PMOStransistor

A PMOS transistor is a type of metal-oxide-semiconductor field-effect transistor (MOSFET) in which the conducting channel is p-type. In complementary metal-oxide-semiconductor (CMOS) technology, PMOS devices are used together with NMOS devices to implement logic circuits, with PMOS forming the pull-up network and NMOS forming the pull-down network.

Structurally, a PMOS transistor is built with source and drain regions doped as p-type in an n-type well (n-well) inside a p-type substrate. The gate is an insulated electrode separated from the channel by a thin oxide layer. The body (substrate or well) is typically tied to the most positive supply, which for many circuits is the source, to minimize body effects and to set the device’s threshold. The p+ source and drain regions create a p-type channel when the gate voltage is suitably applied.

Operation-wise, a PMOS transistor conducts when the gate-to-source voltage (Vgs) is sufficiently negative, below a negative threshold. In contrast to NMOS devices, the carriers are holes, and current flows from source to drain as holes move through the p-type channel. The threshold voltage is negative, and the device turns off as Vgs approaches zero (or becomes positive). In saturation, the drain current depends on the effective gate overdrive, with device behavior influenced by carrier mobility, which for holes is lower than that for electrons.

Applications and characteristics: PMOS transistors are common in CMOS logic as the pull-up devices, providing high-level outputs when the NMOS pull-downs are off. They generally exhibit lower electron mobility and thus lower transconductance than NMOS devices, which affects speed and drive strength. PMOS devices also serve as high-side switches in certain analog and mixed-signal circuits.