The bipolar junction transistor

The bipolar junction transistor is the cornerstone of much of today's semiconductor electronics industry. This form of transistor has been in existence for many years and is still very widely used in electronic circuits. The bipolar transistor is very versatile and finds applications in many applications and at a wide range of frequencies.

Beginnings ....
The bipolar transistor dates back to the middle of the twentieth century when three scientists named Bardeen, Brattain, and Shockley working at Bell Laboratories in the USA discovered it. They had been researching an idea for a semiconductor field effect device, but they had been unable to make it work. They had not succeeded in making this idea work and as a result they decided to follow other lines of research and in doing this they developed the bipolar transistor. They succeeded in making it work in late 1947, and only a week after their initial discovery they demonstrated it in front of a group of executives at Bell.

Today the semiconductor industry is enormous and vast quantities of money are being invested in new semiconductor device developments. Although there are many new types of transistor, the bipolar junction transistor is still in very widespread use.

Structure
The bipolar transistor can be made from a variety of types of semiconductor. The original devices were made from germanium, but silicon is widely used today.

In essence a transistor consists of an area of either p type of n type semiconductor sandwiched between regions of oppositely doped silicon. As such devices can be either a p-n-p or an n-p-n configuration.

There are three connections, namely the emitter, base, and the collector. The base is the one in the centre and it is bounded by the emitter and collector. Of the two outer two the collector is often made larger as this is where most of the heat is dissipated.

The base derives its name from the first point contact transistors where the centre connection also formed the mechanical "base" for the structure. It is essential that this region should be as thin if high levels of current gain are to be achieved. Often it may only be about 1 um across.

The emitter is where the current carriers are "emitted", and the collector is where they are "collected".

Basic operation
The transistor can be considered as two p-n junctions that are placed back to back. In operation, the base emitter junction is forward biased and the base collector junction is reverse biased. When a current flows through the base emitter junction, a current also flows in the collector circuit. This is larger and proportional to the one in the base circuit. In order to explain the way in which this happens, the example of an n-p-n transistor is taken. The same principles are used for the p-n-p transistor except that the current carrier is holes rather than electrons and the voltages are reversed.

Operation of a bipolar junction transistor

The emitter in the n-p-n device is made of n-type material and here the majority carriers are electrons. When the base emitter junction is forward biased the electrons move from the n-type region towards the p-type region and the holes move towards the n-type region. When they reach each other they combine enabling a current to flow across the junction. When the junction is reverse biased the holes and electrons move away from one another resulting in a depletion region between the two areas and no current flows.

When a current flows between the base and emitter, electrons leave the emitter and flow into the base. Normally the electrons would combine when they reach this area. However the doping level in this region is very low and the base is also very thin. This means the most of the electrons are able to travel across this region without recombining with the holes. As a result the electrons migrate towards the collector, because they are attracted by the positive potential. In this way they are able to flow across what is effectively a reverse biased junction, and current flows in the collector circuit.

It is found that the collector current is significantly higher than the base current, and because the proportion of electrons combining with holes remains the same the collector current is always proportional to the base current. In other words varying the base current varies the collector current.

The ratio of the base to collector current is given the Greek symbol B. Typically the ratio B may be between 50 and 500 for a small signal transistor. This means that the collector current will be between 50 and 500 times that flowing in the base. For high power transistors the value of B is likely to be smaller, with figures of 20 not being unusual.

Summary
The transistor is a current operated device. It is the amount of current flowing in the base circuit that controls the amount of current flowing in the collector circuit. The other major three terminal semiconductor device, the field effect transistor is a voltage operated device. Here it is the potential on the gate that controls the current passing between the source and drain.