v_c said:I guess the ideal transistor is unidirectional (supposed to conduct in one direction)
But in reality, this is not always the case. Consider the use of transistors as switches in power conversion circuits.
If we use a power NPN BJT as a controlled switch (current-controlled) it will conduct current from the collector to the emitter but not in the other direction. If we use a power n-channel MOSFET (voltage-controlled), it will conduct drain to source current through the channel but it can also conduct source to drain by means of a body diode (which is an inherent, parasitic diode-connected BJT).
Please take a look at these slides from Bob Erickson's textbook
which I think will guide you in the right direction.