The term MOSFET stands for Metal Oxide Semiconductor Field Effect Transistor, and the name gives a clue to its construction.
The devices had been known about for several years but only became important in mid and late 1960s. Initially semiconductor research had focussed in developing the bipolar transistor, and problems had been experienced in fabricating MOSFETs because process problems, particularly with the insulating oxide layers.
Now the technology is one of the most widely used semiconductor techniques, having become one of the principle elements in integrated circuit technology today. Their performance has enabled power consumptions in ICs to be reduced. This has reduced amount of heat being dissipated and enabled the large ICs we take for granted today to become a reality. As a result of this the MOSFET is the most widely used form of transistor in existence today.
The MOSFET provides some key features for circuit designers in terms of their overall performance.
|Key MOSFET Features|
|Gate construction||gate is physically insulated from the channel by an oxide layer. Voltages applied to the gate control the conductivity of the channel as a result of the electric field induced capacitively across the insulating dielectric layer.|
|N / P channel||Both N-channel and P-channel variants are available|
|Enhancement / depletion||Both enhancement and depletion types are available. As the name suggests the depletion mode MOSFET acts by depleting or removing the current carriers from the channel, whereas the enhancement type increases the number of carriers according to the gate voltage.|
As already implied the key factor of the MOSFET is the fact that the gate is insulated from the channel by a thin oxide layer. This forms one of the key elements of its structure.
For an N-channel device the current flow is carried by electrons and in the diagram below it can be seen that the drain and source are formed using N+ regions which provide good conductivity for these regions.
In some structures the N+ regions are formed using ion implantation after the gate area has been formed. In this way, they are self-aligned to the gate.
The gate to source and gate to drain overlap are required to ensure there is a continuous channel. Also the device is often symmetrical and therefore source and drain can be interchanged. On some higher power designs this may not always be the case.
N channel enhancement mode MOSFET structure
It can be seen from the diagram that the substrate is the opposite type to the channel, i.e. P-type rather than N-type, etc. This is done to achieve source and drain isolation.
The oxide over the channel is normally grown thermally as this ensure good interfacing with the substrate and the most common gate material is polysilicon, although some metals and silicides can be used.
The depletion mode has a slightly different structure. For this a separate N-type channel is set up within the substrate.
N channel depletion mode MOSFET structure
P-channel FETs are not as widely used. The main reason for this is that the holes do not have as high a level of mobility as electrons, and therefore the performance is not as high. However they are often required for use in complementary circuits, and it is mainly for this reason that they are manufactured or incorporated into ICs.
MOSFET circuit design
The MOSFET follows the same basic circuit design principles that are used for all forms of FET. They are essentially high impedance voltage devices and as such they are treated in a slightly different way to bipolar transistors that are current devices.
Note on FET circuit design:
FETs can be used in a whole variety of circuits. Like the bipolar transistor, there are basic circuits. These include the common source, common drain and common gate. These form the basis of FET circuits.
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There are many different circuits in which FETs can be used, from low power amplifiers to high power switching applications. In all these circuit areas FETs can be used and they offer high levels of performance.