On-off switches are one of the most common control elements in electrical circuitry. This has evolved over the years, from the manually operated circuit breaker of the early experiments to the multi-switch integrated circuit of today. In every application, the function of the switch remains the same: to isolate or connect two sections of an electrical circuit. Therefore, an ideal switch should have zero resistance (short circuit) when on and infinite resistance (open circuit) when off. However, in practical applications, a bus switch should have as low resistance as possible when on, for bus connection, and as high resistance as possible when off, for bus isolation.

An insulated-gate field-effect transistor (IGFET) switch is a widely used electronic switch. A metal-oxide semiconductor field-effect transistor (MOSFET) is one type of IGFET. Although the term MOSFET is more commonly used, now most of the electronic switches do not use the metal oxide as the gate. Instead, a more advanced process is being used to form the gate. TI uses advanced poly-silicon gate-enhancement-mode transistor technology to fabricate semiconductor switches, which gives more control of performance characteristics. Throughout this application report the term MOSFET and the associated terms related to MOSFET are used because they are more common in semiconductor literature. When sufficient bias voltage is applied to the gate of a MOSFET, it creates a low-resistance path between its source and drain. When the bias voltage is removed, the resistance of this path becomes very large. MOSFET scan be of two types, n-channel MOSFET (NMOS) and p-channel MOSFET (PMOS).