Transistor Replacement
Like also m.Alin says the DS2501 is a memory IC. It's no longer available, replaced by the DS2502, which appears to have twice the memory. 

The DS250x use a 1-wire serial protocol, where data and power share the same pin, so that there are only two of them required:  

  

(Why they don't put it in a 2-pin TO-92 is beyond me. That's an existing package.)