Switching probability of polycrystalline and amorphous devices dependent on the applied voltage. The dots are measured data points and the solid lines are fits of Eq. (1). The parameters of the fits, i.e. d and V0, are given in the plots. Furthermore, the size of the switching windows ΔVsw is given. In (a, b) the set and reset behaviour of the polycrystalline devices are shown, respectively. The same is depicted in (c, d) for the amorphous devices. The switching probabilities are determined by measuring 128 polycrystalline and 128 amorphous devices with read-out and switching times of 10 µs.