HEMT comparators for ultrahigh-speed A/D converters have been investigated. In particular, the transition times of the D-latch used in the comparator have been analyzed by assuming a 0.1-μm HEMT technology. It is found that for small input signals (<0.1V), the transition time from the track to latch phase dominates the comparator operation speed. As the input signal increases, this time decreases due to the positive feedback in the latch, and the comparator speed is limited by the transition time from the latch to track phase. The transition times of 20ps have been estimated for the present comparator.