The transistor

In 1948 some work was carried out at the Bell Telephone Laboratories in America that has changed our lives. This was the invention of the transistor by Shockley, Brattain and Bardeen.

The transistor, basically a semiconductor triode, consists of a thin central layer of one type of semiconductor between two relatively thick pieces of the other type. The junction transistor can be of two types, as shown in Figure 1: pnp or npn. The pnp transistor consists of a very thin piece of n-type material sandwiched between two pieces of p-type, while the npn transistor has a central piece of p-type. The pieces at either side are called the emitter and the collector while the central part is known as the base. The base is lightly doped compared with the emitter and the collector, and is only about 3-5 μm thick.

From now on we will consider only the npn transistor as it is now in more common use in schools. The npn silicon transistor is connected into the circuit as shown by Figure 2. The emitter-base junction is forward biased and the base- collector junction is reverse biased.

When the base-emitter voltage is 0.6 V current will flow through the transistor, electrons flowing through the base from the emitter to the collector. No current will flow without this base-emitter voltage, since it is needed to overcome the potential barrier formed at the junction. Electrons flow into the collector, although the base- collector junction is reverse biased because the base is very thin.


You should see that the emitter current (I_E) is the sum of the base current (I_B) and the collector current (I_C):

The collector current (I_C) is usually over 99% of I_E and I_B is less than 1 per cent.

The name 'transistor' comes from the words 'transfer of resistance': the emitter-base junction is forward biased and therefore has a low resistance, while the base-collector is reverse biased and has a high resistance.

The properties of the transistor described above lead us to consider it as a current amplifier.

Example circuits with the npn transistor

For its basic operation, the circuit is set up as shown in Figure 4, and the value of R is chosen so that the transistor is switched on, that is, the potential at the base is at least 0.6 V.

Lamp L₂ lights but L₁ does not, showing that the collector current must be much larger than the base current.

If L₁ is removed, however, L₂ goes out because no potential is being applied to the base.

The transistor as a switch

A transistor will not conduct (that is, no current will flow through from the collector to the emitter) unless there is a also potential difference between the emitter and the base of at least 0.6 V. This property enables the transistor to be used as a switch: it is 'on' when the base-emitter potential difference is bigger than 0.6 V and 'off' when it isn't.

If you consider the circuit in Figure 5, then when the transistor is off, that is, there is no current flowing through it, the potential difference across the emitter-collector (V_CE) is high. As soon as the transistor starts to conduct this potential difference falls to very close to zero (Figure 6).

Therefore the output potential difference (V_CE) is small when the input potential difference (V_BE) is large, and large when the input potential difference is small, that is less than 0.6 V. This is the basic NOT logic gate circuit. (see the section on Logic gates in the Foundation level for a further treatment).

We will now consider two circuits in which the switching action of a transistor is important.

(a) Making a light come on in the dark

The circuit includes a light-dependent resistor (LDR), the resistance of which changes with illumination. A table showing the variation for a light-dependent resistor in common use is shown below.

The circuit used is shown in Figure 7. Since the resistance of the light-dependent resistor varies so will the voltage drop across it, and therefore the potential at the base will change.


The less light that shines on the LDR the higher its resistance, and therefore the larger V_BE will be. If this is above 0.6 V the transistor switches on, and so when the LDR is in darkness the transistor conducts and the lamp L comes on.



(b) Moisture detector

The circuit is shown in Figure 8. If the base circuit is broken at XY then the transistor is off, but if the probes XY are placed in a conducting liquid the transistor switches on. This could be used as a liquid level indicator for a blind person, the lamp L being replaced by a buzzer and the two contacts being placed at a suitable level in a cup or bowl.

Saturation

We have seen that when V_BE >0.6 V the transistor switches on, and Figure 9 shows that as V_BE is increased above this value V_CE falls and reaches a steady value (close to zero) when V_BE is about 1.4 V. Any further increase in V_BE does not change V_CE.


In this condition the transistor is said to have bottomed or be saturated.