Logic Gate | Research & Encyclopedia Articles

Logic Gate

A logic gate is an electronic circuit that performs a logical operation on one or more input signals. The logic gate's inputs and outputs are electrical signals (voltages and currents) that represent Boolean values. Boolean values take on only two possible values, 0 and (or TRUE and FALSE). In most logic gates, the 0 or FALSE state is symbolized by a voltage level of approximately 0 volts while the 1 or TRUE state is symbolized by a voltage level of approximately five volts (+5 V).

The instructions that direct a computer's operation are known as machine code and are written as a sequence of binary digits (bits). These bits, physicalized as voltages on wires, switch specific logic gates ON (to high-voltage output) or OFF (to low-voltage output). Most logic gates accept two input bits and output one bit. If the inputs of a logic gate change value, its output also changes value (if appropriate) after a slight delay during which the circuit adjusts to the change.

The seven basic types of logic gates are AND, OR, NOT, XOR, NAND, NOR, and XNOR. The three most common gates are AND, OR, and NOT; all others may be implemented as appropriate networks of AND, OR, and NOT gates. An AND gate takes the value of two input bits and tests them to see if they are both equal to 1. If they are, the output from the AND gate is 1 (ON). If either or both input bits are zero, the AND gate will output 0 (OFF). An OR gate tests two input bits to determine if either is equal to 1. If both or either input bits are equal to 1, the gate outputs 1; if both input bits are 0, it outputs 0. A NOT gate negates (reverses) a single input bit, so an input of 1 results in an output of 0 and vice versa.

Logic gates are commonly built as clusters of metal-oxide semiconductor field-effect transistors (MOSFETs). A MOSFET transistor contains a source (where current may enter the device) and a drain (where current may leave the device), with a region between them called the channel that connects source and drain and through which current may flow. MOSFETs are implemented as tiny volumes of silicon right at the surface of a silicon chip, doped (deliberately contaminated) with precise amounts of various elements. The whole device is covered by a thin coating of nonconductive (insulating) material; above the gate this insulating layer is fused to a metal contact called the gate. For electrons to flow from the source to the drain, a voltage must be applied on the gate. The gate voltage causes the channel, just on the other side of the thin insulating layer from the gate, to shift from a nonconducting state to a conducting state, allowing current to flow from source to drain as if a valve had been turned on to allow water Connecting several MOSFETs appropriately produces a logic gate. Consider, for example, two MOSFETS wired so that the drain of the first transistor is connected to the source of the second. Current can flow from the source of the first transistor, through the channels of both transistors, and out the drain of the second transistor if and only if both channels have been turned on--which is accomplished by placing a high voltage on the gates of both transistors. This circuit behaves like an AND gate, producing logical 1 at its output (i.e., a current that can produce a voltage) if and only if both its logical inputs (the gates of the two transistors) are set to logical 1s (high voltages).

A network of interconnected logic gates form a logic circuit. The output of a logic circuit can provide input to another circuit for further processing or produce the final result of some operation. There is no theoretical limit to the number of logic gates that can be joined together in an individual device. In practice, however, there is always a limit to the number of gates that can be crowded onto a chip. As the manufactured size of each logic gate decreases toward ultimate limits dictated by the atomic structure of matter, digital devices become smaller and capable of performing at faster speeds.