The Gate Rules

A CMOS circuit is built out of two types of transistors, NMOS and PMOS. Each contains four electrodes—source, gate, drain, and substrate—which manipulate electric charge carriers. In a single CMOS device, an NMOS and a PMOS transistor are connected in series to the power supply and are controlled by the same gate voltage. The advantage is that, ideally, the CMOS device consumes power only when it switches. NMOS and PMOS are so-called MOSFETs, or metal-oxide-semiconductor field-effect transistors.

The basic material of CMOS circuits is silicon, but it is doped with other materials, like arsenic or boron , to alter how it conducts electricity. Arsenic is an n-type impurity that releases one of its electrons into the conduction band; so in n-type silicon, electrons carry the current. Boron is a p-type impurity that contains electron vacancies, or holes, which behave like positive electric charge carriers.

NMOS transistors are built on p-type substrates and the drain and source are doped with n-type impurities. Conversely, PMOS transistors are built on n-type substrates and the source and drain are doped with p-type impurities. Between the drain and source is the channel, a thin layer of silicon under the gate that is depleted of carriers. Between the gate and the channel is a layer of insulation. The voltage on the gate turns the transistor on or off by controlling the flow of electrons or holes from the drain to the source.

In normal operation, the goal is to control the flow of charge carriers through the channel. In the case of an NMOS transistor, the drain is biased with a positive voltage, but even so an energy barrier, or voltage, in the channel keeps electrons from flowing from the source to the drain. Biasing the gate with a positive voltage gets things moving, lowering the barrier and attracting electrons into the depleted channel region, until above what is termed the threshold voltage, the carriers begin to flow in appreciable numbers through the channel from the source to the drain. When the gate voltage is brought below the threshold voltage, the flow is shut off. (PMOS devices work the same way, with appropriate changes in voltages and carrier type.)

—L.G.