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The Power MOSFET
Today with the vast number of technical achievements occurring around
the world, many discoveries are overshadowed or obscured by some that
may appear more important to the general media. One such discovery of
importance, to the audiophile at least, is that of the power MOSFET
device.
The MOSFET
The field effect transistor (FET) and then the MOSFET transistor have
been around for a number of years, but only as a small signal-handling
device, mostly employed in radio tuners and communications equipment.
The electrical advantages of these have long been realised by
manufacturers of hi-fi. If only they could be made to handle large amounts
of power – what a benefit to the audiophile.
The term power MOSFET describes a device capable of handling
reasonably large amounts of electrical energy as an amplifier itself – hence
power. MOSFET stands for “Metal Oxide Silicon Field Effect Transistor”,
this in turn means that the device is constructed of Silicon. Similar to a
transistor – but the part that controls the power flow through the device is
insulated from the remainder of the device by a metal oxide insulating
layer and the controlling of the power is achieved by the development of an
electrostatic field between the controlling element and the conducting
element.
In a transistor, the control of the power through the device is effected by
the application of a smaller, but nevertheless, significant amount of power
to the controlling element. Whereas in the power MOSFET, the control of
the power through the device is affected by the application of a very small
and very insignificant amount of power to the controlling element – in fact,
only the amount required to create a small electrostatic field. This makes
the operation of a power MOSFET similar to that of a valve.
Other Field
Effect
Devices
There are basically three types of power field effect device, they are: the
junction FET, the vertical FET and the power MOSFET, all of which were
independently developed by three different hi-fi equipment manufacturers
in Japan and all were major technological breakthroughs in their own right.
The first of these was the junction FET, the second the vertical FET and
lastly, the power MOSFET. Although all these devices are vast
improvements over power transistors, the junction FET and vertical FET
cannot compare with the power MOSFET, in terms of simplicity of the
supporting driver stages and power supply requirements.
The power MOSFET, though having similar characteristics to the valve,
can be divided into 2 types of polarities of device – P-channel and
N-channel. Broadly speaking only one of these types exists in valve
