Hi Prachi,
Regular diodes are not fabricated in the same way as the MMBT3904 transistor so their transfer function is different. Since pn junction can vary quite a bit based on device type (leakage current, speed, power rating etc.) when remote diode sensors were conceived it was found that the native PNP transistor found on a CMOS process could be made to follow the same characteristics as a standard 3904 BJT. Thus most remote diode sensors are tuned for the 3904.
As for the selection of the 3904 just make sure you purchase the 3904 from a reputable supplier that has good process control in place.
Take care,
