The full spelling of MOS FET is
Metal Oxide Semiconductor Field Effect Transistor.
It is the
structure which stacked up Metal, Oxide and Semiconductor.
There are NPN type and PNP type as the semiconductor part. NPN type
is called N-channel and PNP type is called P channel. An oxide film is put
to the semiconductor of NPN or PNP and metal is put onto it as the gate.
In case of NPN, the part of "N" is a source pole and a drain pole. In case
of PNP, the part of "P" are the poles.
Transistor
controls an output current by the input current. However, in case of FET,
it controls an output current by input voltage(Electric field). The input
current doesn't flow.
To handle an MOS FET, it
needs an attention. Because the oxidation insulation film is thin, this
film is easy to destroy in the high voltage of the static electricity and
so on.
 The operation principle of P-N junction
diode
First of all, I will explain about
the operation principle of P-N junction diode simply.
The N-type semiconductor has Electrons(Negative) and the P-type
semiconductor has Electron holes(Positive).
When the positive voltage to the side of
P-type and the negative voltage to the side of N-type are applied
respectively, the electron in N-type is pulled with the positive voltage
on the side of P-type and the electron flows through to the P-type beyond
the boundary of the semiconductor. Also, the hole in P-tyep is pulled with
the negative voltage on the side of N-type and the hole flows through to
the N-type. In this way, the electric current flows through the
semiconductor.
As the opposite case, when the
positive voltage to N-type and the negative voltage to P-type are applied
respectively, the electron in N-type is pulled with the positive voltage
on the side of N-type and the hole in the P-type is pulled with the
negative voltage on the side of P-type. In this case, the electron in the
semiconductor doesn't move through the boundary and the electric current
doesn't flow.
The operation principle of MOS FET
The semiconductor part of MOS FET consists of NPN or PNP. So, when
not applying voltage to the gate, the electric current doesn't flow
between drain and source.
When positive voltage is
applied to the gate of the N-channel MOS FET, the electrons of N-channel
of source and drain are attracted to the gate and go into the P-channel
semiconductor among both. With the move of these electrons, it becomes the
condition like spans a bridge for electrons between drain and source. The
size of this bridge is controlled by the voltage to apply to the
gate.
In
case of P-channel MOS FET, the voltage is opposite but does similar
operation. When negaive voltage is applied to the gate of P-channel MOS
FET, the holes of P-channel of source and drain are attracted to the gate
and go into the N-channel semiconductor among bith. With the move of these
holes, a bridge for holes is spaned and the electric current flows between
drain and source.
Because there is an oxide film between gate and semiconductor, the
electric current doesn't flow through the gate. An electric current flow
between drain and source is controlled only with the voltage which is
applied to the gate.
The operation principle of C-MOS FET
 C-MOS FET is the abbreviation of Complementary MOS
FET. C-MOS FET is the circuit which combined a P-channel MOSFET and a
N-channel MOSFET. When the input is an L level, the P-MOS FET becomes ON
condition and when the input is H level, the N-MOS FET becomes ON
condition. At the C-MOS FET circuit, the N-MOS FET and the P-MOS FET do
the operation which is always opposite.
The
important characteristic of this circuit is that the comparatively big
output current can be controlled. When the input becomes an L level, the
output is connected with the power supply by the P-MOS FET and becomes H
level. Also, when the input becomes H level, the output is connected with
the grounding by the N-MOS FET and becomes an L level. Phase of the input
and the output is opposite.
The drain current of
MOS FET is cut off even if the gate voltage doesn't become 0 V. It may
depend on the kind of the FET but a drain current is cut off when the gate
voltage is lower than 1V or 2V. So, in case of C-MOS circuit, P-MOS FET
and N-MOS FET don't become ON condition at the same time.
|
