While PTC Ohms increments on Temperature rise, the NTC is the opposite its Ohmic Resistance drops with rise in temperature. Both types are very useful in Power Electronics and Electrical Immunity of Telecommunication Units.
Another application in in control systems, Temperature Measurement and Control. Watchdog Controllers to prevent a Burn-out of system, this happens when a software based electronic control system breaksdown or hangs.
Murata's PTC Thermistors called POSISTOR detect overheating to protect a circuit from drawing more than rated current in a equipment. This prevents a fire hazard and most important protects devices and units.
The NTC Thermistor are used in inrush current limiting and also temperature compensation in electronic products.
When the MOSFET temperature goes beyond the design limit the PTC thermistor which is thermally coupled to the device builds up its Ohmic Value, thereby the Emitter-Base Voltage rises to bias the NPN, which drinks away the bias meant for Mosfet. That works as the Foldback protection.