(color online) (a) Schematic of device structure of a few-layer BP back gate FET.^[5] (b) Field-effect mobility (red open circles with error bar) and Hall mobility (filled squares with error bar, three different values of n) as a function of temperature on a logarithmic scale. A power-law dependence (black dashed line) is plotted in the high-temperature region as a guide to the eye.^[5] (c) Transfer curves of a typical few-layer BP transistor with a film thickness of ∼ 5 nm.^[6] (d) Mobility summary of few-layer BP transistors with varying thicknesses.^[6] (e) DC conductivity and IR relative extinction measured along the same six directions on a BP flake in polar coordinates. The optical image of this device was in the inset figure.^[24] (f) Hall mobility measured at a constant hole doping concentration of cm along the x and y directions for BP thin films with a thickness of 8 and 15 nm, respectively.^[24]