Conventional Ge-Sb-Te based phase change memory (PCM) devices offer promising attributes, including fast non-volatile memory operations, higher endurance and sub-ns re-crystallization/re-amorphization capabilities for the next generation high speed non-volatile memory. The threshold switching process in the amorphous phase of the PCM device enables breakdown of electrical resistance followed by rapid flow of device current, leading to Joules heating induced crystallization, known as SET operation. The speed of SET operation is, therefore, governed by voltage dependent threshold switching dynamics and the delay time (t d) characteristics at the timescale of nanoseconds. Although GeSb2Te4 offers faster crystal growth velocity, realization of fast threshold switching capabilities remains unexplored. In this work, the ultrafast threshold switching dynamics of a prototypical GeSb2Te4 PCM device are investigated. The present experimental results demonstrate an exponential reduction of t d upon increasing the applied voltage (V A) above its threshold voltage (V T). The t d has been reduced to a strikingly lower value of 0.5 ns for V A of 1.6 times of V T, which enables faster SET operation within 3 ns as evidenced by a permanent change in the device resistance. These experimental findings of sub-nanosecond threshold switching dynamics and faster SET operation in the GeSb2Te4 device pave a way for designing a PCM device with ns programming speed for future electronics. © 2019 IOP Publishing Ltd.