This month we got involved in the detailed design of a diode. For most analog and RF ASIC designers diodes are pretty trivial as far as design is concerned. The reason is that we get the parameters from the foundry and use the scaling for diodes already fully characterized by the foundry. However, when we get a specification like: " Need a diode with a series resistance of 1 ohm, a capacitance of 0.2pF, with a clamping voltage of 25 volts with a 5 Amp current", things get a little more sticky.

Where do we start? I suppose one set of answers are: (1) Start with the substrate.
(2) Use Irwin's curves to calculate sheet resistance, (3) Use a manual like the Semiconductor QRM design manual to get an initial design for the required capacitance. This involves extracting (a)The concentration gradient (b) The built in voltage (c) zero bias capacitance... Once all of these preliminary parameters are calculated, we use a simulator like Athena or Atlas ( or other simulation tools like the Stanford University TCAD set) or SYNOPSYS. This is the tricky part where optimization becomes so important and it takes a long time!