Transistor is a small semiconductor element of electronic circuits that can be used as an amplifier or a switch, and which is a basic building block of digital electronic computers, integrated circuits and many other electronic devices. Transistors replaced vacuum tubes and relays which were used in primitive computers of the 20th century; transistors can be made much smaller, cheaper, more reliable and, unlike relays, operated purely electronically and therefore much faster. Transistor has become the most manufactured device in history.
Transistor generally has three terminals. Its key principle is that of behaving like an electronically operated amplifier or switch: we can make a transistor open or close (i.e. conduct or not conduct electricity) by applying different voltage or current (and we can also make it something between open and close). The voltage/current by which we control the transistor can be lower than that which we control, so we can see this as an amplifier: we can control high current with low current, i.e. the high current follows the low current but has higher amplitude. We can also see this as a switch: by applying voltage/current we can make a wire connect (low resistivity) or disconnect (high resistivity) similarly to a physical switch. This switch behavior is important for computers because we can exploit it to implement binary (on/off) logic circuits.
A basic division of transistors is following:
Commonly used graphical symbols for transistor are (usually in a circle):
E E D D | | | | B___|.-' B___|.-' G |--' G |--' |'>. |'<. -->-|--. --<-|--. | | | | C C S S BJT (NPN) BJT (PNP) FET (N) FET (P)
First FET transistors were JFETs (junction-gate FET) but by today were mostly replaced by MOSFETs (metal-oxide-semiconductor FET), a transistor using a metal oxide layer for separating the gate terminal which gives it some nice properties over JFET. These transistors are used to implement logic gates e.g. using the CMOS fabrication process which uses complementary pairs of P and N channel FETs so that e.g. one is always off which decreases power consumption.
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