PROPAGATION DELAY IN HIGH SPEED SILICON BIPOLAR AND GaAs HBT DIGITAL CIRCUITS

The needs for multi-gigabits/second digital electronics in advanced lightwave systems have motivated R & D for the next generation of high speed bipolar technology. The speed of the digital circuit may be estimated from the propagation delay of the logic gate. We discuss physics of the delay and show that it is fundamentally limited by the time needed for turning on (off) a transistor and by the time for charging (discharging) of capacitances in the circuit. We present a large-signal theory based on a charge-control model for the calculation of these limits. The results obtained for emitter coupled logic and current mode logic are used to analyze current technologies of silicon bipolar and GaAs HBTs.