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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 operations.
This means that complementary power MOSFETs – P and N channel – can be
used in an audio output stage providing greater linearity of operation than can be
achieved with valves. In addition, further advantages over the valve include
their much smaller size, no filaments and greater reliability with reduced
vulnerability to physical damage.