, which is called an input junction, is always forward biased, and the output junction, which is the base-collector junction, is always reverse biased.
Action of the Transistor:
The action of both types of transistors, therefore n-p-n and p-n-p, is similar except that the majority and minority carriers in the two cases are of opposite natures.
Action of the n-p-n transistor:
Figure 2.20 shows the proper biasing of n-p-n transistors. The n-type emitter is forward-biased by connecting it to the negative pole of the battery Vee (emitter-base battery), and the n-type collector is reverse-biased by connecting it to the positive pole of the battery Vcc (collector-base battery).
The forward bias causes the electrons, which make up the majority of the charge carriers in the emitter, to be attracted to the base. Although holes make up the majority of the charge carriers in the base, they are only mildly doped compared to the emitter or collector, so their number density is low.
As a result, there is a very low chance (5% chance) of electron-hole recombination in the base area. An electron enters the emitter from the negative pole of the emitter-base battery Vee after the majority of electrons (95%) cross into the collector region, where they are whisked away due to the battery’s positive terminal Vcc. As a result, in n-p-n transistors, the electrons carry the current both within the transistors and in the external circuit.
If IE, IB, and IC are the emitter current, the base current, and the collector current, respectively, then
IE = IB + IC
The arrows point in the direction of conventional current or in the direction of the current due to holes, irrespective of the fact that the current is carried out due to the motion of electrons in the n-p-n transistors.
Action of a PNP Transistor:
When the Base Emitter junction is forward-biased and the Base Collector junction is reverse-biased, PNP transistors operate. The PN-Junction will be referred to as forward-biased when a P-type semiconductor is connected to the positive terminal and an N-type semiconductor is connected to the negative terminal of the battery. Also, the PN-Junction will be referred to as reverse-biased when the P-type semiconductors are connected to the negative terminal and the N-type semiconductors are connected to the positive terminal of the battery.
In this instance, the majority of the charge carriers in the emitter are holes, and the forward bias causes them to be attracted to the base. Due to its thinness and minor doping, the base has a very low dopant concentration compared to the collector and emitter, which results in a low electron density. Only approximately 5% of electron-hole recombination occurs when the holes reach the base area.
Under the influence of reverse bias, the majority of the holes—roughly 95%—arrive at the collector. An electron leaves the negative pole of the collector-base battery, VCC, and joins one hole as it approaches the collector. A hole is made in the emitter at the same time that an electron is liberated from a covalent bond there. The electrons that are so liberated enter the emitter-base battery VCE‘s positive pole. As a result, the holes in the PNP transistor carry the current while also maintaining, as previously mentioned, their concentration.
In this case, the emitter current flowing will be given as the sum of the base current and of the collector current, thus,
IE = IB + IC
IC = IE -IB
IC = beta. I
I = IC/beta
As is evident from the band gap energies for silicon semiconductors and germanium semiconductors, the base is roughly 0.7 volts and the emitter is about 0.3 volts more negative, respectively.
It is concluded that the Emitter-Base junction barrier is reduced by increasing the forward bias voltage. This allows more carriers to reach the collector, which in turn increases the current flow from Emitter to collector, implying that reducing the forward bias voltage decreases the current flow.
Characteristics of Transistors:
The relationship between DC currents and voltages is represented graphically, which is known as a characteristic of the transistors. The two important characteristics of a PNP Transistor are
Characteristics of NPN transistors in common emitter mode:
The common emitter characteristics of a transistor are the graphical relations that are observed between the voltage and the current. When the emitter is earthed, the base is used as the input terminal and the collector as the output terminal.
The common emitter characteristics of an NPN transistor are studied using the experimental arrangement shown in Figure 9.2.15 given above.
Base-emitter battery VBE is used to forward-bias the base-emitter junction, and collector-emitter battery Vcc is used to reverse-bias the base-collector junction. The base current IB and the voltage between the base and emitter (VBE) are measured using the microampere and voltmeter in the base-emitter circuit. The collector-emitter circuit’s millimeter and voltmeter both measure the voltage between the collector and emitter (VCE) and the collector current (IC).
The following common emitter characteristics are to be studied:
Input Characteristics:
The input characteristics of transistors are represented in terms of the graph illustrating the fluctuation of base current (IB) with base-emitter voltage (VBE) at constant collector-emitter voltage (VCE).
The input characteristics are arrived at following the points given below.
- Using a rheostat, Rh2, keep the collector-emitter voltage (VCE) at some constant value, say 4 volts.
- Now, using the rheostat, Rh1, we go on changing the base-emitter voltage in steps, and the corresponding values of base currents IB are noted as shown in Table 9.2.16
- The graphs are plotted between the different values of VBE and the corresponding values of IB. The curves so obtained represent the input characteristics of transistors.
- Draw similar characteristic curves at different values of collector-emitter voltage (VCE).
Conclusions:
The following inferences can be made from the NPN transistor input characteristic curves shown in Figure 9.2.17:
- The input characteristics resemble the forward-biased characteristics of a PN-Junction diode quite fairly.
- The base current (IB) falls with an increase in the value of collector emitter voltage (VCE) for some given value of base emitter voltage (VBE).
Input Resistance (ri):
The ratio of the slight fluctuation in base-emitter voltage VBE to the variation in base current IB, maintaining collector-emitter voltage VCE at some constant value, is the definition of input resistance for a transistor operating in common-emitter mode.
The input resistance in this case changes continuously as the input characteristics are non-linear, and at any point, the slope of the tangent to the curve represents it at that point.
Output Characteristics:
These are the graphs between the collector voltage (VCE) and the collector current (IC) at different constant values of the base current (IB).
Figure 9.2.18, which contains the curves that demonstrate the output characteristics of the transistors corresponding to the different values of base current (IB), was obtained using the following procedures:
- Using Rh1, keep the base current (IB) constant at some value, say 10 µA.
- Using Rh2, go on changing collector-emitter voltage (VCE) continuously in steps, and the corresponding values of collector current (IC) are noted.
- The output characteristics of the transistors are obtained by plotting the graphs between the various values of VCE and IC.
- Draw such curves corresponding to the different fixed values of base current, say 10, 20, 30, 40, and 50 microamperes.
Conclusions:
The following inferences can be made from the NPN transistor output characteristic curves shown in Figure 9.2.18:
- When the collector-emitter voltage (VCE) is increased for any given value of base current, the collector current initially rises quickly, but at higher VCE values, the collector current stabilizes.
- The collector current (IC) for any given value of collector-emitter voltage (VCE) is greater for larger values of base current.
There are three regions where the output characteristics of transistors in common emitter mode consist of:
a. Active region:
The area located above IB = 0 is the active region of the curve and is the non-shaded core area. In this region, the collector-emitter junction is reverse-biased, while the emitter-base junction is forward-biased. Collector current rises as collector-emitter voltage VCE rises for a certain value of base current IB. In this area, the transistors act as audio amplifiers.
b. Cut-off Region:
A cut-off region is the shaded area that is located below the curve for IB = 0. In this location, IB and IC are both equal to zero since both junctions are reverse-biased. Just setting IB = 0 won’t result in the collector current being cut off. The Junction must be somewhat reverse-biased and IB = 0 for the cutoff to occur. Transistors in this region can function as switches because they can quickly transition from a cut-off state, where IC is 0, to an on state, where IC is maximum.
c. Saturation Region:
The saturation region refers to the shaded area to the left of the line OP, and the saturation line refers to the line itself. VCE > VBE in this area. Both junctions develop a forward bias. In this case, IC is independent of the input current, IB.
AC output resistance:
The AC output resistance of the transistor in a common emitter arrangement is defined as the ratio of a small change in collector voltage to the small change in collector current at a fixed value of base current.
Rout = [dVce /dIc]Ib=constant
Transfer Characteristics:
Here are graphs showing the relationship between the collector current (IC) and the base current (IB) at various constant collector voltage (VCE) values.
Plotting transfer characteristics will be done at, say, VCE = 3 volts. To do this, the potential divider configuration of the emitter-collector circuit is changed such that the linked voltmeter displays 3 V. Now, the base-emitter voltage VBE is changed by utilizing the potential divider configuration in the emitter-base circuit. As a result, the base current will change, which will affect the collector current.
The base current IB and matching collector current IC values are recorded each time. The voltage between the collector and emitter is guaranteed to be 3 V at all times. The transfer characteristic of the transistor at VCE = 3 V is therefore depicted in the picture as the line drawn between IB and IC. The graph will be discovered to be a straight line. The output current IC is said to change linearly with the input current IB.
AC current Gain:
The ratio of variation in collector current to the change in base current at some constant value of collector voltage is called ac current gain. It is represented in terms of ß and is also called the current transfer ratio. Therefore,
ß= [(dIC/dIB)]VCE=constant