Properties of phosphorene are intensively researched by many research groups since it was discovered in 2014. This new 2D material has a direct band-gap, depending on the number of layers (0.3 eV for bulk to 2.0 eV for monolayer) similar to MoS
. In this paper, we report layer-by-layer (LbL) deposition method to deposit a few layers of phosphorene film on the substrate, and successfully fabricate phosphorene field effect transistor (FET) with LbL black phosphorus (BP) film. By using the FET characteristics of the device, we fabricate biosensors with Poly-L-lysine (PLL) linker molecules and Alpha-fetoprotein (AFP) antibody to detect different concentration of AFP antigen (0.1 ppb to 1 ppm). The results indicate that BP can be a promising material for biosensing applications.