Transistors
Generally, in a circuit a resistor is used to convert the changing current to a changing voltage, and the transistor is being used to amplify voltage. A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on). The amount of current amplification is called the current gain.
There are two types of standard transistors are available. They are NPN (Negative-Positive -Negative) and PNP (Positive-Negative-Positive), with different circuit symbols. The letters “P” and “N” refer to the layers of semiconductor material used to make the transistor. NPN transistor is the most commonly used transistor because, this is easiest to make from silicon.
Transistors are composed of three parts named, Base (B), Collector (C) and Emitter (E). The base is the gate controller device for the larger electrical supply. The collector is the larger electrical supply, and the emitter is the outlet for that supply.
Generally transistors fall into two categories. They are:
• Bipolar Junction Transistor
• Field Effect transistor (FET)
• Junction Field Effect Transistor (JFET)
• Metal Oxide Semiconductor Field Effect Transistor (MOSFET)
FET based silcon chips are easier to construct than their bipolar counterparts. FETs switch a little slower than bipolar transistors, but use less power. By comparison, a bipolar transistor requires a small amount of current flowing to keep the transistor on. The most widely used and widely known FETs are MOSFETs (metal oxide semiconductor FETs), which come in NMOS (n-channel) and PMOS (p-channel) varieties. On a chip, NMOS and PMOS transistors are wired together in a complementary fashion to create CMOS logic.