THE IGFET/MOSFET

The second (and most important) family of FETs are those known under the general title of IGFET or MOSFET. In these FETs, the gate terminal is insulated from the semiconductor body by a very thin layer of silicon dioxide, hence the title ‘Insulated Gate Field Effect Transistor,’ or IGFET. Also, the devices generally use a ‘Metal-Oxide Silicon’ semiconductor material in their construction, hence the alternative title of MOSFET.

Shows the basic construction and the standard symbol of the n-channel depletion-mode FET. It resembles the JFET, except that its gate is fully insulated from the body of the FET (as indicated by the symbol) but, in fact, operates on a slightly different principle to the JFET. It has a normally-open n-type channel between drain and source, but the channel width is controlled by the electrostatic field of the gate bias. The channel can be closed by applying suitable negative bias, or can be increased by applying positive bias.

         In practice, the FET substrate may be externally available, making a fourterminal device, or may be internally connected to the source, making a three-terminal device. An important point about the          IGFET/MOSFET is that it is also available as an enhancement-mode device, in which its conduction channel is normally closed but can be opened by applying forward bias to its gate. Shows the basic construction and the symbol of the n channel version of such a device. Here, no n-channel drain-to-source conduction path exists through the p type substrate, so with zero gate bias there is no conduction between drain and source; this feature is indicated in the symbol of Figure 13(b) by the gaps between source and drain. To turn the device on, significant positive gate bias is needed, and when this is of sufficient magnitude, it starts to convert the p-type substrate material under the gate into an nchannel, enabling conduction to take place. Shows the typical transfer characteristics of an n-channel enhancement-mode IGFET/MOSFET, and Figure 15 shows the VGS/ID curves of the same device when powered from a 15V supply. Note that no ID current flows until the gate voltage reaches a ‘threshold’ (VTH) value of a few volts, but that beyond this value, the drain current rises in a non-linear fashion. Also note that the transfer graph is divided into two characteristic regions, as indicated (in Figure 14) by the dotted line, these being the ‘triode’ region and the ‘saturated’ region. In the triode region, the device acts like a voltage-controlled resistor; in the saturated region, it acts like a voltage-controlled constant-current generator.

        The basic n-channel MOSFETs and 13 can — in principle — be converted to p-channel devices by simply transposing their p and n materials, in which case their symbols must be changed by reversing the directions of their substrate arrows. A number of sub-variants of the MOSFET are in common use. The type known as ‘DMOS’ uses a double-diffused manufacturing technique to provide it with a very short conduction channel and a consequent ability to operate at very high switching speeds.

     Several other MOSFET variants are described in the remainder of this opening episode. Note that the very high gate impedance of MOSFET devices makes them liable to damage from electrostatic discharges and, for this reason, they are often provided with internal protection via integral diodes or zeners, as shown in the example MOSFET, the main signal current flows ‘laterally’ through the device’s conductive channel. This channel is very thin, and maximum operating currents are consequently very limited (typically to maximum values in the range 2 to 40mA). In post-1970 times, many manufacturers have tried to produce viable high-power/high-current versions of the FET, and the most successful of these have relied on the use of a ‘vertical’ (rather than lateral) flow of current through the conductive channel of the device. One of the best known of these devices is the ‘VFET,’ an enhancement-mode power MOSFET which was first introduced by Siliconix way back in 1976.