Question

What is junction transistor? Make the necessary diagram and explain the working of a p-n-p transistor.

Answer 1

Junction Transistor:

A simple junction transistor generally is a unit crystal of an extrinsic semiconductor (silicon or germanium) in which three regions of different conductivity are present. The width of the middle region is less then the other two regions width and the nature of the semiconductor in this region is different than the other two regions. In this way we get two type of junction transistors which are called PNP and NPN respectively. In any PNP transistor, there are two P-types semiconductor regions and a N-types semiconductor is sandwiched between them with very less width. Similarly in an NPN transistor there are two N types semiconductor regions and a P-type semiconductor is sandwiched between these two with very less width.

The middle region in both the types of transistors is called the base (B) and out of the outer parts one is called the emitter (E) and the other is called the collector (C).[Figure]. In all the three regions there is a metallic electrode and a lead through which the transistor can be connected to external electric field. To whichever part the lead is connected according to that, the name is kept E lead, B lead or C lead.

The emitter and collector are of the same type of semiconductor (P or N) but the quantity of impurities mixed in them is different; also the collector region is bigger in size than the emitter. Due to this physical and electrical properties are different meaning, they individually have their special work. And hence they cannot be interchanged and used.

Emitter is basically doped because its work is to provide majority charge carrier in large numbers to the base. There is very less doping in base; also it is of very less width so that it does not provide opposite nature charge carrier in large number for recombination to the majority charge carriers coming from emitter. The work of the collector is to collect the majority charge carriers which pass through the base. Here the doping is less than the emitter but more than the base meaning it is medium.

The connecting region of collector-base is bigger then the connecting region of emitter-base. Due to this it does the collection of the charge carriers very well and the transistor when used in the form of an amplifier helps in rapid diffusion of the generated heat. In this way there are two P-N junctions in these transistors which are E-B junction (emitter-base junction) and B-C junction (base-collector junction). Principally, both these junction have a relationship between them. (Figure).

But here it is necessary to acknowledge that if two different P-N junction are joined according to the figure then the transistor will never be received, because in transistor, both these junctions are in the same crystal, whereas two diodes when joint a single crystal is not possible. The width of the base of the transistors is in micrometer, hence both these junctions are very close.

Transistor when used in the form of an amplifier the emitter-base junction is always in forward biasing and base collector junction is always in reverse biasing. Due to this the majority charge carriers always flow from the emitter towards the base, but the current flowing in this junction can be of the nature flowing from emitter to base (E to B) or base to emitter (B to E), These two different directions of the electric current are used to differentiate between the signs of PNP and NPN transistor. These symbols are shown in the figure.

The line part having an arrow sign represents the emitter part, the middle line part represents the base and third line part represents the collector. The arrow head shows the flow of current. Since in forward biasing state the emitter base junction of the PNP transistor the majority of holes from the P-type emitter move towards the N-type base hence, the current will flow in the junction from E to B. Therefore, in the symbol of PNP transistor the arrow head shows from E to B. Similarly, in NPN transistor in reverse biasing state the majority number of electrons move form N-type emitter to P-type base hence the current will be in the direction from B to E.

Operation of Transistor:

For any transistor to work efficiently E-B junction is placed in forward hosing and B-G junction is placed in reverse biasing. In this state the conduction of the art he biasing output for transistor is in active state. Figure shows NPN and PNP transistor, in both the figure the depletion region related to both the junctions are shown.

Since, E-B junction is forward biased and the doping in emitter is intense hence, E-B junction will be narrow. Whereas B-C junction is reverse biased hence it is broader. The applied forward biased potential difference V_E value is small at E-B junction (0.5 – 1 V) and the applied reverse biased potential difference V_CB value is more at B – C junction (5 to 15 V).

If we consider the circuits of PNP transistor shown in the figure. Since, E-B junction is forward biased hence E region’s (P-type) majority charge carriers ‘holes’ will diffuse through this junction in large numbers. This is known as the injection of the holes from the emitter to the base. Similarly in the N-type semiconductor the electrons pass through the junction and reach emitter. The direction of both charge carriers is in opposite direction but the current related to them flows from E to B. This current is called emitting current I_E which is both due to the holes and electrons. But due to less doping in base region for PNP transistor this current is basically due to holes.

The nature of the injected holes from the emitter to the base is of recombination with the electrons present in the base. But. due to the thin base and less dopped base very less hole (lees than 5%) recombine with the electrons, and majority of holes pass base collector junction and reach the collector. Since the collector terminal is negative hence these holes reach the collector terminal very easily and in this way generate collector current I_C.

Some holes injected form the emitter to the base recombine with the electrons in the base and generate the base current I_B of very less value. For every electron destroyed in the process of recombination the negative end joined with the base of battery V_EB release one electron in the base. Therefore, here base current I_B flows through the base terminal B. For a PNP transistor the I_s current direction I_E , I_B and I_C are shown in the figure.

For the complete transistor shown in the figure if Kirchhoffs current law is used then, it is very clear that emitter current will be equal to the sum of base current and collector current, meaning;

I_E = I_B + I_C …………… (14)
Here I_B << I_E and I_B << I_C Since, E-B junction is forward biased hence its forward resistance, is very less, and B-C junction is reverse biased hence its reverse resistance is maximum. Due to this it is observed that at emitter junction current I_E will be more than the current collector junction. But from equation (14) I_B is very less hence; I_C ≈ I_E. Hence from the view of operation transistor is a device which transfers the current I_E from the circuit of very low resistance forward biased E-B junction) to the high resistance (reverse biased B-C junction keeping the same value I_C (I_C ≈ I_E). For the transfer process of current this device is named by the combination of two words transfer resistor which is very well known as transistor.

In other words, it can also be said that in transistor the process of operation of current at B-E junction is very high on the collector current. Since the value of V_EB is high hence both emitter current and collector current are more.

The above description can also be used for the active operation of NPN transistor. In this transistor majority charge carriers will be electron; which when injected form the emitter of N-type to the base of P-type reach the collector and produce collector current I_C. Figure shows the directions for I_E, I_B and I_C for an NPN transistor.

The emitter current in a NPN transistor is mainly because of electrons. Whereas both transistors PNP and NPN are used but due to the mobility of electrons being higher than the holes, in high frequency circuits NPN transistors are more effective.