Long-term reliability of Si-Si/sub 0.7/Ge/sub 0.3/-Si HBTs from accelerated lifetime testing

Accelerated lifetime tests were performed on double-mesa structure Si-Si/sub 0.7/Ge/sub 0.3/-Si npn heterojunction bipolar transistors, grown by molecular beam epitaxy, in the temperature range of 175/spl deg/C-275/spl deg/C. The transistors (with 5/spl times/20 /spl mu/m/sup 2/ emitter area) have DC current gains /spl sim/40-50 and f/sub T/ and f/sub max/ of up to 22 GHz and 25 GHz, respectively. It is found that a gradual degradation in these devices is caused by the recombination enhanced impurity diffusion (REID) of boron atoms from the p-type base region and the associated formation of parasitic energy barriers to electron transport from the emitter to collector layers. This REID has been quantitatively modeled and explained to the first order of approximation, and the agreement with measured data is good. The mean time to failure (MTTF) of these devices at room temperature under 1.35/spl times/10/sup 4/ A/cm/sup 2/ current density operation is estimated from extrapolation of the Arrhenius plots of device lifetime versus reciprocal temperature. The results of the reliability tests offer valuable feedback for SiGe heterostructure design in order to improve the long-term reliability of the devices and circuits made with them.