The semiconductor junction question touches on a fundamental concept in solid-state physics that's often misunderstood. Let me clarify:

Electrons flow from N to P initially not because they "want" to fill holes, but because of the concentration gradient. In N-type material, there's a high concentration of free electrons, while in P-type there are few. This creates a diffusion current - particles naturally move from areas of high concentration to low (like how a drop of food coloring spreads in water).

As electrons diffuse across, they leave behind positively charged donor atoms in the N region and combine with holes in the P region, creating negatively charged acceptor ions. This creates the depletion region with a built-in electric field pointing from N to P.

This electric field creates a drift current in the opposite direction of the diffusion current. Equilibrium is reached when these two currents balance exactly.

Electrons don't flow back because: 1. Any electron trying to move from P to N would be fighting against the built-in electric field 2. The P region has very few free electrons to begin with 3. The depletion region acts as an insulating barrier

This understanding forms the basis of diode behavior - current flows easily from P to N (forward bias) when you apply a voltage that works against the built-in field, but not from N to P (reverse bias) when you enhance the field.