In the realm of next generation computing, chalcogenide based phase change memory (PCM) offers promising features for a â€˜universal memoryâ€™ owing toall-round characteristics including high-speed and non-volatility [1-3]. However, realizing an ultrafast switching is still a key challenge for faster programming. This talk will present exhaustive experimental results on electrical switching of Ge-Sb-Te, Ag, In-doped Sb2Te and In-Sb-Tebased PCM devices including ultrafast electrical switching dynamics, voltage-dependent transient characteristics in picosecond timescale using a custom-built advanced programmable electrical test setup [4-9]. Furthermore, a trajectory map for defining the ultimate speed of PCM devices will be discussed on the basis of field-dependent transient dynamics in picosecond timescale [6-9]. This map is developed based on a systematic understanding of the voltage-dependent, time-resolved transient characteristics using voltage pulses of various short pulse-widths (full width half maximum, FWHM) down to 1.5 ns. Our experimental results show that the delay time, td decreases exponentially for increasing the applied voltage, VA above the threshold voltage, VT. With sufficient over-voltage, the device switches remarkably fast at its VT,revealing an ideal â€˜zero-delay-timeâ€™. On the other hand, Ag,In doped Sb2Te device reveals a strikingly differentthreshold switching behavior at VT without delay (sub-50 ps).The switching speed is primarily governed by the rate of VT and it is independent of VA. These novel findings of unique switching behavior of Ag,In-doped Sb2Te and the trajectory map for enabling the ultimate speed of PCM devices would pave a way towards realizing â€˜universal memoryâ€™ for future computing.