Abstract
A new high-speed bipolar transistor structure, the ELOBJT-3, is proposed as a novel application of selective epitaxy technology. The new structure is greatly suited to high-speed ECL circuits, where Ccb, C,, and Rbx are of prime importance. The reduction of these parasitics to their nearly theoretical minimums is accomplished through the use of dielectric isolation and concentric contacting. For extremely high speed operation, dimensions can be scaled to sub-micron size due to the completely self-aligned emitter-base region. Simulation was used to compare important device parameters of the ELOBJT-3 device and a comparably sized existing high speed bipolar structure. Results showed significant improvement in all three of the investigated parameters. Rbx, Ccb, and C, had reductions of 77, 58, and 43 percent respectively. These simulated values were used in a circuit simulation where ELOBJT-3 devices provided a 37% reduction of propagation delay. The device simulations verified the ELOBJT-3's significantly reduced parasitics and propensity for high speed operation. The ELOBJT-3 self-aligned pedestal structure was obtained following considerable process development. It was found that CLSEG could be grown within- an oxide cavity without the use of nitride. If nitride was used, a passivation technique was developed which virtually eliminated nucleation and clogging at the via holes. A PNP configured ELOBJT-3 device with N+ doped CLSEG base contacts was built to establish feasibility of the self-aligned structure. Also, fully functional NPN devices were built in a simplified structure with current gains up to 90. Dislocations and defects at the SEG edge produced unacceptable emitter-collector leakage currents unless the emitter was moved away from the SEG edge. The reported problems with junctions located at SEG sidewalls were avoided by moving the junction out of the sidewall area. Finally, parasitic measurements were correlated with computer simulation to validate the previous comparison simulations.
Date of this Version
July 1992