What is a MOS Transistor?
The metal oxide semiconductor (MOS) transistor is the building block of most modern digital memories, processors and logic chips. It is also a common element in many analog and mixed-signal integrated circuits. These transistors are found in any number of electronic devices from cell phones and computers to digitally-controlled refrigerators and electronic medical equipment. The MOS transistor is quite versatile and can function as a switch, an amplifier or a resistor. It is also known as a particular type of field-effect transistor (FET) called insulated-gate (IGFET) or MOS (MOSFET). Field-effect refers to the electric field from the charge at the gate of the transistor.
The MOS transistor is fabricated on a semiconductor crystal substrate, usually made of silicon. The substrate is topped with a thin insulating layer, often made of silicon dioxide. Above this layer is the gate, typically made of either metal or polycrystalline silicon. The crystal region on one side of the gate is called the source, while the other is the drain. The source and drain are generally "doped" with the same type of silicon; the channel underneath the gate is "doped" with the opposite type. This forms a structure similar to a standard NPN or PNP transistor.
An MOS transistor is generally manufactured as either a PMOS or an NMOS transistor. A PMOS transistor has a source and a drain made of p-type silicon; the channel underneath the gate is n-type. When a negative voltage is applied to the gate, the transistor switches on. This allows a current to flow between the source and drain. When a positive voltage is applied to the gate, it shuts off.
An NMOS transistor is the opposite: a p-type channel with an n-type source and drain. When a negative voltage is applied at the gate of an NMOS transistor, it switches off; a positive voltage turns it on. One advantage that NMOS has over PMOS is switching speed — NMOS is generally quicker.
Many integrated circuits use complementary MOS (CMOS) logic gates. A CMOS gate is composed of two types of transistors wired together: one NMOS and one PMOS. These gates are often favored where power consumption is critical. They typically use no power until the transistors switch from one state to the other.
The depletion-mode MOSFET is a special type of MOS transistor that can be used as a resistor. Its gate area is fabricated with an extra layer between the silicon dioxide insulator and the substrate. The layer is "doped" with the same type of silicon as the drain and source regions. When there is no charge at the gate, this layer conducts current. The resistance is determined by the size of the transistor when it is created. The presence of a gate charge switches this type of MOS transistor off.
Like most other transistors, an MOS transistor can amplify a signal. The amount of current flowing between the source and drain varies with the gate signal. Some MOS transistors are constructed and individually packaged to handle large currents. These can be used in switching power supplies, high-power amplifiers, coil drivers and other analog or mixed-signal applications. Most MOS transistors are used in low-power, low-current digital circuits. These are typically included inside chips with other parts, rather than standing alone.
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