Patent ID: 8229683

Claim:
A test circuit for measuring time-varying change in threshold voltage in a device under test due to voltage stress-induced bias temperature effect, the test circuit comprising: a plurality of delay elements having delays substantially sensitive to only a stress-induced bias temperature effect of a single polarity, wherein the plurality of delay elements are connected in cascade to form a delay line; a stress voltage supply for applying a voltage stress to particular devices within the delay elements; a pulse generator for supplying a pulse to an input of a first one of the plurality of delay elements; an edge-detecting circuit for detecting the position of an edge of the pulse within the delay line after a predetermined period after a start of the pulse; and a test interface for transmitting the detected position of the pulse edge to an external measurement system, and wherein the pulse generator has an input coupled to an output of a last one of the delay elements, whereby the delay line forms a ring oscillator, wherein the test circuit further comprises a counter for counting oscillations of the ring oscillator during the predetermined period, wherein the test interface further transmits a count value of the counter to the external measurement system.