In previous studies of tunneling through a MOSa tunnel junction, where Sa was a-Si : H, it was shown that their characteristics resemble those of MOSc devices where Sc was crystalline silicon. In the present work we would like to report a demonstration of fundamental tunneling processes in such tunnel junctions. In particular, the transition from semiconductor controlled regime to tunneling controlled regime can be clearly distinguished. The present results represent one of the rare cases where a fundamental semiconductor process is demonstrated more clearly in an amorphous semiconductor device than in a crystalline semiconductor device. Following the above findings we have identified tunneling into forbidden-gap states and we got a qualitative map of the state distribution in the forbidden-gap