On the basis of polarization arguments, the equality &tgr;d=&tgr;lis proposed as the critical condition for the threshold point of chalcogenide glass switches, &tgr;dbeing the dielectric relaxation time and &tgr;lthe carrier lifetime. It is shown that this switching criterion is in good agreement with measurements and that it explains the temperature dependence of the threshold voltage as well as the voltage dependence of the delay time.