In transition metal dichalcogenides （TMD） flakes， the geometry， such as layer thickness， significantly tune the electronic properties， including bandgap， electron affinity and Fermi level. Such characteristic offers a high degree of freedom to tune the functionality of semiconductor device， once the volatile electronic properties are precisely determined. However， to date， there are still significant uncertainties in determining the Fermi-level alignment of TMD homo- or hetero- junctions， which might lead to significant deviations of band-bending and thus device performance. Here， we utilize the Scanning Kelvin Probe Microscopy （SKPM） to characterize the surface-potential/Fermi-level alignment of TMD homo- or hetero- junctions. Through this effort， a distinct phenomenon is verified where the Fermi-levels of MoS₂ and MoTe₂ shift towards the intrinsic level with an increasing layer thickness （in other words， the background doping concentration is continuously lowering）. Moreover， we show the significant impact of surface contamination （molecular scale） on the surface potential of monolayer TMD. Finally， we fabricate a MoTe₂/MoS₂ heterojunction， in which we observe the wide depletion region and large photoresponse. Together， those findings might offer a reference to precisely stack van der Waals （vdW） layers as designed for both electronic and optoelectronic applications.