This part is a kind of semiconductor called AN103.

Function of this product has The FET Constant-Current Source/Limiter.

Manufacturers : Siliconix

Image and pinout :

Some of the text within the PDF file :

AN103 The FET Constant-Current Source/Limiter Introduction The combination of low associated operating voltage and high output impedance makes the FET attractive as a constant-current source. An adjustable-current source (Figure 1) may be built with a FET, a variable resistor, and a small battery. For optimum thermal stability, the FET should be biased near the zero temperature coefficient point. D S RL RS – + A change in supply voltage or a change in load impedance, will change ID by only a small factor because of the low output conductance goss. DID = (DVDS)(goss) (3) The value of goss is an important consideration in the accuracy of a constant-current source where the supply voltage may vary. As goss may range from less than 1 mS to more than 50 mS according t [ ... ]

AN103 PDF Datasheet Download