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What is a CMOS Transistor?

G.W. Poulos
G.W. Poulos

The term CMOS stands for complimentary metal-oxide semiconductor and refers to a technology that uses metal oxides to build electronic integrated circuits as well as the devices created with that technology. Informally, these types of devices are often called chips or ICs, and some people use the term CMOS transistor to refer to a transistor contained in a CMOS device. CMOS devices are primarily used for high-speed digital logic applications such as computer CPUs, memory chips, and other integrated circuits that rely on digital logic to perform their assigned tasks. As a result, transistors are used heavily in CMOS devices as switching transistors that perform the high-speed switching operations needed in logic circuits.

A switching transistor is a device intended to conduct an electrical signal under a given set of circumstances where it will switch on or off and, by doing so, perform a logical function. A CMOS transistor is made of three components: a collector, an emitter, and a base. When the transistor has a signal at its collector but not at its base, or its base but not its collector, it will not conduct a signal and remains off. It will only switch on and send a signal when receiving a signal at both its collector and base. Hence, transistors can be arranged to perform numerous logical functions. These arrangements of transistors in CMOS devices are referred to as logic gates.

Man holding computer
Man holding computer

Logic gates come in a number of types such as AND, NAND, OR, or NOR, and depending on their order in the CMOS device, allow it to respond to various input signals in different but predictable ways. An AND gate will only switch on if it receives two specific signals. A NAND gate will only switch on if it does not receive two specific signals. An OR gate switches on if it receives either one or both of two specific signals, but not both at the same time. A NOR gate will only switch on if it does not receive either of two specific signals.

The transistors used in CMOS devices are referred to individually in a number of ways—all of which identify certain characteristics of the devices. Collectively, the transistors are usually called a name that includes the prefix MOS, which stands for metal-oxide semiconductor, identifying the material and construction method of the device. The transistors in CMOS devices are typically field effect transistors, and it is common to see them referred to as MOSFETs.

A CMOS transistor can also be referred to by its order of electrical charge. The three main components of the transistors, the collector, emitter, and base, have a specific electrical charge that is either ordered as positive-negative-positive or negative-positive-negative respectively. The terms N-type and P-type are often used to identify the order of charge in the transistor. Additionally, a CMOS transistor can be referred to as a PMOS/pMOSFET or NMOS/nMOSFET, with the first letter identifying what the order of charge is in the transistor. Knowing the order of charge is important when connecting CMOS transistors to other circuits, and in understanding how they function in given types of logic gates.

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