Differences in ion mass of lighter ions (having a higher mobility) and heavier ions are utilized in conjunction with bias voltage modulation of an atomic layer etch (ALE) to provide a fast ALE process. The difference in ion mobility achieves surface modification with reactive neutral species in the absence of a bias voltage, and ion bombardment with lighter ions (e.g., inert or less reactive ions) in the presence of a bias voltage. By modulating the bias voltage, preferential ion bombardment is achieved with lighter ions without the need to physically separate or purge the reactive precursors and inert gases supplied to the process chamber for a given ALE cycle. A “fast” plasma ALE process is provided which improves etch rate, throughput and cost-efficiency by enabling the same gas chemistry composition (e.g., reactive precursor and inert gas combination) to be kept in the process chamber during a given ALE cycle.