The invention of the transistor may be the most important invention of the 20th century. - Gordon Moore
The Bipolar Junction Transistor (BJT) was invented in 1948 while the point contact transistor was invented in 1947. All these inventions happened at Bell Labs. Since the focus is to build a computer probably the only type of transistor which will be used is the BJT NPN transistor.
As the name suggests this type of transistor has a P-type doped silicon semiconductor sandwiched between two N-type doped silicon. It is referred to as bipolar because there are two PN Junctions in this device.
There are three terminals in an NPN BJT, each connecting to the N, P, and N. They are called Emitter, Base, and Collector. The middle P-type is called the base. The base is very thin and extremely lightly doped of P-type. The collector is physically the largest part of the NPN BJT, it surrounds the entire transistor as shown in the simplified schematic below. It is called collector because it makes sure that the electrons which enter the base cannot escape without being collected. The emitter is heavily doped but smaller in physical dimension. The heavily doped emitter increases the emitter injection efficiency which is the ratio of carriers injected by emitter to those injected by the base.
The transistor works as a switch in principle. When no potential difference is applied the two PN Junctions are in equilibrium and there is a large depletion zone where there is no conduction. The depletion zones are wide and there is no movement of electrons or holes.
But when a small voltage is applied to the base terminal (approximately 0.6V to 0.7V) the transistor switches to an active mode. There is an injection of electrons from the emitter to the base and these move on (technically called diffusing) into the collector. The circuit diagram for the active mode is shown below
From the band diagram we can see the energy levels and the diffusion of electrons. The mental model here is that a small voltage to the base causes the depletion region at the two PN Junctions to shrink thus allowing conduction to begin. This threshold voltage is 0.6V to 0.7V. Thus, an NPN BJT acts as a switch for electrons to be emitted from emitter to the collector or in other words current to flow from collector to emitter. We also say that a small base current leads to amplification of the collector-emitter current flow which is much larger in value.
I have built this very basic circuit on a breadboard to show how an NPN BJT can act as a switch. This is where we see for the first time where we get our 0s and 1s from! We have gone all the way from the lepton, the electron to generating a binary signal.
