Memory curves with different regions measured at pressurep= 200 Pa, glow currentI_g= 130 μA: (a) at working voltageU_w= 250 V and inter-electrode distanced= 1 cm, and (b) at 50% overvoltage, i.e.,U_w= 315 V, 345 V, and 375 V andd= 1, 1.5, and 2 cm (t̄_d: mean value of electrical breakdown time delay,t̄_f: formative time delay,t̄_s: statistical time delay,σ(t_d): standard deviation of time delay, andτ: relaxation time).