Bipolar Junction Transistors (BJTs)

Introduction

The transistor is a solid state device, whose operation depends upon the flow of electric charge carriers within the solid. Transistor is capable of amplification and in most respect it is analogous to a vacuum triode. The main difference between the two is that the transistor is a current controlled device whereas vacuum triode is a voltage controlled device. The transistor replaced vacuum tubes in almost all applications. The reasons are obviously its advantages over vacuum tubes such as compact size, light weight, rugged construction, more resistive to shocks and vibrations, instantaneous operation (no heating required), low operating voltage, high operating efficiency (no heat loss) and long life with essentially no ageing effect if operated within permissible limits of temperature and frequency. However, transistors, in comparison to vacuum triodes, have some drawbacks also such as loud hum noise, restricted operating temperature (up to 75^oC) and operating frequency (up to a few MHz only).

It consists of a silicon or germanium (preferably silicon because of its smaller cutoff current I_CBO, smaller variations in I_CBO due to variations in temperature and higher operating temperature) crystal in which a layer of N-type material is sandwiched between two layers of P-type material, as shown in Figure 1 (a). Alternatively a transistor may consist of a layer of P-type material sandwiched between two layers of N-type material sandwiched between two layers of N-type material as shown in Figure 1 (b). In the former case the transistor is referred to as a P-N-P transistor and in latter case, as an N-P-N transistor. Each type of transistor has two P-N junctions - one junction between the emitter and base, called the 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, as shown in Figure 1. The emitter, base and collector correspond in a general way to the cathode, grid and plate or anode of a vacuum triode.

Transistor Terminals

As mentioned earlier , transistor is a single crystal in which there are two P-N junctions as shown in Figure 1. The idea behind is to have first section to supply the charges (either holes or electrons) to be collected by the third section through middle section. One side section supplying free charges is called the emitter, other side section collecting these charges is called the collector and the middle section which is formed between the emitter and collector is called the base.

1. Emitter: It is left hand section (or region) of the transistor and its main function is to supply majority charge carriers (electrons in case of N-P-N transistors and holes in case of P-N-P transistors) to the base. The emitter is always forward biased biased w.r.t. base so that it is able to supply majority charge carriers to the base. The emitter is heavily doped so that it may able to inject majority charge carriers. It is of moderate size in order to maintain heavy doping without diluting it or mesh formation in it.

2. 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 moderately doped to avoid the chances of mesh formation even after taking the carriers from the emitter. Large in size to withstand the temperature generated at the collector.

3. 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 the collector current and is very thin (of the order of µm) in comparison to either emitter or collector so that it may pass most of the injected charge carriers to the collector.

Transistor Action or Working

Transistors of both types (P-N-P and N-P-N) behave exactly the same way except change in biasing and majority carriers. In P-N-P transistors the conduction is by holes whereas in N-P-N transistors conduction is by electrons. However, NPN transistors are preferred due to their high frequency response. Basic connections and biasing for N-P-N and P-N-P type transistors are shown in Figure 2 (a) & (b) respectively. Let us discuss an N-P-N transistor, which is in common use.

The emitter is forward biased and as a result a large forward current flows across the emitter junction due to flow of majority carriers i.e., electrons from the emitter region enter the base region and holes from base region into the emitter region. However, since the conductivity of emitter region is larger than that of base region, the electrons from the emitter region out the number of holes from the base region. It may be assumed with good accuracy that forward current in the emitter junction flows due to the movement of electrons from the emitter region to the base. We can, therefore, say that electrons from the emitter region due to forward bias in the emitter junction cross into the base region where electrons are in minority. Injection of electrons makes the electrons concentration on the emitter junction very large and on the collector junction the concentration of the electrons is extremely small and as such the gradient of electrons concentration is very large in base. Injected electrons diffuse into collector region due to extremely small thickness of base which is much less than the diffusion length. Most of the electrons cross into the collector region. Collector is reverse biased and creates a strong electrostatic field between base and collector. The field immediately collects the diffused electrons which enter the collector junction. Flow of electrons into the base region when confronted with the holes, a few electrons (say 1 to 10%) combined and neutralize and rest of the electrons say 90 to 99% of the injected electrons diffuse into the collector region and collected by the collector electrode. To maintain the base neutrality base electrode provides equal number of electrons which have combined with holes and results in a base current. Thus the emitter current I_E is equal to the sum of collector current I_C and base current I_B. The ratio of collector current I_C to emitter current I_E is called the α and it is measure of the possible current amplification. In a transistor a cannot be higher than unity, but practical values of 0.95 to 0.99 are attained in commercial transistors.

For connecting batteries properly in the transistor circuits like following scheme may be remembered.

The battery polarities to the emitter and base are the same as the letter of the emitter ans base are the same as the letter of the emitter and base are impurity designations. Thus, in the P-N-P type, the emitter receives the P (positive) terminal, and base receives the N (negative) terminal. Similarly, in N-P-N type, the emitter receives the N (negative) terminal, the base receives the P (positive) terminal. The collector receives a battery polarity opposite to its impurity type designation, Thus P-type collector receives the N (negative) terminal of the battery and the N-type collector receives the P (positive) terminal.

Operation of P-N-P Transistor: The P-N-P transistor behaves exactly the same way as an N-P-N device, with the exception that the majority charge carriers are holes. As illustrated in Figure 2 (b), holes are emitted from the P-type emitter across the forward-biased emitter -base junction into the base. In the lightly doped N-type base, the holes find few electrons to recombine with. Some of the holes flow out via the base terminal, but most are drawn across the collector by the positive-negative electric field at the reverse-biased collector-base junction. As in the case of N-P-N device, the forward bias at the emitter base junction controls the collector and emitter currents.