For a BJT which you're talking about, the structure resembles two diodes back to back (like this: -|<--->|-), but the middle material has to be lightly doped an

For a BJT which you're talking about, the structure resembles two diodes back to back (like this: -|<--->|-), but the middle material has to be lightly doped and must be very thin as well. The rough mechanism of operation is that when you pass current through one of the diodes, the charge carrier current doesn't all get siphoned off through the middle terminal (base), and most of it goes to the other "reverse biased" diode and gets siphoned off through the reverse bias electric field.