What is a Transistor

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             Variations of FET technology are based on different ways of generating the electric field. In all of these, however, electrons at the gate are used only for their charge in order to create an electric field around the channel, and there is a minimal flow of electrons through the gate. This leads to a very high de input resistance in devices that use FETs for their input circuitry. There may be quite a bit of capacitance between the gate and the other FET terminals, however. The input impedance may be quite low at RF. .
             The current through an FET only has to pass through a single type of semiconductor material. There is very little resistance in the absence of an electric field (no bias voltage). The drain-source resistance (rDS ON) is between a few hundred ohms to less than an ohm. The output impedance of devices made with FETs is generally quite low. If a gate bias voltage is added to operate the transistor near cut off, the circuit output impedance may be much higher. .
             FET devices are constructed on a substrate of doped semiconductor material. The channel is formed within the substrate and has the opposite polarity (a P-channel FET has N-type substrate). Most FETs are constructed with silicon. In order to achieve a higher gain-bandwidth product, other materials have been used.
             Placing an insulating layer between the gate and the channel allows for a wider range of control (gate) voltages and further decreases the gate current (and thus increases the device input resistance). The insulator is typically made of an oxide (such as silicon dioxide, SiO2), This type of device is called a metal-oxide-semiconductor FET (MOSFET) or insulated-gate FET (IGFET). The substrate is often connected to the source internally. The insulated gate is on the opposite side of the channel from the substrate. The bias voltage on the gate terminal either attracts or repels the majority carriers of the substrate across the PN junction with the channel.