Comparison of silicon bipolar and GaAlAs/GaAs heterojunction bipolar technologies using a propagation delay expression

A performance comparison is presented of silicon bipolar and GaAlAs/GaAs heterojunction technologies for high-speed ECL (emitter-coupled logic) circuits. The propagation delays for state-of-the-art technologies are calculated using a quasianalytical equation which expresses the propagation delay in terms of all the time constants of the circuit. An idealized self-aligned transistor layout is used to eliminate geometry differences from the comparison, and hence to allow the roles of the heterojunction and of the fundamental material, and the device properties to be identified. The gate delays of GaAlAs/GaAs and silicon circuits are predicted. It is shown that the lower gate delays of GaAlAs/GaAs circuits arise primarily from the use of a heterojunction emitter, and also because the peak f/sub T/ of the GaAlAs/GaAs transistor occurs at high collector current density. The limiting time constants are identified for both technologies, and the best approaches for further optimization discussed.<>