PREDICTED PROPAGATION DELAY OF SI/SIGE HETEROJUNCTION BIPOLAR ECL CIRCUITS

This paper presents a comparison between the predicted propagation delays of ECL circuits composed of Si/SiGe heterojunction and silicon homojunction bipolar transistors. Important transistor parameters, such as the current gain, base transit time, base resistance, and emitter delay, are calculated for the heterojunction transistor as a function of the Ge concentration in the SiGe base. This allows the important design trade-offs for the heterojunction device to be identified. These calculations show that a Ge concentration of 12% is sufficient to allow the reversal of the usual emitter and base doping concentrations in a transistor with a base width of 0.02 pm. The resulting transistor has a gain of > 50 and an emitter delay of <1 ps. A quasi-analytical expression is then used to calculate the propagation delay of 1-μm ECL circuits incorporating the above transistor. A propagation delay of 15 ps is predicted for fully optimized Si/SiGe heterojunction circuits, compared with 29 ps for fully optimized silicon homojunction circuits. On scaling to geometries below 0.5 pm, a propagation delay of 10 ps is predicted for Si/SiGe heterojunction circuits. © 1990 IEEE