From a circuit point of view a unipolar switching is preferable, since a normal diode can be used as a select device to block snack currents of the other paths in a cross point array. However, when it comes to the physical mechanism, a unipolar switching means that somehow the conductive path has to be destroyed by passing a current through it. As a consequence unipolar switching effects normally come with significant reset currents and it is much easier to design a low power and high endurance bipolar switching.

Ahh.. now I understand.

Thank you for your reply sir.

see this paper: https://www.researchgate.net/file.PostFileLoader.html?id=5167b180d3df3e203e000001&key=504635167b18072e05