Audio Amplifi ers 305A further benefi t enjoyed by the MOSFET is that it is a majority carrier device, which
means that it is free from the hole-storage effects that can impair the performance of
power junction transistors and make them sluggish in their turn-off characteristics at
high collector current levels. However, on the debit side, the slope of the Vg / Id curve of
the MOSFET is less steep than that of theVb / Ic curve of the BJT, which means that the
output impedance of power MOSFETs used as source followers is higher than that of an
equivalent power BJT used as an emitter follower. Other things being equal, a greater
amount of overall negative feedback (i.e., a higher loop gain) must therefore be used to
achieve the same low amplifi er output impedance with a power MOSFET design than
would be needed with a power BJT one. If a pair of push–pull output source/emitter
followers is to be used in a class AB output stage, more forward bias will be needed with
the MOSFET than with the BJT to achieve the optimum level of quiescent operational
current, and the discontinuity in the push–pull transfer characteristic will be larger in size,
although likely to introduce, in the amplifi er output signal, lower rather than higher order
crossover harmonics.
9.10 U and D MOSFETs ...............................................................................................
I have, so far, lumped all power MOSFETs together in considering their performance.
However, there are, in practice, two different and distinct categories of these, based
on their constructional form, and these are illustrated in Figure 9.22. In the V or U
MOS devices—these are just different names for what is essentially the same system,
depending on the profi le of the etched slot—the current fl ow, when the gate layer has
been made suffi ciently positive (in the case of an N-channel device) to induce a mobile
electron layer, will be essentially vertical in direction, whereas in D-MOS or T-MOS
construction the current fl ow is T shaped from the source metallization pads across
the exposed face of the very lightly doped P region to the vertical N / N drain sink.
Because it is easier to manufacture a very thin diffused layer ( short channel) in the
vertical sense than to control the lateral diffusion width, in the case of a T-MOS device,
by surface masking, the U-MOS devices are usually much faster in response than the
T-MOS versions, but the T-MOS equivalents are more rugged and more readily available
in complementary (N-channel/P-channel) forms.
All power MOSFETs have a high input capacitance, typically in the range of
500–2500 pF, and because devices with a lower conducting resistance ( Rds/on ) will have
achieved this quality because of the connection of a large number of channels in parallel,