Time-varying low-frequency noise in InGaP/GaAs HBTs

Time-varying low-frequency noise is observed in InGaP/GaAs heterojunction bipolar transistors from two different foundries. Frequency-domain noise spectral density measurements are repeated in regular intervals and plotted in 3-D to reveal variations both in frequency and time. The low-frequency noise models obtained from the frequency-domain noise measurements are used for predicting phase noise but, as the instantaneous values are not preserved in frequency-domain averaging, they have limited validity.