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Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

Schematic of a MOSFET

The metal oxide semiconductor field effect transistor (MOSFET) is one of the cornerstones of modern semiconductor technology. The general structure is a lightly doped p-type substrate, into which two regions, the source and the drain, both of heavily doped n-type semiconductor have been embedded.The symbol n+ is used to denote this heavy doping.

The source and the drain are about 1 μm apart. Metallized contacts are made to both source and drain, generally using aluminium. The rest of the substrate surface is covered with a thin oxide film, typically about 0.05 μm thick. The gate electrode is laid on top of the insulating oxide layer, and the body electrode in the above diagram provides a counter electrode to the Gate. The thin oxide film contains silicon dioxide (SiO2), but it may well also contain silicon nitride (Si3N4) and silicon oxynitride (Si2N2O).

The p-type doped substrate is only very lightly doped, and so it has a very high electrical resistance, and current cannot pass between the source and drain if there is zero voltage on the gate. Application of a positive potential to the gate electrode creates a strong electric field across the p-type material even for relatively small voltages, as the device thickness is very small and the field strength is given by the potential difference divided by the separation of the gate and body electrodes.

Since the gate electrode is positively charged, it will therefore repel the holes in the p-type region. For high enough electrical fields, the resulting deformation of the energy bands will cause the bands of the p-type region to curve up so much that electrons will begin to populate the conduction band. This is depicted in the animation below which shows a cross section through the region of the p-type material near the gate electrode. Click the button to increase the voltage applied to the gate electrode.

Note: This animation requires Adobe Flash Player 8 and later, which can be downloaded here.

The population of the p-type substrate conduction bands in the region near to the oxide layer creates a conducting channel between the source and drain electrodes, permitting a current to pass through the device. The population of the conduction band begins above a critical voltage, VT, below which there is no conducting channel and no current flows. In this way the MOSFET may be used as a switch. Above the critical voltage, the gate voltage modulates the flow of current between source and drain, and may be used for signal amplification.

This is just one type of MOSFET, called 'normally -off' because it is only the application of a positive gate voltage above the critical voltage which allows it to pass current. Another type of MOSFET is the 'normally-on', which has a conductive channel of less heavily doped n-type material between the source and drain electrodes. This channel can be depleted of electrons by applying a negative voltage to the gate electrode. A large enough negative voltage will cause the channel to be closed off entirely.

'Normally-off' MOSFETs are used in a wide variety of integrated circuit applications. AND gates, NOT gates and NAND gates are all made from these type of MOSFETs and are essential components of memory devices.

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