Audio Engineering

Audio Amplifi ers 301

techniques in a lateral MOSFET will be too long to allow a low channel “ on ” resistance.
For high current applications, the semiconductor manufacturers have therefore evolved
a range of vertical MOSFETs. In these, very short channel lengths are achieved by
sequential diffusion processes from the surface, which are then followed by etching a
V- or U-shaped trough inward from the surface so that the active channel is formed across
the exposed edge of a thin diffused region. Because this channel is short in length, its
resistance will be low, and because the manufacturers generally adopt device structures
that allow a multiplicity of channels to be connected electrically in parallel, channel “ on ”
resistances as low as 0.008 Ω have been achieved.

Like a JFET, the MOSFET would, left to itself, have a square-law relationship between
gate voltage and drain current. However, in practice, this is affected by the device
geometry, and many modern devices have a quite linear Id / Vg characteristic, as shown in
Figure 9.18 for an IRF520 power MOSFET.

The basic problem with the MOSFET is that of gate/channel overvoltage breakdown,
in which the thin insulating layer of silicon oxide or silicon nitride between the gate
electrode and the channel breaks down. If this happens the gate voltage will no longer

10

I (amperes)d

IRF520

8

6

4

2

0

0510

Vg (volts)

Figure 9.18 : Power MOSFET.