Electrothermal limitations on the current density of high-frequency bipolar transistors

In this paper, electrothermal consequences of downscaling bipolar transistors, reducing the emitter resistance and implementing substrate modifications are examined by means of electrical measurements, numerical simulations and analytical calculations. A formulation is given for the optimum current density that can be run through the device and still maintain both sufficient transconductance and thermal stability. This expression sets a theoretical limit on the current density and therefore also on the speed of the given technology node. Particularly the lowering of the emitter resistivity is a trade-off between transconductance and thermal stability, and the optimum choice can be estimated from these results along with the maximum emitter area that will allow unconditional thermal stability.