We have investigated the effect of rate window, transient time, and reverse bias field on the deep levels in molecular beam epitaxy (MBE) grown, Si-doped, low temperature (LT) GaAs by field effect deep level transient spectroscopy (FEDLTS). As far as we know, this is the first successful paper on LT-GaAs investigated by FEDLTS. The applied rate window (R=t2/t1) ranged from 5 to 8 with interval 1, transient time from 5 to 50 s, and reverse bias field from -0.5 to -4×105 V/cm. The capacitance transients (CT) were recorded from 70 to 380 K at temperature intervals of 0.5, 1, or 2 K and analyzed by modulating function waveform analysis. The CT were relatively strong marked by very short growth time (∼5 s). At the measurement transient time greater than 5 s, the CT began to deteriorate showing no further resolution. On the other hand, the FEDLTS spectra were quite dense due to a large number of overlapping deep levels which were found to be very sensitive to the selection of rate window, measurement transient time, and reverse bias electric field. LT-GaAs was found to posses as many as nine electron trap levels in contrast to five deep trap levels observed in MBE-grown, Si-doped, high temperature GaAs studied earlier. © 1997 American Vacuum Society.