Why is substrate terminal needed in electronic components like transistors?
Modern transistors are built on a piece of silicon. To get isolation between transistors, the substrate is diffused so that the collector substrate junction can be reverse biased in normal operation. There is a collector substrate diode, which has capacitance. If you don’t model this connection, your simulations won’t be correct. More importantly, if you don’t bias the substrate properly in the real world, all you transistors will talk to each other and your chip won’t work properly. You could experience SCR latchup, etc. Integrated bipolar transistors are 4 terminal devices!
