Electric field effect in ultrathin black phosphorus

Black phosphorus exhibits a layered structure similar to graphene, allowing mechanical exfoliation of ultrathin single crystals. Here we demonstrate few-layer black phosphorus field effect devices on Si/SiO$_2$ and measure charge carrier mobility in a four-probe configuration as well as drain current modulation in a two-point configuration. We find room-temperature mobilities of up to 300 cm$^2$/Vs and drain current modulation of over 10$^3$. At low temperatures the on-off ratio exceeds 10$^5$ and the device exhibits both electron and hole conduction. Using atomic force microscopy we observe significant surface roughening of thin black phosphorus crystals over the course of 1 hour after exfoliation.