The MMBT5551-B is an NPN transistor manufactured by Blue Rocket. It is housed in a SOT-23 package and is compliant with the RoHS standard. It has a maximum current rating of 600mA and a maximum voltage of 160V. The device is used to amplify signals in various consumer electronics and other electronic applications. It is commonly used for switching, amplification, and other functions. The MMBT5551-B can also be used as a switch, amplifying devices, and even for low-noise audio applications. It comes in both through-hole and surface-mount models for easy mounting and installation.