For bulk WSe2, we found that the dielectric constant of the substrate not only alters the response time but also the speed-limiting mechanism. Glass has a larger dielectric constant of 4.5 -8, in comparison to the weaker screening on polyimide with 3.7. For polyimide, the WSe2 detectors are faster and limited by the RC time. For the detectors on glass, the response time is slower than the RC constant, presumably now limited due to drift limitation as a consequence of a shrunken depletion layer induced by the higher dielectric screening (Figure ). The maximal achieved bandwidth is 2.6 MHz.