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 & 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, & there are two modes of
operating a MOSFET called Depletion mode & Enhancement mode.
In depletion mode, when a gate voltage is applied the metal contact acts
as a capacitor & 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 & neutralize the holes; & also as the voltage at the gate
decreases, the electrons are free to move again, the channel widens &
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 & the depletion zones return to
their normal neutral net charge state, insulating the layers &
preventing current flow.
