Predicted propagation delay of Si/SiGe heterojunction bipolar ECL circuits

A comparison between the predicted propagation delays of ECL (emitter coupled logic) circuits composed of Si/SiGe heterojunction and silicon homojunction bipolar transistors is presented. 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 tradeoffs for the heterojunction device to be identified. The 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 μm. The resulting transistor has a gain of >50 and an emitter delay of <1 ps. A quasi-analytical expression is 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 μm, a propagation delay of 10 ps is predicted for Si/SiGe heterojunction circuits