Bipolar junction transistors
ALTHOUGH in principle consisting merely of two p-n junction diodes back to back, in practice the bipolar junction transistor (BJT) is a completely different class of device; whereas the diode is a passive device, the transistor is active — it can be used to amplify voltages (signals). The BJT is termed bipolar because its operation — unlike the field effect transistor’s (FET) — depends on both positive (holes) and negative (electrons) charge carriers. Before the transistor was invented, amplifiers used vacuum tubes (’valves’) and vacuum tubes used a great deal of power, most of which was wasted as heat. Vacuum tubes were also prone to failure by filament burn out, loss of vacuum and just plain breakage; they were also difficult to miniaturise. A ‘large’ computer (by the standards of its day, 1950) such as EDSAC at Cambridge occupied a fair-sized room, required a large amount of power and cooling, and was prone to break down as the tubes failed, though ingenious methods were devised to minimise the stoppages. But it was not only high-technology products like computers which suffered from valve technology; radios for example were large and clumsy, and batteries for ‘portable’ radios were enormous, heavy and costly. Above all, the transistor could be made smaller and smaller, and cheaper and cheaper — and more and more reliable. The story of the transistor is far from finished 45 years after its invention.
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