App note from Nexperia on how MOSFETs’ imperfection can cause imbalance current when paralleled, and how these problems are solved. Link here (PDF)
In today’s automotive and power industries, higher power requirements are leading to more designs that require lower RDSon. Sometimes this is not achievable with a single packaged MOSFET and the design will need to make use of two or more devices in parallel. Higher power applications could also require the use of high performance substrates like heavy copper PCB, IMS (Insulated Metal Substrate) or DBC (Direct Bonded Copper) and even bare dies. By paralleling, the total current and thus dissipation is shared between each device. However, this is not as simple as applying Kirchhoff’s current law: MOSFETs are not identical and thus they don’t share equally.