Two dimensional (2D) semiconductor materials haven been extensively studied for the last decade, regarded as one kind of the most important breakthrough materials toward future device technologies. Band gap of monolayer and few layer in 2D semiconductors has been reported, measured by optical probing such as photoluminescence (PL). However, if their exfoliated thickness is as large as multilayer over 10 layers (10L), PL measurements become less effective and inaccurate because the optical transition of 2D semiconductor is changed from direct to indirect mode. At the moment, the energy band gap also becomes smaller with the semiconductir thickness. In fact, for thick multilayer 2D semiconductors, practical band gap measurement is very difficult due to lack of measurementment technique, so that density function theory (DFT)-based calculations might be only way to conjecture the bandgap. Here, we introduce another way to measure such bandgap of multilayer 2D semiconductors; that is using field effect transistor (FET) as a platform. We have thus fabricated FETs with multilayer 2D channel using graphene contact and top passivation for this study. Because graphene contact would secure ambipolar behavior and Schottky contact barrier tuning of FETs with the assitance of top passivation, bandgap measurement was successfully carried out. Our FETs with WSe2, MoTe2, BP channel, show ambipolar behavior and their maximum Schottky barrier height are measured by temperature-dependent transfer curve characteristics. Bandgap values from WSe2 FET1, WSe2 FET2, MoTe2 FET, and BP FET were 1.16,1.44, 0.92, 0.42 eV, respectively obtained by twice the maximum Schottky barrier height. The values from WSe2 FETs were compared with those of PL measurement, to be confirmed for their compatibilities and according to literature, our values seems reasonable.