hi dear ,it specifically depends upon the diode,for ex. PNJunction Diode have characteristics in forward and reverse bias,ZENER has characteristics in only reverse bias ,Tunnel & Gunn diode have Negative resistance phenomenon
Varactor has some different characteristics
so firstly identify the type of diode/ number of diode?
and then get the datasheet.
still the problem persists
just get the JPG. of the circuit/whatever it may be to upload in this forum:grin::grin: and we will try to answer it