Ring-oscillator-based test structures that can separately measure the negative bias temperature instability (NBTI) and positive bias temperature instability (PBTI) degradation effects in digital circuits are presented for high-k metal gate devices. The mathematical derivation also shows that the structure for frequency degradation measurement can directly be used for estimating the portion of the NBTI and PBTI in the conventional ring oscillator. The proposed test structures including frequency degradation sensing circuitry have been implemented in an experimental high-k/metal gate SoI process.