Bipolar Junction Transistor: Current Flow Mechanism in NPN and PNP, Common Base and Emitter Configuration

Introduction TO Bipolar Junction Transistor

The transistor is a solid- state device whose operation depends upon the flow of electric charge carries within the solid. A transistor consists of two P-N junctions formed by sandwiching either P-type or N-type semiconductor between a pair of opposite types. Accordingly, there are two types of transistors, namely:

1. N-P-N transistor

2. P-N-P transistor

A  N-P-N transistor is compared of two N-type semiconductors separated by a thin section of P-type as shown in the figure below.

However, a P-N-P transistor is formed by two P-sections separated by a thin section of N-type.

Each type of transistor has two P-N junctions-one junction between the emitter and base called emitter-base junction or simply the emitter junction and the other junction between the base and collector called the collector-base junction or simply the collector junction. Thus, a transistor is like two P-N junction diodes connected back to back. The two junctions give rise to three regions provided with three terminals called the emitter, base and collector.

Transistor Terminals

1. Emitter

Its main function is to supply majority charge carriers (electrons in case of NPN transistors and holes in case of PNP transistors) to the base. The emitter is always forward biased with respect to base so that it is able to supply majority charge carries to the base. The emitter is heavily doped so that it may be able to inject a large number of charge carriers. It is of moderate size in order to maintain heavy doping without diluting it or mesh formation in it.

• Collector

It is the right hand section of the transistor and its main function is to collect majority charge carriers. Collector is always reverse biased so as to remove the charge carriers away from its junction with the base. It is large in size to withstand the temperature generated at the collector and is moderately doped.

• Base

It is the middle section of the transistor and is very lightly doped to reduce the recombination within the base so as to increase collector current and is very thin in comparison to either emitter or collector so that it may pass most of the injected charge carriers to the collector.

Current Flow Mechanism In NPN and PNP Transistor

• Current Flow Mechanism In NPN Transistor:

Figure below shows the NPN transistor with forward bias to emitter base junction and reverse bias to collector base junction. The forward bias causes the electrons in the N-type emitter to flow towards the bias.

This constitutes the emitter current I_E. As these electrons flow through the P-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore only a few electrons combine with holes to constitute base current I_B. The remainder electrons cross over into the collector region to constitute collector current I_c. In this way, almost the entire emitter current flows in the collector circuit. It is clear that emitter current is the sum of collector and base currents I.e.  I_E=I_B+I_C.

In actual practice, a very little current (a few μA) would flow in the collector current. This is called collector cut off current and is due to minority carriers.

• Current Flow Mechanism In PNP Transistor

Figure below shows the basic component of a PNP transistor.

This constitutes the emitter current I_E. As these holes cross into N-type base, they tend to combine with the electrons. As the base is lightly doped and very thin, therefore, only a few holes combine with electrons. The remainder cross into the collector region to constitute collector current I_C. In this way, almost the entire emitter current flows in the collector circuit. It may be noted that current conducting within PNP transistor is by holes. However, in the external connecting wires, the current is still by electrons.

Input And Output Characteristics Of CB And CE Transistor

A. Common Base (CB) Configuration

In common base configuration, input is connected between emitter and base and output is taken across collector and base. Thus, base is common between input and output circuits.

In this arrangement, the emitter- base junction is forward biased and collector- base junction is reversed biased. This emitter current I_E flows in the input circuit and collector current I_C in the output circuit.

The ratio of collector current, I_C to the emitter current, I_E is called current amplification factor, α.

The collector current consists the current produced by normal transistor action i.e. component depending upon emitter current (αI_E) which is produced by majority carriers and the leakage current due to movement of minority carriers across base-collector junction on account of it being reverse biased. This leakage current flows in the same direction as that of I_CBO and is denoted by I_CBO.

∴ Total collector current (I_C) = αI_E+ I_CBO

The current I_CBO sometimes referred to as I_CO, is usually very small and may be neglected intransistor 

calculation.

So,

α = ( I_C – I_CO)/ I_E = I_C / I_E

Neglecting I_CO in comparison with I_C; we have,

I_B=I_E-I_C = I_E– (αI_E+ I_CBO) = ( 1 – α) * I_E – I_CO  [∴ I_C = αI_E+ I_CBO]

1. Input Characteristics

The curve drawn between emitter current I_E and emitter base voltage V_EB for a given value of collector base voltage V_CB is known as input characteristics. For determination of the input characteristics, the output voltage V_CB is maintained constant and the input voltage V_EB is set at several convenient levels. Figure shows the input characteristics of a typical transistor in CB arrangement.

The following points can be noted from these characteristics:

• Emitter current , I_E increases rapidly with small increase in emitter base voltage V_EB. It means that input resistance is very small.
• The emitter current is almost independent of collector base voltage V_CB. This leads to the conclusion that emitter current is almost independent of collector voltage.

Input resistance =  (Change in emitter – Base voltage)/( Change in emitter current)

∴ r_i = ▲V_EB / ▲I_E

As a very small V_EB is sufficient to produce a large flow os emitter current I_E, therfore input resistance is quite small of the order of a few ohms.

2. Output Characteristics

The curve drawn between collector current I_C and collector-base voltage V_CB for a given value of emitter current I_E is known as output characteristics.

For determination of output characteristics, the emitter current is held constant at each of several fixed levels. Figure aside shows the output characteristics of a typical transistor in CB arrangement.

The noteworthy points regarding output characteristics of common base configuration are given below:

• The collector current I_C varies with V_CB only for very low voltage but transistor is never operated in this region.
• In cutoff region (emitter and collector junctions forward biased) small collector current I_C flow even when emitter current I_E = 0. This is the collector leakage current I_CBO or I_CO.
• In active region ( emitter forward biased and collector reverse biased) collector current I_C is almost equal to I_E and appears to remain constant when V_CB is increased. In fact , there is very small increase in I_C with increase in V_CB. This is because the increase in V_CB expands the collector- base depletion region and thus shortens the distance between the two depletion regions. Transistor is normally operated in this active region.
• A very large change in collector voltage causes a very small change in collector current I.e. output resistance of CB configuration is very high-the dynamic output resistance r_out being given as the ratio of ▲V_CB and ▲I_C for a given value of I_E.

I.e. r₀ = ▲V_CB / ▲I_C  at constant I_E

• In saturation region ( both emitter and collector junction forward biased) collector current I_C flows even when V_CB = 0. Collector current I_C is reduced to zero when V_CB is increased negatively.

B. Common Emitter (CE) Configuration

In common emitter configuration, input is connected between base and emitter while the output is taken between collector and emitter. Thus, emitter is common to input and output circuits. In this configuration, the bias voltage are applied between collector and emitter and base and emitter. Emitter-base junction is forward biased and the base is made more positive than the emitter by V_BB, collector emitter junction is reverse biased and the collector is made more positive than emitter by V_CC. The value of V_CC must be greater than that of V_BB.

In this arrangement, the base current I_B flows in the input circuit and collector current I_C flows in the output circuit.

The ratio of change in collector current (output current) and change in base current (input current) is called the base current amplification factor, β.

Also,

β = I_C / I_B = I_C / (I_E – I_C) = (I_C / I_E) / (1- I_C/ I_E) = α / ( 1- α)

In CE configuration, a small collector current flows even when base current is zero. This is the collector cutoff current and denoted by I_CEO. I_CEO must be larger than I_CBO. The current equations are:

I_E = I_B + I_C

And,

I_C = αI_E + I_CBO  = α(I_B + I_C) + I_CBO

Or, I_C(1- α ) = αI_B + I_CBO

Or, I_C = {α/(1- α) } * I_B + I_CBO/ (1- α) ———–(1)

And,

Total collector current,(I_C) =  βI_B + I_CEO —————(2)

Comparing equation (1) and (2); we have,(1

β = α / ( 1- α)

And,

I_CEO = {α /(1- α) } * I_CBO

Putting the value of I_CEO = {α /(1- α) } * I_CBO  in the equation (2); we have

I_C = βI_B +  {α /(1- α) } * I_CBO  

Or, I_C = βI_B +  (1+β )  * I_CBO  

1. Input Characteristics

The curve drawn between base current I_B and base-emitter voltage V_BE for a given value of collector- emitter voltage V_CE is known as the input characteristics. For determiantion of input characteristics, collector emitter voltage V_CE is held constant and base current i_B  is recorded for different values of base-emitter voltage V_BE.

The noteworthy points regarding input characteristics are given below:

• The input characteristics of CE transistors are quite similar to those of a forward biased diode because the base-emitter region of the transistor is a diode and it is forward biased.
• In comparison to common base arrangement base current increases less rapidly with the increases less rapidly with the increase  base- emitter voltage, V_BE. This indicates that input resistance is larger in common emitter configuration that in common base configuration. Due to initial non-linearity of the curve, input resistance varies from point to point in the initial part of the characteristics. Its value over the linear part of the curve is of the order of few hundred ohms.
• An increment in value of V_CE  causes the input current I_B to be lower for a given level of V_BE. This is because the higher levels of V_CE provide greater collector-base junction reverse bias causing depletion region penetration into the base and thus reducing the distance between the collector- base and emitter-base regions. As a resuilt, more of the charge carriers from the emitter flows across the collector-base junction, and few flow out across the base lead.

The ratio of change in base-emitter voltage to the resulting change in base current at constant collector-emitter voltage is known as dynamic input resistance.

I.e. r_in = ▲V_BE / ▲I_B  at constant V_CE

2. Output Characteristics

Output characteristics for a common emitter transistor in the curve  drawn between collector current I_C and collector- emitter voltage V_CE for a given value of base current I_B.

The noteworthy points regarding output characteristics are given below:

• The collector current I_C varies with V_CE for V_CE between 0 to 1 V  and then become almost constant and independent of V_CE . The transistor are always operate above 1V.
• Output characteristics in CE configuration has same slope while CB configuration has almost horizontal characteristics. This indicates the output reistance in case of CE configuration is less than in CB configuration.
• Moderate output to input impedence ratio makes this configuration an ideal one for coupling between various transistor stages.
• With low values ( ideally zero) or V_CE, the transistor is said to be operated in saturation region and in this region base current I_B does not cause a corresponding change in collector current I_C.
• With much higher V_CE, the collector-base junction completely breaks down and because of this avalanche breakdown collector current I_C increases rapidly and the transistor gets damaged.
• In cutoff region, small amount of collector current I_C flows even when base current I_B =  0. This is called I_CEO. Since, main current I_C is zero, the transistor is said to be cutoff.

The ratio of change in collector-emitter voltage to the change in collector current at constant base current is known as dynamic output resistance. It is calculated as the reciprocal of the slope of output characteristic at a given V_CE.

i.e. r_out = ▲V_BE / ▲I_C  at constant I_B

DC current gain, (β) = I_C / I_B

AC current gain, (β₀) = ▲I_C / ▲I_B  at constant V_CE