Principle of Operation of Bipolar Junction Transistors (BJT)

Principle of Operation of Transistors

1. Unbiased Transistor

• An unbiased transistor means a transistor with no external voltage (biasing) is applied. Obviously, there will be no current flowing from any of the transistor leads.

• Since transistor is like two pn junction diodes connected back to back, there are depletion regions at both the junctions, emitter junction and collector junction, as shown in Fig. 4.3.1.

• During diffusion process, depletion region penetrates more deeply into the lightly doped side in order to include an equal number of impurity atoms in the each side of the junction.

• As shown in Fig. 4.3.1, depletion region at emitter junction penetrates less in the heavily doped emitter and extends more in the base region.

• Similarly, depletion region at collector junction penetrates less in the heavily doped collector and extends more in the base region.

• As collector is slightly less doped than the emitter, the depletion layer width at the collector junction is slightly more than the depletion layer width at the emitter junction.

• Barrier voltage is the voltage necessary to cause electrical conduction in a junction of two dissimilar materials.

• Like diodes, a barrier voltage exists within the transistor.

• The barrier voltage at each junction is positive on the n‒side and negative on p‒side.

• The barrier voltage across the junction in a silicon. transistor is about 0.7 volt and approximately 0.3 volt in a germanium transistor.

2. Biased Transistor

The transistor can be operated in four possible bias combinations depending on whether forward or reverse bias is applied to each junction. These are listed in Table 4.3.1.

Table 4.3.1 Operating regions and bias conditions

Inverse Active Mode

• The active mode of transistor is further classified as forward active and inverse active.

• In inverse active mode, B‒E junction (J_BE) is reverse biased and B‒C junction (J_BC) is forward bias.

• In this operating mode, the transistor is operating upside down; that is, the emitter is acting as the collector and the collector is operating as the emitter.

• Fig. 4.3.3 shows the bias conditions for four modes of operations of an npn transistor.

Fig. 4.3.3 Biasing conditions for four operating modes of npn transistor

• To bias the transistor in its active region, the emitter is base junction forward biased, while the collector‒base junction in reverse‒biased as shown in Fig:4.3.4.

• Fig. 4.3.4 shows the circuit connections for active npn region for both and pnp transistors.

• The externally applied bias voltages are V_EE and V_CC, as shown in Fig. 4.3.4, which bias the transistor in its active region. The operation of the pnp is the same as for the npn except that the roles of the electrons and holes, the bias voltage polarities and the current directions are all reversed.

• Note that in both cases the base‒emitter (J_E) junction is forward biased and the collector‒base junction (J_C) is reversed biased.

3. Operation of npn Transistor

• The base to emitter junction is forward biased by the d.c. source V_EE. Thus, the depletion region at this junction is reduced. The collector to base junction is reverse biased, increasing depletion region at collector to base junction as shown in Fig. 4.3.5.

• The forward biased EB junction causes the electrons in the n‒type emitter to flow towards the base. This constitutes the emitter current I_E. As these electrons flow through the p‒type base, they tend to combine with holes in p‒region (base).

• Due to light doping, very few of the electrons injected into the base from the emitter recombine with holes to constitute base current, I_B and the remaining large number of electrons cross the base region and move through the collector region to the positive terminal of the external d.c. source.

• This constitutes collector current I_C. Thus the electron flow constitutes the dominant current in an npn transistor.

• Since, the most of the electrons from emitter flow in the collector circuit and very few combine with holes in the base. Thus, the collector current is larger than the base current. The relationship between these current is given by

 I_E = I_C + I_B

• This relationship is also obtained by applying Kirchhoff's current law to the transistor of Fig. 4.3.4.

• Since it is a bipolar device, the collector current comprises two components: Majority and minority.

• The minority current component is called the leakage current and is given the symbol Ico (Ic current with emitter terminal open).

• The collector current, therefore, is determined in total by

 I_C = I_Cmajority +I_{CO minority}

 = I_C(INJ) + I_CO

 I_C(INT): It is an injected collector current due to majority carriers crossing the collector base junction.

4. Operation of pnp Transistor

• The pnp transistor has its bias voltages V_EE and V_CC reversed from those in the npn transistor. This is necessary to forward‒bias the emitter‒base junction and reverse‒bias the collector base junction.

• The forward biased EB junction causes the holes in the p‒type emitter to flow towards the base. This constitutes the emitter current I_E.

• As these holes flow through the n‒type base, they tend to combine with electrons in n‒region (base). As the base is very thin and lightly doped, very few of the holes injected into the base from the emitter recombine with electrons to constitute base current, I_B.

• The remaining large number of holes cross the depletion region and move through the collector region to the negative terminal of the external d.c. source. This constitutes collector current I_C. Thus the hole flow constitutes the dominant current in an pnp transistor.

5. Transistor Voltages

npn Transistor

• Fig. 4.3.7 shows the terminal voltages and its polarities for an npn transistor. The voltage between base and emitter is denoted as V_BE. For V_BE, base is positive than emitter because for npn transistor, the base is biased positive with respect to the emitter.

 Fig. 4.3.7 Voltage source connections for npn transistor

• The voltage between the collector and the emitter is denoted as V_CE and the voltage between the collector and the base is denoted as V_CB. Since collector is positive with respect to base and emitter the polarities are as shown in Fig. 4.3.7.

• Fig. 4.3.7 shows the npn transistor with voltage connections. The voltage sources are connected to the transistor with series resistors. These resistors are called current limiting resistors.

• The base supply voltage V_BB is connected via resistor R_B, and the collector supply voltage, V_CC is connected via resistor R_C.

• The negative terminals of both the supply voltages are connected to emitter terminal of the transistor.

• To make CB junction reverse biased, the supply voltage V_CC is always much larger than supply voltage V_BB.

pnp Transistor

• Fig. 4.3.8 shows the terminal voltages and its polarities for a pnp transistor. For a pnp transistor, the base is biased negative with respect to the emitter, and the collector is made more negative than the base.

• Fig. 4.3.8 shows the pnp transistor with voltage source connections. Like npn transistor voltage sources are connected with series resistors. The source voltage positive terminals are connected at the emitter with V_CC larger than V_BB to keep collector‒base junction reverse biased.

Junction Voltages

• In different conditions such as active, saturation and cutoff there are different junction voltages. The junction voltages for a typical npn transistor at 25 °C are given in Table 4.3.2.

• The entries in the table are appropriate for an npn transistor. For pnp transistor the signs of all entries should be reversed.

6. Transistor Currents

• The directions of conventional currents in an npn transistor are as shown in Fig. 4.3.9 (a) and those for a pnp are shown in Fig. 4.3.9 (b).

• It can be noticed that the arrow at the emitter of the transistor's symbol points in the direction of conventional current.

• Let us consider pnp transistor. The current flowing into the emitter terminal is referred to as the emitter current and identified as I_E. The currents flowing out of the collector and base terminals are referred to as collector current and base current, respectively.

• The collector current is identified as I_C and base current as I_B. For both npn and pnp transistors,

 I_E = I_B + I_C.

• Since I_B is very small, I_E and I_C are nearly equal; however, I_E > I_C.

7. Transistor Current Equations

• Fig. 4.3.10 shows an npn bipolar transistor biased in the forward‒active mode. Since the B‒E junction is forward biased, electrons from the emitter are injected across the B‒E junction into the base, creating an excess minority carrier concentration in the base.

Fig. 4.3.10 An npn bipolar transistor blased In the forward‒active mode; base‒emitter junction forward

• The base region is very narrow so that, in the ideal case, the injected electrons will not recombine with any of the majority carrier holes in the base. In this case, the minority carrier electron concentration in the base is a linear function of distance as shown in Fig. 4.3.11.

Fig. 4.3.11 Minority carrier electron concentration across the base region of an npn bipolar transistor blased in the forward‒active mode

• The electrons that are injected from the emitter region diffuse across the base, are swept across the base‒collector space‒charge region by the electric field, and are collected in the collector region creating the collector current.

• However, some carrier recombination does occur in the base. In this case, the electron concentration will deviate from the ideal linear curve, as shown in Fig. 4.3.11.

• To minimize recombination effects, the width of the neutral base region is kept small compared to the minority carrier diffusion length.

• Emitter current: Since the B‒E junction is forward biased, the current through this junction is an exponential function of B‒E voltage. Thus, we can write the current at the emitter terminal as

 I_E = I_SE exp (V_BE /V_T)

• V_T is the usual thermal voltage. The multiplying constant, I_SE, contains electrical parameters of the junction. It is directly proportional to the active B‒E cross sectional area.

• Collector current: It is proportional to the emitter current and is given by

 I_C = α_F I_E = α_F I_SE е^vBE/VT

• Base current: It is given as

 I_B = I_C/B_F = [ α_F I_SE е^vBE/VT ] / βF

Review Questions

1. Explain various voltage components of transistor.

2. Explain the various current components of the transistor.

3. Write equations for collector current (I_C) in terms of emitter current (I_E) and α_dc, and in terms of base current (I_B) and α_dc.

4. Draw a sketch to show the various current components in a NPN transistor and deduce the relation between various current components.

5. Sketch the symbols of transistors and mark the current directions.

6. Give the biasing arrangement for an NPN transistor to operate in the active region.

7. Sketch and explain the minority carrier electron concentration across the base region of an npn bipolar transistor biased in the forward‒active mode.

8. State expressions of I_C, I_E and I_B.

9. What is the need for biasing in the transistor?

10. Describe the working of PNP transistor.