Lecture notes Dr V Seetha Rama Raju Ph.D. 1 Unit – IV Transistors part - I The transistor is a multijunction semiconductor device that, in conjunction with other circuit elements, is capable of current gain, voltage gain, and signal power gain. The transistor is therefore referred to as an active device, whereas the diode is passive. Transistor Terminals The transistor has three terminals namely, emitter, collector and base. The terminals of the diode are explained below in details. Emitter – The section that supplies the large section of majority charge carrier is called emitter. The emitter is always connected in forward biased with respect to the base so that it supplies the majority charge carrier to the base. The emitter junction injects a large amount of majority charge carrier into the base because it is heavily doped and moderate in size. Collector – The section which collects the major portion of the majority charge carrier supplied by the emitter is called a collector. The collector-base junction is always in reverse bias. Its main function is to collect the majority charges from its junction with the base. The collector section of the transistor is moderately doped, but larger in size so that it can collect most of the charge carriers supplied by the emitter. Base – The middle section of the transistor is known as the base. The base forms two circuits, the input circuit with the emitter and the output circuit with the collector. The emitter-base circuit is in forward biased and offered the low resistance to the circuit. The collector-base junction is in reverse bias and offers the higher resistance to the circuit. The base of the transistor is lightly doped and very thin due to which it offers the majority charge carrier to the collector. Transistor Symbols: There are two types of transistor, namely NPN transistor and PNP transistor. The transistor which has two blocks of n-type semiconductor material and one block of P-type semiconductor material is known as NPN transistor. Similarly, if the material has one layer of N-type material and two layers of P-type material then it is called PNP transistor. The symbol of NPN and PNP is shown in the figure below. The arrow in the symbol indicates the direction of flow of conventional current in the emitter with forward biasing applied to the emitter-base junction. The only difference between the NPN and PNP transistor is in the direction of the current. Lecture notes Dr V Seetha Rama Raju Ph.D. 2 Transistor Operating Conditions: The transistor has two junctions which can be biased in different ways. The different working conduction of the transistor is shown in the table below. Condition Emitter Junction (EB) Collector Junction (CB) Region of Operation FR Forward-biased Reversed-biased Active FF Forward-biased Forward-biased Saturation RR Reversed-biased Reversed-biased Cut-off RF Reversed-biased Forward-biased Inverted FR – In this case, the emitter-base junction is connected in forward biased and the collector-base junction is connected in reverse biased. The transistor, which operates in this region is used for amplification. FF – In this condition, both the junction is in forward biased. The transistor is in saturation. The transistors act like a closed switch. RR – Both the junctions are in reverse biased. The emitter does not supply the majority charge carrier to the base. Thus the transistors act like a closed switch. RF – The emitter-base junction is in reverse bias and the collector-base junction is kept in forward biased. As the collector is lightly doped as compared to the emitter junction it does not supply the majority charge carrier to the base. Hence poor transistor action is achieved. Transistor Current Components: The emitter current consists of hole current (holes crossing from the emitter into the base) and electron current (electrons crossing from base into emitter). The holes crossing the emitter-base junction and reaching collector-base junction constitutes collector current . Not all the holes crossing the emitter-base junction reach the collector-base junction, because some of them combine with the electrons in the n-type base. Since the base width is very small, most of the holes cross the collector-base junction and very few recombine, constituting the recombination current Lecture notes Dr V Seetha Rama Raju Ph.D. 3 During recombination, base region loses its electrons, if electrons are continuously lost than base region become positive in nature and it may start opposing movement of holes passing from Emitter to Collector. Therefore compensating electrons should be supplied to the base region from an external biasing supply. These compensating electrons flowing into the base, generate a current equal to recombination current. In BJT base current is almost equal to recombination current. If electrons supplied to the base are fixed → Base region remains neutral and fixed no. of holes travel from Emitter to Collector. If electrons supplied to the base are increased → Base becomes negative in nature and hence attract greater no. of holes from Emitter, due to which net flow from E to C increases. If electron supplied to the base are reduced → Base becomes positive in nature and attract less no. of holes from Emitter, due to which net flow from E to C decreases. Thus electrons supplied to the base or base current controls the flow of holes from E to C, therefore BJT is called current controlled device. Emitter Injunction Efficiency ( ): It is the ratio of the current through the emitter junction due to the carrier of the emitter and the total current through the emitter junction. Base Transport Factor : It is the ratio of hole current through collector junction and hole current through emitter junction. It is the measurement of efficient transportation of carrier through base region. Large Signal Current Gain ( ) : It is the ratio of the current due to injected carriers to the total emitter current Lecture notes Dr V Seetha Rama Raju Ph.D. 4 Lecture notes Dr V Seetha Rama Raju Ph.D. 5 Lecture notes Working of a Transistor: The emitter- base junction is connected in forward biased due to which the emitter pushes the holes in the base region. These holes constitute the emitter current. When these electrons move into the N-type semiconductor material or base, they combined with the electrons. The base of the transistor is thin and very lightly doped. Hence only a few holes combined with the electrons and the remaining are moved towards the collector space charge l ayer. Hence develops the base current. The collector base region is connected in reverse biased. The holes which collect around the depletion region when coming under the impact of negative polarity collected or attracted by the collector. This develops th e collector current. The complete emitter current flows through the collector current I C Common Base (CB) Configuration: The configuration in which the base of the common between emitter and collector circuit is called a common base configuration . The common base circuit arrangement for PNP transistor is shown in the figure. In common base-emitter con nection, the input is connected between emitter and base while the output is taken across collector and base. Current Amplification factor ( α ) The ratio of output current to input current is known as a common base configuration, the collector current I I E is the input current. = ------- (1) The I C = α I E and I B = I E - I C Total collector current: The total collector current consists 1. The large percentage of emitter current that reaches the collector terminal, i.e., 2. Leakage current across the base ∴ Total collector current, The above expression shows that if current flow in the collector circuit called leakage current. Dr V Seetha Rama Raju 6 base junction is connected in forward biased due to which the emitter pushes the holes in the base region. These holes constitute the emitter current. When these electrons move material or base, they combined with the electrons. The base of the transistor is thin and very lightly doped. Hence only a few holes combined with the electrons and the remaining are moved towards ayer. Hence develops The collector base region is connected in reverse biased. The holes which collect around the depletion region when coming under the impact of negative polarity collected or attracted by the e collector current. The complete emitter current flows through the Common Base (CB) Configuration: The configuration in which the base of the transistor is common between emitter and collector circuit is called . The common base circuit arrangement for PNP transistor is shown in the figure. In nection, the input is connected between emitter and base while the output is taken across The ratio of output current to input current is known as a current amplification factor configuration, the collector current I C is the output current, and the emitter current The range of α is from 0.9 to 0.99 C => I B = I E - α I E => I B = I E (1 The total collector current consists 1. The large percentage of emitter current that reaches the collector terminal, i.e., α 2. Leakage current across the base -collector junction, I CB o. Total collector current, I C = α I E + I CB o The above expression shows that if I E = 0 (when the emitter circuit is open) then still a small current flow in the collector circuit called leakage current. I C = I CB o Dr V Seetha Rama Raju Ph.D. base junction is connected in forward biased due to which the emitter pushes the depletion region when coming under the impact of negative polarity collected or attracted by the e collector current. The complete emitter current flows through the current amplification factor . In the is the output current, and the emitter current (1 - α ) 1. The large percentage of emitter current that reaches the collector terminal, i.e., α I E = 0 (when the emitter circuit is open) then still a small Lecture notes Dr V Seetha Rama Raju Ph.D. 7 Characteristics of Common Base (CB) Configuration: The circuit diagram for determining the common base characteristic is shown in the figure below. In the circuit milli-ammeters are connected in series with the emitter and collector to measure emitter current I E and collector current I C respectively. Similarly voltmeters are connected in parallel across E and B to measure the voltage V EB and across C and B to measure the voltage V CB respectively. Input characteristics : The curve plotted between emitter current I E and the emitter- base voltage V EB at constant collector base voltage V CB is called input characteristic curve. The input characteristic curve is shown in the figure. 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. Output characteristics: The curve plotted between the collector current and collector base voltage V CB at constant emitter current I E is called output characteristic. The CB configuration of PNP transistor is shown in the figure. Lecture notes Dr V Seetha Rama Raju Ph.D. 8 The transistor consists of two diodes placed in series back to back (with two cathodes connected together). The complete characteristic can be divided into three regions. 1. Active region: In this region the collector junction is reverse biased and the emitter junction is forward biased. Consider first that the emitter current I E = 0. Then the collector current, I C is small and equals the reverse saturation current I CB o of the collector junction. If the forward current I B is increased, then a fraction of I E ie. α 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 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 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. 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 junction is forward biased, there is a 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 the 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 CB o. It means collector to base current with emitter open. This is also temperature dependent. Common Emitter (CE) Configuration: In this configuration the emitter is common to both the input and the output circuit, and hence the name is the common emitter configuration. The common emitter arrangement for PNP transistor is shown in the figure. Base Current Amplification Factor ( β ): The base current amplification factor is defined as the ratio of Lecture notes the output and input current in a common emitter configuration. In common emitter amplification, the output current is the collector current I current I B = The relation between Β and α can be derived as, Substituting I B value we have Total Collector Current: In CE configuration, the input current I related by the equation shown below. = + , where If the base current is open (i.e., I B = 0). There is a I CEO we know that Characteristics of Common Emitter (CE) Configuration: The circuit diagram for determining the common emitter characteristic is shown in the figure below. In the circuit, the battery V BB provides forward bias to emitter –base junction and V BE is a measure of voltage drop across base and emitter. A battery V between collector and emitter and V voltage drop across collector and emitter. Input Characteristics: The curve plotted between base current I base-emitter voltage V EB for different value of in the figure below. The curve for CE configuration is similar to a forward diode characteristic. The base current I with the increases in the emitter - With higher values of V CE collector gathers slightly more electrons and therefore base current reduces due to effect. Dr V Seetha Rama Raju 9 the output and input current in a common emitter configuration. In common emitter amplification, the output current is the collector current I C, and the input current is the base α can be derived as, I E = I B + I C => I B = I E - I C = => => In CE configuration, the input current I B and the output current I related by the equation shown below. , where is the leakage current. = 0). There is a leakage current from collector to the emitter, and this current is abbreviated as = + Characteristics of Common Emitter (CE) Configuration: The circuit diagram for determining the common characteristic is shown in the figure below. provides forward bias to is a measure of voltage drop across base and emitter. A battery V CC is connected between collector and emitter and V CE is a measure of voltage drop across collector and emitter. The curve plotted between base current I B and the different value of V CE is shown figure below. The curve for CE configuration is similar to a forward diode characteristic. The base current I B increases - base voltage V BE collector gathers slightly more current reduces due to Early Dr V Seetha Rama Raju Ph.D. the output and input current in a common emitter configuration. In common emitter input current is the base and the output current I C are leakage current from collector to the emitter, and this current is abbreviated as 1 Lecture notes Dr V Seetha Rama Raju Ph.D. 10 Output Characteristics: In CE configuration the curve draws between collector current I C and collector-emitter voltage V CE at a constant base current I B is called output characteristic. The O/P characteristic curve for the CE configuration is shown in the figure. In the active region, the collector current increases slightly as collector-emitter V CE current increases. The slope of the curve is quite more than the output characteristic of CB configuration. The output resistance of the common base connection is more than that of CE connection. 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. = + 1 If is truly constant then I C would be independent of V CE But because of Early effect, increases by 0.1% (0.001) e.g. from 0.995 to 0.996 as V CE increases from a few volts to 10V. Then increases from . . = 200 to . . = 250 or about 25%. This shows that small change in reflects large change in 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, = 1 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 0.2 V is sufficient for cut off a transistor. 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, Lecture notes Dr V Seetha Rama Raju Ph.D. 11 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. Common Collector (CC) Configuration: The configuration in which the collector is common between emitter and base is known as CC configuration In CC configuration, the input circuit is connected between emitter and base and the output is taken from the collector and emitter. The collector is common to both the input and output circuit and hence the name common collector connection or common collector configuration. Current Amplifier Factor ( ): The current amplification factor is defined as the ratio of the output current to the input current. In common emitter configuration, the output current is emitter current I E , whereas the input current is base current I B = Relation Between and : The is the current amplification factor of common collector configuration and the is current amplification factor of common base connection. = and = We know that I E = I B + I C => I B = I E - I C Substituting the value of I B we get, This relation shows that the value of is nearly equal to β . This circuit is mainly used for amplification because of this arrangement input resistance is high, and output resistance is very low. This circuit arrangement is mainly used for impedance matching. Collector Current: We know that, Lecture notes Dr V Seetha Rama Raju Ph.D. 12 Input Characteristics: The input characteristic of the common collector configuration is drawn between collector base voltage V CE and base current I B at constant emitter current voltage V CE . The value of the output voltage V CE changes with respect to the input voltage V BC and I B With the help of these values, input characteristic curve is drawn. The input characteristic curve is shown below. Output Characteristics: The output characteristic of the common emitter circuit is drawn between the emitter-collector voltage V EC and output current I E at constant input current I B . If the input current I B is zero, then the collector current also becomes zero, and no current flows through the transistor. The transistor operates in active region when the base current increases and reaches to saturation region. The graph is plotted by keeping the base current I B constant and varying the emitter-collector voltage V CE , the values of output current I E are noticed with respect to V CE . By using the V CE and I E at constant I B the output characteristic curve is drawn.