let's answer this question using the example below:
Let's imagine an ideal condition, where a 5V ideal voltage source with no internal resistance is connected to a diode, the voltage source can only supply max of 1A of current, how will the circuit behave? what will be the voltage and current in the diode?
My thinking which is probably wrong:
In Fermi band diagram, to put it simply, you need to increase the fermi level of electrons in the n region to overcome the reverse bias, so that some electrons can be higher than the conduction band in the P region and spill over, thus conduction follows.
My misunderstanding is probably surrounding the cause and effect of the fermi level change. I thought the elevation of fermi level governs the maximum
I that can be allowed to flow thru the junction. The actual
I is determined by the how much the circuit allows. In a sense the voltage across diode is like opening an adjustable valve in a water pipe, if you only have trickle of flow in the pipe, opening a valve any further won't get your more water.
however in reality, I suspect,
I are strictly coupled. In another word Fermi levels can NOT be manipulated purely by external voltage source without having the charges available to pile up to cause that voltage increase. you need electrons to pile up on the n side to cause the fermi level to increase on the n side.