<\/span><\/h3>\nThe bipolar transistor has three separately doped regions and two p-n junctions. Figure shows the basic structure of an npn bipolar transistor and pnp bipolar transistor, along with the circuit symbols. The three terminal connections are called the emitter emitter, base and collector collector. The width of the base region is small compared to the minority carrier diffusion length. Generally the sequence of length of the three regions are WB<\/sub> < WE<\/sub> < WC<\/sub>.<\/p>\n![\"Device](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/device-structure.jpg\")
<\/p>\n
Note:<\/strong><\/p>\nAn important point to note from the devices shown in figure is that bipolar transistor is not a symmetrical device. Although the transistor may contain two n-regions or two p-regions, the impurity doping concentrations in the emitter and collector are different and the geometry of these regions can be vastly different.
\n<\/strong><\/p>\nThere are few specific features that differentiate one region from the other region. That are<\/p>\n
<\/span>Emitter Region<\/span><\/h3>\n\n- A region which supplies or emits majority carriers, for example in pnp transistor emitter will supply holes and in npn transistor it supplies electrons.<\/li>\n
- Emitter is heavily doped, so that it can emit large number of carriers.<\/li>\n
- Impurities are added in the ratio 1 : 103<\/sup>.<\/li>\n<\/ul>\n
<\/span>Collector Region<\/span><\/h3>\n\n- A region which receives or collects majority carriers coming from emitter.<\/li>\n
- It is moderately doped and largest in size.<\/li>\n
- Large collector will help in rapid transfer of heat to the surroundings.<\/li>\n
- In a transistor collector junction develops large amount of heat because it operates at higher current and higher voltage.<\/li>\n
- If collector is lightly doped it\u2019s conductivity will decrease, which is undesired.<\/li>\n
- If collector is heavily doped breakdown voltage of collector junction will decrease which is also undesired. Therefore collector is moderately doped so that it has better conductivity and collector junction has higher breakdown voltage.<\/li>\n<\/ul>\n
<\/span>Base Region<\/span><\/h3>\n\n- A region through which majority carriers travel from emitter to collector.<\/li>\n
- Base is lightly doped and small in size, i.e. it has narrow width (10\u20136 m).<\/li>\n
- The number of electron-hole recombination inside base will be reduced by keeping it narrow in width and by doping it lightly.<\/li>\n
- Impurities are added in ratio 1 : 108<\/sup>.<\/li>\n<\/ul>\n
<\/span>Early Effect ![\"Early](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/early-effect.jpg\")
<\/span><\/h3>\nWhen BJT is biased in active region the emitter junction (JE<\/sub>) is forward-biased but the collector junction is reverse-biased, then in figure the barrier width at JE<\/sub> is negligible as compared to space charge width W at JC<\/sub>.
\nThe transition region at junction is a region of uncovered charges on both sides of junction at positions occupied by impurity atoms. As the voltage applied across the junction increases, transition region penetrates deeper into collector and base. As neutrality of charges must be maintained, so the number of uncovered charges on each side remains equal. Since the doping in base is substantially smaller than that of collector, the penetration of the transition region into the base is much larger than that in collector. Hence the collector depletion region is neglected in figure, and all immobile charges are indicated in base region.<\/p>\nIf metallurgical base width is WB, then the effective electrical base width is . W’B<\/sub> = WB<\/sub> – W. This modulation of effective base width by reverse bias VCB<\/sub> is known as the Early effect Early effect or Base width modulation.<\/p>\n<\/span>Consequences of Early Effect<\/span><\/h4>\nThe decrease in W’B<\/sub> with increase in reverse collector voltage has three consequences.<\/p>\n1. When |VCB<\/sub>| is increased the effective base width of transistor decreases so there are less chances of recombination of charge carriers within the base region. As a result \u03b1 increases with increasing\u00a0|VCB<\/sub>|.<\/p>\n2. With decrease in base width the concentration gradient of minority carriers is increased within the base. As we have,
\n![\"Diffusion\"](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/diffusion.jpg\")
where W’B<\/sub> is effective basewidth.<\/p>\n![\"](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/conceptual-circuit.jpg\")
<\/p>\n
consequently IE<\/sub> also increases. Hence we see that IE<\/sub>\u00a0increases due to increase in gradient of concentration of holes.
\nAlso in BJT, voltage applied across one junction has effect on current passing through other junction therefore junctions JE<\/sub> and JC<\/sub> are called interactive junctions.<\/p>\n3. At large value of |VCB<\/sub>| depletion region can fully occupy the base region or in other words for extremely large voltages, W’B<\/sub> may be reduced to zero. This phenomenon is known as punch through or reach through.<\/p>\nWhen punch-through occurs effective base width becomes zero and collector region gets electrically shorted to emitter. Due to this shorting, the negative voltage applied at collector reaches emitter also. This results in heavy current flow which can damage the transistor.<\/p>\n
<\/span>Instability in Collector Current<\/span><\/h3>\nIn the preceding section we have seen that for faithful reproduction of the input signal we need a stable Q-point. In other words we can say that dc collector current should be fixed in a particular circuit. But as we know that collector current can be given by the following equation:<\/p>\n
IC<\/sub> = \u03b2IV<\/sub> + (1 + \u03b2)ICO<\/sub><\/p>\nIC<\/sub> can be unstable due to following reasons.<\/p>\n1. Variations in ICO<\/sub> :
\n<\/strong>ICO<\/sub>\u00a0is reverse saturation current of collector junction. It increases with increase in temperature due to increase in the concentration of minority carriers<\/p>\nIf temperature increases by 1\u00b0C then ICO<\/sub>\u00a0increases by 7%
\nFor each 10\u00b0C rise in temperature ICO<\/sub> doubles.<\/p>\nHence we see that variation in temperature causes change in ICO<\/sub> and as a result IC<\/sub>\u00a0also varies.<\/p>\n2. Variations in VBE<\/sub>:
\n<\/strong>VBE<\/sub> is forward voltage of emitter junction. When temperature increases by 1\u00b0C, VBE<\/sub> decreases by 2.5 mV.
\n<\/strong><\/p>\n![\"Variations\"](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/variations.jpg\")
<\/p>\n
Hence changes in temperature create change in VBE<\/sub> due to which base current (IB<\/sub>) changes. Hence, from above equation we conclude that collector current IC<\/sub>\u00a0also varies.<\/p>\n3. Variations in \u03b2:
\n<\/strong>\u03b2 varies either due to transistor replacement or due to variations in temperature.
\n(a) When a transistor is replaced with another transistor, \u03b2 value will vary because it is practically difficult to find two transistors having exactly equal \u03b2.
\n(b) For a given transistor \u03b2 also increases with increase in temperature.<\/p>\n\n- Hence from above discussion we conclude that Ic<\/sub> is unstable due to variations in Ico<\/sub>, \u03b2, and VBE<\/sub> i.e.<\/li>\n<\/ul>\n
Ic<\/sub> = f(Ico<\/sub>, VBE<\/sub>, \u03b2)<\/p>\n\u201cThe above equation is read as Ic<\/sub> is a function of Ico<\/sub>, \u03b2 and VBE<\/sub>.\u201d<\/p>\nBy using partial differentiation, we have<\/p>\n
![\"Variations\"](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/variations-1.jpg\")
<\/p>\n
where S, S\u2032 and S\u2033 are the stability factors.<\/p>\n
From above equation we conclude that for greater stability in Ic<\/sub>, \u2206Ic<\/sub> should be smaller which is possible when stability factors are smaller.<\/p>\nInstability in Ic<\/sub> has two undesired effects:<\/p>\n(a) Operating point will drift or vary along the load line which may result in distorted output and in worst case operating point may move into saturation or cut-off region.<\/p>\n
(b) Thermal runaway may occur which damages a BJT.<\/p>\n
Hence collector current (Ic<\/sub>) should be made stable with either stabilization or compensation methods.<\/p>\n<\/span>Calculation of Stability Factor (S)<\/span><\/h4>\nStability factor (S) is rate of change of Ic<\/sub> with respect to reverse saturation current Ico<\/sub>,<\/p>\n![\"Stability](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/stability-factor.jpg\")
<\/p>\n
Differentiating above equation with respect to Ic<\/sub> assuming \u03b2 and VBE<\/sub> are constant<\/p>\n![\"Stability](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/stability-factor-1.jpg\")
<\/p>\n
<\/span>BJT Biasing<\/span><\/h3>\nBiasing refers to providing appropriate DC voltages and DC currents to an electronic device to operate it in a desired way. Biasing of BJT is done for following purposes:<\/p>\n
(i)To operate the BJT in active region so that it can be used as an amplifier.
\n(ii) To maintain Ic<\/sub>\u00a0stable so that operating point does not drift and thermal runaway cannot take place.
\nCommonly used biasing circuits are:
\n(i) Fixed bias circuit.
\n(ii) Collector to base bias circuit.
\n(iii) Self bias circuit.<\/p>\n<\/span>Fixed Bias Circuit<\/span><\/h3>\nIn figure below, the base current is derived from supply voltage Vcc<\/sub> by resistor RB<\/sub>. This type of biasing is called fixed bias as Vcc<\/sub> and RB<\/sub> are fixed quantities.<\/p>\n![\"](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/fixed-bias-circuit.jpg\")
<\/p>\n
![\"Stability](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/stability-factor-2.jpg\")
<\/p>\n
<\/span>Collector to Base Bias Circuit<\/span><\/h3>\nAn improvement in bias stability is obtained if tapping for bias is taken from the collector terminal instead of from the collector supply point as shown in figure.<\/p>\n
![\"Base](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/base-bias-circuit.jpg\")
<\/p>\n
<\/span>Stability Factor<\/span><\/h4>\nTo calculate stability factor, differentiating equation (3.11) with respect to IC<\/sub> ; we have<\/p>\n\u2202IB<\/sub>\/\u2202IC<\/sub>= -RC<\/sub>\/RB<\/sub> + RC<\/sub><\/p>\nPutting above value in equation (3.7), we have<\/p>\n
![\"Stability](\"https:\/\/study.madeeasy.in\/wp-content\/uploads\/2024\/07\/stability-factor-3.jpg\")
<\/p>\n
<\/span>Advantage<\/span><\/h4>\nHere stability factor is smaller than (\u03b2 + 1), hence an improvement in stability is obtained over fixed bias circuit.<\/p>\n
<\/span>Disadvantage<\/span><\/h4>\n\n- Stability factor depends upon RC<\/sub>. If RC<\/sub> becomes smaller or zero then stability factor becomes very large and IC<\/sub>\u00a0does not remain stable.<\/li>\n
- Resistance RB<\/sub> connected from collector to base causes negative feedback due to which voltage gain of the amplifier circuit decreases.<\/li>\n<\/ul>\n