The MOSFET: Metal Oxide Semiconductor Field Effect Transistor

Sometimes, even that small amount of current is too much, so a new FET design came into being. The Insulated Gate FET (IGFET) is another type of field effect transistor. This time, the P material is completely dumped and replaced by a metal contact. The metal does not come in direct contact with the N material, instead it is insulated by a thin layer of Silicon Dioxide (In other words, glass).

This configuration of materials gives this type of transistor its more common name: Metal-Oxide-Semiconductor FET, or MOSFET for short.

The internal working of the MOSFET is somewhat different from that of the junction FET in action, not in principle, and there are two modes of operating a MOSFET called Depletion mode and Enhancement mode.

In depletion mode, when a gate voltage is applied the metal contact acts as a capacitor and start charging positively. This charge draws electrons to the other side of the oxide insulator, which recombine with the holes of the P material, resulting in a zone of neutral net charge.

This region acts in exactly the same way as the depletion zone of the reverse biased diode, which in effect is a neutral net charge zone inside the semiconductor. As you can see, the net effect is the same, as the gate voltage is increased, more electrons are drawn to towards the gate and neutralize the holes; and also as the voltage at the gate decreases, the electrons are free to move again, the channel widens and more current flows.

In enhancement mode, a layer of N material is built inside the P bar, in a structure similar to the bipolar transistor. This intrinsic layer creates two depletion regions inside the bar, insulating the from each other so no current can flow.

In P channel enhancement mode MOSFETs, the applied voltage is negative, opposite of how it was in depletion mode. When a negative voltage is applied to the gate, it pushes electrons away from that region, leaving only the holes.

In the area where the gate meets either depletion zone, the result is a net positive charge in the material, as if in that zone the material was the same P type material. The free electrons of the intrinsic N type layer are pushed away from the gate, also leaving a zone of free holes that act as P type material.

As you can see, in this mode a channel is created near the gate that connects both ends of the P material, pushing the N middle layer away, allowing current to flow through it. When the voltage is removed, the free electrons again fill the holes and the depletion zones return to their normal neutral net charge state, insulating the layers and preventing current flow.