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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

There are basically three types of power field effect device, they are: the

Effect

junction FET, the vertical FET and the power MOSFET, all of which were

Devices

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

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