MOSFET transistors are used in a variety of electronics to regulate electricity throughout the device. If you have a MOSFET transistor that needs to be replaced, you will need to identify the transistor before you can put an order in for a replacement. You don't have to be an electrical engineer to identify your transistor -- use the part number and an identification chart for the replacement. There are two kinds of transistors: enhancement transistors increase the voltage to a device, while depletion transistors decrease the voltage to a device.
Determine if the transistor is an enhancement or depletion mode transistor. All MOSFET enhancement transistors come from the n-channel series. The p-channel resistors are depletion mode transistors. Look to the bottom of the transistor for an "N-CH" or a "P-U" labeling to determine which type of transistor you need.
Determine the voltage rating of the transistor. Look at the bottom of the transistor for the voltage rating. You should see "120V" or something similar.
Identify the series of the transistor. All MOSFET transistors use letter series at the beginning of the part number. For instance, the BU series has a "BU" at the beginning of the part number.