AN EPITAXIAL EMITTER-CAP SIGE-BASE BIPOLAR TECHNOLOGY OPTIMIZED FOR LIQUID-NITROGEN TEMPERATURE OPERATION

We give the first demonstration that a properly designed silicon bipolar technology can achieve faster unloaded circuit speed at liquid-nitrogen temperature than at room temperature. Transistors were fabricated using a reduced-temperature process employing an in situ arsenic-doped polysilicon emitter contact, a lightly phosphorus-doped epitaxial emitter-cap layer, and a graded SiGe base. At 84K, transistors have a current gain of 500, with a cutoff frequency of 61 GHz, and a maximum oscillation frequency of 50 GHz. ECL circuits switch at a record 21.9 ps at 84K, 3.5-ps faster than at room temperature. Circuits which were optimized for low-power operation achieve a minimum power-delay product of 61 fJ (41.3 ps at 1.47 mW), nearly a factor of two smaller than the best achieved to date at 84K. The unprecedented performance of these transistors suggests that SiGe-base bipolar technology is a promising candidate for cryogenic applications requiring the fastest possible devices together with the processing maturity and integration level achievable with silicon fabrication.