where α is the carrier transfer effi ciency from ZnO QDs to graphene, q is the elementary charge, W is the sample width, and v = μ V DS L − 1 is the carrier drift velocity in graphene with the carrier mobility μ and the sample length L . T t = L 2 μ − 1 V − 1 is the carrier transit time. In our work, with W = 30 μ m, L = 10 μ m, V DS = 1 m V , and μ = 3000 cm 2 V − 1 s − 1 , T t is estimated to be 330 ns. Assuming η = 1 and α = 1, [ 26 ] we estimate the gain by