An apparatus for determining alternating current (AC) delay variation of a transistor device under test includes a ring oscillator, the ring oscillator having the transistor device under test configured within a feedback path of the ring oscillator; and circuitry configured to measure a difference between a first signal delay path and a second signal delay path, the first signal delay path being between a gate terminal and a drain terminal of the transistor device under test, and the second signal delay path being between a source terminal and the drain terminal of the transistor device under test.