A MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is a semiconductor device. A MOSFET is most commonly used in the field of power electronics. A semiconductor is made of manufactured material that acts neither like a insulator nor a conductor. An insulator is a natural material that will not conduct electricity, such as a dry piece of wood. A conductor is a natural material that conducts or passes electricity. Metals are the most common examples of conductors. Semiconductor material from which devices like a MOSFET are made exhibit both insulation like properties and conduction like properties. Most importantly, semiconductors are designed such that the conduction or insulation properties can be controlled.
The transistor is perhaps the best known semiconductor device. Early transistors use a technology referred to as bi-polar material. Pure silicon can be doctored or "corrupted"--a process that is referred to as "doping". It is possible to make either p type (positive) material or n type (negative) material depending upon material used to "dope" or corrupt the pure silicon. If you combine p type material and n type material, you have a bipolar device. The transistor is a basic example of a bipolar device. The transistor has three terminals, the collector, the emitter, and the base. The current in the base terminal is used to control the flow of current between the emitter and the collector.
MOSFET technology is an enhancement on bipolar technology. Both n and p type material are still used but metal oxide insulators are added to provide some performance enhancements. There are still typically only three terminals but they now have the following names, the source, the drain, and the gate. The field effect portion of the name refers to the method used to control the electron or current flow through the device. The current is proportional to the electrical field developed between the gate and the drain.
One other very significant enhancement over bipolar technology is that a MOSFET has a positive temperature co-efficient. This means that as the temperature of the device increases its tendency to conduct current decreases. This feature allows the designer to easily use it in parallel to increase the system's capacity. A bipolar deice has the opposite effect. With MOSFET technology, devices in parallel will naturally share current between them. If one device tries to conduct more than its share it will heat up and the tendency to conduct current will decrease causing the current through the device to decrease until all devices are again sharing evenly. Bipolar devices in parallel, on the other hand, increase in temperature if one device starts to conduct more current. This means more current will switch to this device which will result in a further increase in temperature, and a further increase in current. This is a runaway condition that quickly destroys the device. For this reason it is much more difficult to connect bipolar devices in parallel and the reason MOSFET devices are now the most popular power semiconductor type transistor.