An electronic circuit that consists of elements, which may be transistors, diodes, or resistors, combined in such a manner that they perform a logic operation. Gate circuits are the most basic building blocks of a digital system. These circuits have one or more inputs and one output which is a boolean function of the inputs. The input and output signals can have only two discrete values, low (for example, 0 V) and high (for example, 3.3 V). These values are usually represented as 0 and 1, or “false” and “true,” respectively.
Whereas the early gate circuits consisted of diodes, resistors, and transistors, the majority of gate circuits nowadays are built exclusively with transistors. The dominant technology for fabricating gate circuits is the metal oxide semiconductor (MOS) silicon method, followed by the silicon bipolar and gallium arsenide (GaAs) techniques. The manufacturing process has become so sophisticated that transistors smaller than 1 square micrometer can be fabricated, allowing the placement of millions of gate circuits on a silicon chip the size of a fingernail. The main advantage of MOS technology is that it gives rise to very low power circuits that can still operate at relatively high clock speed. It is these characteristics that have allowed the fabrication of very complex digital systems such as microprocessors and memories. See Integrated circuits
The transistors in a gate circuit are used as ON-OFF switches. By combining these transistors in a certain way, it is possible to realize logical, arithmetical, and memory functions. There are two type of MOS transistors: NMOS and PMOS field-effect transistors (FETs), corresponding to a normally-OFF or normally-ON switch, respectively. Circuits in which both types of transistors are used are called CMOS (complementary MOS) circuits. CMOS circuits now constitute the majority of gate and logic circuits.
In switch circuits an MOS transistor is used to pass or block the flow of information in a similar fashion to a mechanical switch. By placing these switches in a network, it is possible to realize different logic functions. A transistor used in this fashion is often called a pass-transistor. In order to improve the switching characteristics, an NMOS switch and a PMOS switch are placed in parallel, each clocked at opposite clock signals. Such a combination is called a CMOS transmission gate. Several transmission gates can be combined to form a logic AND circuit.
An alternative way to realize logic functions is to make use of logic gates. The simplest gate circuit, the inverter, takes an input signal and presents the inverted signal at the output. See Logic circuits, Transistor