The Circuit Designer's Companion - diagramas.diagram...

Active components 139Reduce V Dto keepV GDbelow breakpointV DCascodeconnectionV D(a)(b)Figure 4.31 Reducing the effect of gate current breakpointrange of the fet is required then a better solution is to add a second fet in cascode withthe first (b). The drain-gate voltage of the input fet is maintained at a constant lowvoltage over the whole input range by the source of the cascode fet. The latter is stillsubject to operation above the breakpoint at the negative end of the input voltage range,but the excess gate current is now added to the operating current with negligible effect.4.5 MOSFETsThe distinguishing feature of the MOSFET, or metal oxide semiconductor FET, is thatits gate is insulated from the semiconducting channel by a thin layer of oxide. It is nota p-n junction so, unlike the JFET, no gate leakage current flows.4.5.1 Low-power MOSFETsLow-power MOSFETs have existed for some time, aimed at niche applications similarto JFETs, particularly high input impedance amplifiers, RF circuits and analogueswitches. These devices are planar fabricated, with the source and drain terminalsdiffused into one side of the substrate. Development of the planar MOSFET into thedouble-diffused (DMOS) structure has allowed smaller channel dimensions whichresult in lower capacitances and faster switching times. DMOS devices have useableperformance as analogue switches in the sub-nanosecond range, and as RF amplifiersinto the GHz range, but they are not widely sourced.Gate breakdownThe drawback of low-power MOSFETs centres around the extremely thin insulatinglayer between gate and channel. Generally manufacturers specify a breakdown voltagefor the gate of between ±15V and ±40V. At the same time the gate-channel capacitanceis a few pF, and since there is no discharge path for this capacitance when the device isout of circuit the charge needed to exceed the breakdown voltage is very small (arounda hundred pico-coulombs) and can easily be generated electrostatically (cf section9.2.2). As a result low-power MOSFETs can be destroyed simply by handling themprior to insertion into the circuit.There are two solutions to this problem. One is to impose rigorous anti-statichandling precautions throughout production. The safest procedure is to short togetherall leads of the device, for example by wrapping a wire link around them, until the boardassembly is complete. This is labour-intensive compared to most other assemblyoperations, is prone to error and is also incompatible with surface mount packages.Unprotected-gate MOSFETs are therefore not popular for high volume production.