Amorphous semiconductors such as a-C:H and a-Si:H deposited via conventional commercially available plasma enhanced chemical vapour deposition systems (PECVD) have been shown to be capable of electron emission at low threshold fields. Using PECVD it is possible to produce 'mirror like' thin films over large areas which if deposited under optimised conditions of film thickness and dopant content can produce good field electron emission characteristics. This paper will discuss the results of different experiments, which show that by a suitable choice of substrates, deposition and post-deposition treatments it is possible to optimise the threshold electric field. The role of defects on the electron emission properties of the films is discussed and the electron emission process will be discussed in terms of a space-charge induced emission mechanism.