Transistors
Electronics guide > Transistors

In previous chapters weve looked at semiconductors, spending some time on diodes (the simplest semiconductor device) and taking brief glimpses of integrated circuits. Its the turn of the transistor now to come under the magnifying glass perhaps the most important semiconductor of all.

You dont need many components this chapter, just a handful of resistors:

  • 1 x 100 kΩ
  • 1 x 220 kΩ
  • 1 x 47 kΩ miniature horizontal preset

and, of course, one transistor.

The type of transistor you need is shown in Photo 8.1 and is identified as a 2N3053.

A transistor next to a UK fifty pence piece

Photo 8.1 A transistor next to a UK fifty pence piece

Youll see that the transistor has three terminals called, incidentally, base, collector and emitter (often shortened to B, C and E). When you use transistors in electronic circuits it is essential that these three terminals go the right way round. The 2N3053 transistor terminals are identified by holding the transistor with its terminals pointing towards you from the body and comparing the transistors underneath with the diagram in Figure 8.1. The terminal closest to the tab on the body is the emitter, then in a clockwise direction are the base and the collector.

Other transistor varieties may have different body types so its important to check with reference books or manufacturers data regarding the transistor terminals before use. All 2N3053 transistors, however, are in the same body type known as a TO-5 body and follow the diagram in Figure 8.1.

How to recognise the location of the base, emitter and collector connections on a common transistor body

Figure 8.1 How to recognise the location of the base, emitter and collector connections on a common transistor body

For the previous two chapters, weve looked closely at diodes the simplest of the group of components known as semiconductors. The many different types of diodes are all formed by combining doped layers of semiconductor material at a junction. The PN junction (as one layer is N-type semiconductor material and the other layer is P-type) forms the basis of all other semiconductor-based electronic components. The transistor the component were going to look at now is, in fact, made of two PN junctions, back to back. Figure 8.2 shows how we may simply consider a transistor as being two back-to-back diodes, and we can verify this using our multimeter to check resistances between the three terminals of a transistor: the base, emitter and collector.

To do the experiment, put a transistor into your breadboard, then use the meter to test the resistance between transistor terminals. Using our brains here (I know its hard) we can work out that if there are three terminals there must be six different ways the meter leads can connect to pairs of the terminals. Table 8.1 lists all six ways, but the results column is left blank for you to fill in as you perform the experiment.

A symbol for a transistor considered as two diodes, back to back

Figure 8.2 A symbol for a transistor considered as two diodes, back to back

Its not necessary for you to measure the exact resistance obtained between terminals, its sufficient just to find out if the resistance is high or low.

Take note

One final point before you start the casing of the transistor body (sometimes called the can) is metal and therefore conducts. When you touch the meter test probes to the transistor terminals make sure, therefore, they dont touch the can. Its all too easy to do without realising youve got a short circuit which, of course, gives an incorrect result. The can of a TO-5 bodied transistor such as the 2N3053 is also electrically connected to the transistors collector, so be careful!

Six different ways to connect the meter leads

Table 8.1 Six different ways to connect the meter leads

Your results should show that low resistances occur in only two cases, indicating forward current flow between base and emitter, and base and collector. This corresponds as we should expect to the diagram of Figure 8.2.

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