App Notes

PARASITIC CAPACITANCE IN TRANSIENT VOLTAGE SUPPRESSORS & LOW CAPACITANCE OPTIONS

A silicon transient voltage suppressor (TVS) has an inherent capacitance resulting from mobile electrons and holes on opposite sides of the p-n junction and depletion layer.  This is equivalent to parallel plates having an intervening dielectric layer of silicon in between thus providing all the elements of a capacitor.  When reverse bias is applied, the depletion region widens and decreases the capacitance as the voltage bias is increased.

Low voltage TVS devices have a high concentration of dopant resulting in a narrow depletion region thus producing higher capacitance values.  Progressively higher voltage devices have exponentially decreasing levels of dopant and wider depletion regions with a corresponding reduction in capacitance.  Figure 1 depicts capacitance versus rated Working Standoff Voltage (V_WM) for both unidirectional and bidirectional devices in an example 1500 W rated series of TVSs.  The bidirectional TVSs have two p-n junctions in series that further reduces capacitance.