The source and body are connected to ground.įigure 4. For the NMOS in Figure 4, the gate and drain terminals are connected to independent voltage sources. The electrical behavior of the MOSFET is determined by the voltage levels at each of its four terminals. Now that we’ve examined the basic structure of the MOSFET, let’s dive into the theory of operation. Three-dimensional NMOS transistor structure. This can be hard to see in two-dimensional diagrams, so I’ve included one that shows a three-dimensional view (Figure 3).įigure 3. The width (W) and length (L) of a transistor affect its other characteristics. The PMOS body is a lightly N-doped region within the total P-type substrate, forming what’s referred to as an N-well. The PMOS device has the same structure as the NMOS, but with opposite dopant polarities. So as to provide a good electrical connection, it’s heavily doped with the same polarity as the substrate. Note that there’s no physical difference between the source and drain terminals, making them interchangeable. This is meant to prevent leakage from the gate terminal to the source, drain, and/or body terminals.įor the NMOS, the source and drain terminals are implemented by creating highly N-doped areas within the substrate. Image used courtesy of UT DallasĪn insulator (normally silicon dioxide) is placed on top of the silicon substrate, and a gate terminal made of either polysilicon or metal is placed on top of the insulator.

Physical structure of NMOS and PMOS transistors. This is true of any transistor within an integrated circuit.įigure 2. Both transistors are built on top of a lightly P-doped silicon substrate. For this reason, you may see schematics that show the MOSFET as a three-terminal device with only the gate, source, and drain.įigure 2 compares the basic physical structure of an NMOS transistor, on the left, with that of a PMOS transistor, on the right. In digital circuits, the source and body terminals are typically tied together. The body terminal for the NMOS transistor is tied to the lowest possible voltage in the circuit (ground in a single-supply system) whereas the PMOS body terminal is connected to the highest voltage level ( V DD of the circuit). Lastly, we have the body terminal, which connects to the substrate that the transistor is built on. In an NMOS transistor, the drain is normally at a higher voltage than the source the opposite is true for a PMOS. The voltage level determines the drain current, which is the current that flows from the drain to the source terminals.

The input voltage is connected to the gate terminal. Generally speaking, these two channel types behave as the inverse of one another.įigure 1. Figure 1 illustrates the two types of MOS transistor: the N-channel MOSFET (NMOS) and the P-channel MOSFET (PMOS). In this article, we’ll introduce the physical implementation of MOSFETs and their theory of operation.Īn MOS transistor is a four-terminal device consisting of a gate (G), drain (D), source (S), and body (B). Today, analog ICs primarily use metal-oxide semiconductor field effect transistors, better known as MOS transistors or MOSFETs. Transistors-devices that translate an input voltage to an output current, or vice versa-are what make analog integrated circuit (IC) design possible.

Nmos transistor theory