nMOS

An nMOS transistor is a metal-oxide-semiconductor field-effect transistor (MOSFET) in which the conductive channel between source and drain is formed from n-type semiconductor material, meaning it is doped with donor impurities (typically phosphorus or arsenic in silicon) that provide a surplus of free electrons. When a positive voltage is applied to the gate terminal, it attracts electrons into the channel, turning the device on and allowing current to flow from drain to source. The electron mobility in n-type silicon is roughly twice that of holes in p-type material, which is why nMOS devices switch faster and conduct more current for a given die area than their pMOS counterparts.

The name reflects the physical structure: the gate electrode sits atop a thin silicon dioxide insulator (the oxide), which lies above the semiconductor channel. In enhancement-mode nMOS, the most common variant in integrated circuits, the channel does not exist in its off-state; applying gate voltage creates it. In depletion-mode nMOS, a channel exists by default and gate voltage pinches it off, but this variant is now rare outside specialized RF and analog circuits. Modern nMOS transistors in logic and memory are typically built with gate lengths between 3 nanometers and several hundred nanometers, with channel widths varied to achieve desired on-resistance and switching speed.

Where nMOS dominates

NMOS logic formed the basis of early microprocessors and remains the faster, more power-efficient component in CMOS (complementary MOS) circuits, where nMOS and pMOS transistors work in push-pull pairs. In a CMOS inverter or gate, nMOS pulls the output toward ground (logic 0) while pMOS pulls toward the supply rail (logic 1). This arrangement minimizes static power dissipation because one transistor is always off. For this reason, all high-volume digital logic, microcontrollers, and processors since the 1980s have been built in CMOS, not pure nMOS. Pure nMOS logic draws significant current even at rest and generates heat, which made it impractical for large chips.

In memory, nMOS transistors serve as access gates in DRAM cells and as the programmable components in flash storage. A floating gate nMOS transistor in flash memory can be charged or discharged by applying high voltages during programming and erase cycles, and the presence or absence of charge on that gate determines whether the cell reads as 0 or 1. The threshold voltage of such a transistor determines its state; voltage thresholds in a modern flash cell can be controlled to within tens of millivolts.

Physical degradation of nMOS devices occurs through hot-carrier injection (energetic carriers striking the gate oxide and becoming trapped), electromigration in metal interconnects, and negative bias temperature instability (NBTI), which affects the threshold voltage over time. Designers must account for these mechanisms when setting voltage and thermal margins. The nMOS device remains the dominant switching element in semiconductor manufacturing because of its speed, small footprint, and compatibility with scaled processes down to the smallest feature sizes used in production today.