The coefficient of rectification,arect(&ohgr;), for a resonant tunneling diode is obtained from an exact solution for the transmission probabilityT(&egr;) through an oscillating resonant level. The experimentally observed broadening and lowering of the peaks inarect(&ohgr;) with increasing frequency &ohgr; are explained by the increase in spacing between the sidebands inT(&egr;). Most important,arect(&ohgr;) is entirely determined by the dc current. Consequently, any mechanism which broadens the negative differential conductance region will cause a dc‐like coefficient of rectification to persist to frequencies higher than the inverse charge‐transport time.