Latchup is the most common problem in the CMOS transistor. Mainly causes due to the formation of BJTs (PNP and NPN) and can be prevented using Guard Rings. First of all, this is the most important VLSI interview question. Most of the interview guys prefer to ask this question to check the basics of the candidate regarding MOS and its second-order effects. You must also read these topics which I am listing below. 1. Non-ideal characteristics of MOSFET such as Velocity Saturation, Mobility degradation, Channel length modulation (CLM), body effect, subthreshold conduction, DIBL (Drain induced barrier lowering), BTBT (Band to band tunneling), GIDL (Gate induced barrier lowering), 2. the Tunneling effect, 3. Latchup, 4. Stack effect, 5. Charge sharing effect, 6. Short channel effects, and Narrow channel effects. Latchup: Latchup