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Ok. There are mainly two effects in semiconductor (those is a MUST to know) which causes current. the one is by diffusion. suppose a the areas having different levels of free charges. If electrons can travell, this effect will stop if both areas have the same q. this effect is known as diffusion and in semiconductors like the diode it is caused by different dopping!
The other effect is called mobility caused by an electric field. A electric field caused electrons to flow.
If you connect a p and a n doped silicon is connected together, than due to the different concentrations there will be a diffusion mechanism. But the electrons which move leave a positive charged atom back. Therefore a electric field wil be created.-- working against the diffusion mechansim. Therefore if you want to let electrons flow through a diode , you have to put a electric field (a voltage) across the pn section. There is no diffusion effect in the neutral zone, because there are no free cariier.
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