Amorphous oxide thin-film transistors (TFTs) have provided a new route to transparent, low power consumption, large-area display technology. The properties of these thin films are crucially dependent on material composition and process variations which give rise to sub-gap defect states that may act as dopants and can be used to tune the electronic properties of devices. Because these devices are sensitive to small changes in processing, there is a need for a high-throughput, reliable method for extracting sub-gap defect concentrations of amorphous oxides. We present a method, ultra-broadband photoconduction microscopy (UBPC), to extract the density of states of defects throughout the sub-gap of amorphous In-Ga-Zn-O (a-IGZO) TFTs from 0.3 – 3.5 eV. By utilizing continuously tunable, focused laser excitation, we maintain high signal-to-noise over ~5 orders of magnitude of photoconduction, while removing transfer curve drift effects by subtracting dark current in real-time via lock-in amplification.