Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
As leo told, you can use several P+ diodes on Nwell, if they may share their common cathodes (or N+ diodes on p substrate, if they can share their common anodes). I they can't share a common node, use N+ diodes in separate Nwells. The layout is arbitrary -- can be circular, square, rectangular -- whatever your design system allows for. Diodes' size depends on your application. With a std. CMOS process, however, you can't get a very good PIN diode, as the "I" zone will be rather thin, in the order of only one or a few microns, not much for a good radiation capture sensitivity, if this should be your application.