Self-aligned gate

In semiconductor electronics fabrication technology, a self-aligned gate is a transistor manufacturing approach whereby the gate electrode of a MOSFET (metal–oxide–semiconductor field-effect transistor) is used as a mask for the doping of the source and drain regions. This technique ensures that the gate is naturally and precisely aligned to the edges of the source and drain. The use of self-aligned gates in MOS transistors is one of the key innovations that led to the large increase in computing power in the 1970s. Self-aligned gates are still used in most modern integrated circuit processes. Semiconductor device fabrication Integrated circuits (ICs, or "chips") are produced in a multi-step process that builds up multiple layers on the surface of a disk of silicon known as a "wafer". Each layer is patterned by coating the wafer in photoresist and then exposing it to ultraviolet light being shone through a stencil-like "mask". Depending on the process, the photoresist that was exposed to light either hardens or softens, and in either case, the softer parts are then washed away. The result is a microscopic pattern on the surface of the wafer where a portion of the top layer is exposed while the rest is protected under the remaining photoresist. The wafer is then exposed to a variety of processes that add or remove materials from the portions of the wafer that are unprotected by the photoresist. In one common process, the wafer is heated to around 1000 C and then exposed to a gas containing a doping material (commonly boron or phosphorus) that changes the electrical properties of the silicon. This allows the silicon to become an electron donor, electron receptor, or near-insulator depending on the type and/or amount of the dopant. In a typical IC this process is used to produce the individual transistors that make up the key elements of an IC. In the MOSFET, the three parts of a transistor are the source, the drain, and the gate (see diagram). The "field effect" in the name refers to changes to the conductivity that occur when a voltage is placed on the gate.