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The Power MOSFET
Today with the vast number of technical achievements occurring around th
e
world, many discoveries are overshadowed or obscured by some that ma
y
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 bee
n
around for a number of years, but only as a small signal-handling device, mostl
y
employed in radio tuners and communications equipment. The electrica
l
advantages of these have long been realised by manufacturers of hi-fi. If onl
y
they could be made to handle large amounts of power – what a benefit to th
e
audiophile.
The term power MOSFET describes a device capable of handling reasonabl
y
large amounts of electrical energy as an amplifier itself – hence power.
MOSFET stands for “Metal Oxide Silicon Field Effect Transistor”, this in tur
n
means that the device is constructed of Silicon. Similar to a transistor – but th
e
part that controls the power flow through the device is insulated from th
e
remainder of the device by a metal oxide insulating layer and the controlling o
f
the power is achieved by the development of an electrostatic field between th
e
controlling element and the conducting element.
In a transistor, the control of the power through the device is effected by th
e
application of a smaller, but nevertheless, significant amount of power to th
e
controlling element. Whereas in the power MOSFET, the control of the powe
r
through the device is affected by the application of a very small and ver
y
insignificant amount of power to the controlling element – in fact, only th
e
amount required to create a small electrostatic field. This makes the operation o
f
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: th
e
junction FET, the vertical FET and the power MOSFET, all of which wer
e
independently developed by three different hi-fi equipment manufacturers i
n
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 ove
r
power transistors, the junction FET and vertical FET cannot compare with th
e
power MOSFET, in terms of simplicity of the supporting driver stages an
d
power supply requirements.
The power MOSFET, though having similar characteristics to the valve, can b
e
divided into 2 types of polarities of device – P-channel an
d
N-channel. Broadly speaking only one of these types exists in valve operations.
This means that complementary power MOSFETs – P and N channel – can b
e
used in an audio output stage providing greater linearity of operation than can b
e
achieved with valves. In addition, further advantages over the valve includ
e