11.1. CONSTRUCTION OF A TRANSISTOR

Transistors consist of three sections of a semiconducting base material. The two outer sections being doped in opposite sense, the centre section - called the BASE. The outer two sections are called:

COLLECTOR and EMITTER.

fig. 119

Between these three sections and obviously between the three terminals too, there are two junctions of PN-type, these two junctions can be looked at as two diodes connected in opposite direction.

Connection basically:

If the transistor is used in an electronic circuit generally the collector-base junction is connected in reverse direction and the base-emitter junction is connected in forward direction.

Function (simplified):

If there flows a considerable small current through the base-emitter junction, this allows a considerable strong current to flow from collector to emitter. The sketch in fig. 122 shows this behaviour very simplified.

fig. 122a

fig. 122b

Notation of transistors:

To find out which type of transistors we deal with we have to know the notation of transistors.

In most cases you find a combination of letters and figures on the case of the transistor. To find out exactly the specifications of this very transistor. You have actually to use a databook.

But very often the notation gives us hints about what the transistor is normally used for.

How does the transistor do its job?

Incidental remark:

Here will be derived the function of an NPN-transistor and an PNP-transistor in a similar manner.

It must be kept in mind, that the base layer in any transistor is very tiny in comparison with the two other layers. To explain the function of a transistor it must be dealt with as being connected to voltage sources. There are different possibilities to connect it two voltage sources. The possibility used here is the so called COMMON EMITTER CONNECTION.

This connection is the most often used one in practice. Therefore the other connections will not be dealt with here.

By the way: the transistor technology is still developing and this development led to new transistors, which can be used in common-emitter connection even in cases where in former times another connection was necessary.

Derivation of the function:

1. If a transistor is connected to a voltage-source as shown in fig. 123 it will not conduct any current, because one of the PN-junctions is always biased in reverse direction.

fig. 123

2. As long as the voltage between base and emitter is connected in reverse direction there will not flow any current as well - as we know already from our explanations about a diode.

fig. 124

3. As soon as a voltage source is connected to the base-emitter junction in forward direction, and the voltage reaches a level higher than the so-called threshold voltage, of this junction, there will start to flow a current through this junction.

This current causes within the transistor a very special effect:

The chargecarriers (electrons or “holes”) enter the base-region. But as the base region is very narrow, these chargecarriers comming in big numbers from the wide emitter-region cannot be channelled totally through the base terminal.

So they invade the depletion-layer of the collector-base junction. But at the collector terminal with a strong polarity of the voltage-source connected to the collector and the emitter-terminals waiting for those charge-carriers, and attracts them through the depletion layer causing a current to flow through a PN-juncion (collector-base junction) in reverse direction.

fig. 126

The current channeled from collector to emitter (or vice versa) by the base current is called the COLLECTOR CURRENT.

This collector current is depending on the amount of BASE-CURRENT which was the origin of the effect.

As soon as the base current will be changed or interrupted, the collector current will change proportionally or will be interrupted as well.

The big advantage of the transistor is: the collector current is between 20 to 200 times bigger than the base - current.

SUMMING UP:

The transistor is a device which makes it possible to control.

A big current (COLLECTOR CURRENT) by a very small current (BASE CURRENT).