Amorphous Silicon alloy thin films were deposited by plasma enhanced chemical vapor deposition technique. In order to make optoelectronic measurements, diode structures were fabricated by depositing transparent metal electrodes. Theoretical background of localized density of states in the mobility gap and photocurrent mechanisms has been revisited. In light of this, time of flight technique, using transient photocurrent, was utilized to determine mobility in extended states and characteristic energy of tail states in the film. The actual density of states (DOS) in the mobility gap of the deposited films was determined by using absorption coefficients obtained via constant photocurrent measurements. Finally, adverse effects of small Oxygen incorporation on mobility and DOS were observed.