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 their much smaller size, no filaments and greater reliability with reduced vulnerability to physical dam

When used in an audio power amplifier, the advantages of the power MOSFET over the power transistor are much more difficult to describe and would require greater complexity than can be gone into here. However, they can be summarised as follows ? the most important has a negative temperature coefficient whereas the power transistor

has a positive temperature coefficient. This means that when a power transistor is handling power it heats up further and consumes more power. This characteristic, called thermal runaway, will result in the destruction of the power transistor if some means is not provided to control it. The power MOSFET on the other hand, although heating up due to the power flow through the device

does not continue to draw more and more power just because its temperature has risen. But in fact has a tendency to stabilize itself ? provided adequate head sinking is available to remove the heat generated during normal operation. Incidentally this is less heat sinking than is required for a similarly power rated standard transistor.

Then there is the appearance of secondary breakdown and ?hot spots? in a power transistor. This is related to thermal runaway. In order to understand this, one must imagine that the chip silicon inside the power transistor is in fact many smaller transistors connected in parallel. Now, if one of these smaller transistors or a spot on the chip has a greater gain (or amplification factor) than the rest, then that spot will heat up faster and to a greater temperature than the remainder of the transistor chip. This means that whole power dissipation capability of

the transistor has been severely reduced and is a major cause of these unexplained output stage failures in large power amplifiers, i.e. over 80Wrms.

The power MOSFET is largely immune to this problem because if a small part of this chip has a higher gain than the rest then its temperature will rise slightly causing that spot to reduce gain and hence stabilization occurs. The power is more evenly distributed throughout the chip and therefore reliability is maintained. It can be seen from the above that the transistor power amplifier has to have a much larger

margin of power dissipation capability and heat sinking in its output stage than the power MOSFET amplifier.

The transistor power amplifier of 100Wrms output into 8??can require a driver stage capable of delivering 10W at 1kHz and up to 20W at 20kHz into the input of the output device. The power MOSFET only requires a maximum of 0.01W so a major saving in driver stage componentry and associated noise and distortion can be eliminated.

Probably, from the sonic quality point of view, the most important improvement is the power MOSFETs vastly superior high frequency response. A large proportion of the power transistors used in modern hi-fi amplifiers start to show a decline in efficiency from 10kHz upwards. The efficiency of the power MOSFET does not start to decline until about 2MHz and is only down 3dB at 30MHz. This is due to the energy transfer being accomplished with minority charged

carriers in the power MOSFET as opposed to majority charged carriers within the transistor, and results in hole storage at high frequencies causing the transistor to dissipate increasing amounts of energy within itself as the frequency increases.

Further sonic degradation of the transistor power amplifier occurs due to hole storage of the output transistors. As the output distortion increases with increasing signal frequency, it is obvious that the distortion products in the negative feedback path also increase.

Because the negative feedback system is employed to reduce distortion by cancellation, at high frequencies it causes even more power to be consumed within the output transistor just to cancel out the distortion.

Transient intermodulation (TIM) is also more prevalent in transistor power amplifiers because the signal transition in time is relatively slow. This means the distortion products in the signal of, say, a fast transient will not travel through the negative

Further sonic improvement is achieved in power MOSFET amplifiers due to reduced crossover distortion, as power MOSFETs have a sharper ?knee? than transistors at cut-off and provide a greater linearity when crossing over from one device to the other. Because crossover distortion is a major cause of odd order harmonic distortion in transistor amplifiers (be it small, i.e. 0.05% total) they are usually considered to sound more harsh

than valve amplifiers which generally have large amounts of even order harmonic distortion up to 5% and are thought to sound more pleasant and musical.

However, which is more accurate? The valve amplifier at 5% THD with a pleasant sound and even order harmonics; the transistor amplifier with 0.05% THD with relatively unpleasant sound with even and odd harmonic output, or a power MOSFET amplifier with 0.02% THD

and relatively pleasant even order harmonic distortion? In our opinion, the power MOSFET amplifier because the THD generated is virtually all second or even order harmonic distortion total 0.02% or less at 20kHz and down to 0.004% or less at 1kHz.

It can be seen that power MOSFETs are here to stay and that there are major sonic and electrical improvements to be had over other output devices.