MOS transistor is a FET can be manufactured as a depletion type or enhancement type, a P-type or N-type, whether it is the kind which works essentially the same. MOS tube and many types of action, which is mainly used in the switching action of the power used.
MOS transistor often applied gate voltage at the input current to the drain of the control output, through the characteristics of the gate voltage applied to the control device, the charge storage time of the image due to the base current of the transistor due to switching do not occur effect. MOS transistors used in the switching power supply circuit in an open-drain, a drain connected to the load intact, called open drain. Open drain circuit connected to the load regardless of how high the voltage can be switched on and off the load current, the analog switch devices is desirable. 
MOS transistor operating state of the process can be divided into open, conducting state, and an off state during turn-off. It can be divided into loss switching loss, the conduction loss, energy loss and avalanche-off loss. There are common causes of damage to the overcurrent, overvoltage and static electricity, etc., following on from Li Xin Electronics deep and work with you to explore the mysteries of one, make you more thorough understanding of the factors that lead to hot MOS transistor.
Hot MOS transistor of four factors:
1, circuit design problem. The MOS transistor operates in a linear operating state, rather than in the off state. If you do NMOS switch G than the high power supply voltage level several V to full conduction, and the P-MOS opposite. Is not fully opened and the pressure drop caused by excessive power consumption, equivalent relatively large DC resistance, pressure drop increases, the U * I also increases, it means that the heat loss.
2, frequency is too high. The excessive pursuit of volume, resulting in frequency increases the losses in the MOS transistor is increased, heat generation is also increased.
3, do not have enough thermal design, current is too high, the current nominal value of the MOS transistor, generally required in order to achieve good heat dissipation. Therefore, the current ID is less than the maximum, it is also possible heat, so require adequate auxiliary fins.
4, the selection MOS transistor are not. The error power is determined, the MOS tube resistance is not sufficiently considered, causes the switch resistance increases.