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

140 The Circuit Designer’s CompanionProtection for the gateThe other solution is to prevent the gate voltage from reaching the breakdown level byincorporating extra protection, in the form of a zener diode rated to just below thebreakdown voltage (Figure 4.32), within the package of the device. Manufacturers mayNo protection gate-source diode gate-body diodeFigure 4.32 MOSFET gate protectionoffer versions of the same device with and without gate zener protection. The greatadvantage of including protection is that the device can now be treated in the assemblyprocess just like any other semiconductor component, and no additional assemblyprecautions need be taken.The trade-off is in the circuit performance, of course. The zener restricts negativegate voltage swings (for an n-channel device) to one diode drop, which can affect thedesign of the gate drive circuit. More importantly, it adds a component of gate leakagecurrent which exhibits all the properties of diode leakage outlined earlier. In someapplications this can nullify the advantage of using a MOSFET.MOSFET tradeoffsThe choice facing designers who want to use low-power MOSFETs is fairly simple.Either specify a gate-protected device and accept the performance limitations that thisimplies, but make yourself popular with the production department. Or, insist on usingunprotected devices because you need ultra-high impedance, and impose extra costs inproduction, along with possible reliability penalties if the production procedures are notrigorously enforced.Remember also that the susceptibility of low-power MOSFETs to gate breakdownmay not end after they are soldered into circuit. Especially if they are used in highimpedanceinput circuits, the gate can still be vulnerable if its biasing resistor is in thehigh megohm range (or worse still, absent) and stringent handling precautions mayneed to be observed at the board or equipment level.4.5.2 VMOS Power FETsThe constraint of channel on-resistance (typically tens to hundreds of ohms) andconsequent restriction on power handling of the conventional planar MOSFET wasremoved with the development of the double-diffused vertical MOSFET or VMOS.This range of devices, introduced by International Rectifier in the late 70s under theHEXFET trademark and subsequently widely second-sourced, can achieve “on”resistances in the milliohm region, and drain-source breakdown voltages up to 500Vand higher. It is therefore a direct competitor to the power bipolar transistor andoutperforms it in many respects. Table 4.2 compares the major differences betweenthese two and their hybrid offspring, the IGBT (see section 4.6). The necessary