MoS2 and WS2 trilayers were grown on p-type Si wafers using atmospheric pressure plasma processes. Current-voltage measurements of MoS2/Si and WS2/Si heterojunctions showed rectifying behaviors, indicating formation of diodes. It should be noted that the very large shunt resistance indicated uniform MoS2 layer formation on the Si wafers. Relatively large dark current of the MoS2 and WS2 heterojunctions under reverse bias indicated the band to band tunneling and avalanche multiplication processes. Temperature dependence of the diode ideality factor was also studied to reveal the major recombination processes, based on conventional 3D semiconductor models. Photocurrent characteristics of the junctions were studied using green lasers (wavelength: 532 nm). Large photocurrent was observed under reverse bias, whereas photocurrent was negligibly small under forward bias. The measured photocurrent was linearly proportional to the laser power. This suggested that trapping and detrapping of the photo-generated carriers at interface defects and surface adsorbates did not much limit the collection of photo-carriers. Both MoS2/Si and WS2/Si heterojunctions showed fast photoresponse: the rising and decaying time constants were less than 0.1 ms. All these results showed that our processes could prepare high quality 2D/3D hybrid semiconductor heterojunctions with clean interfaces.