Perreaux Prisma 350 owner manual The Power MOSFET, The MOSFET Other Field Effect Devices

8 The Power MOSFET

The MOSFET

Other Field

Effect

Devices

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 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.

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

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