Connects one with a p-n-doped semiconductor layer head, so that it can realize a diode. This allows a stream of transport in only one direction.
To explain this behavior, considering the transition area between these layers, the so-called pn-transition

pn transition in equilibrium (no external voltage)
At the border area between the two semiconductor layers, a concentration compensation in motion. From the p-doped field pushing holes, while electrons from the n-doped range aspire. In the transition area therefore the two types of charge one another and to neutralize (electrons are bound in the holes; electron-hole recombination). It will remain there so loaded donor and acceptor ions surrounding free without charge. It creates an area known as the impoverishment zone or as a space charge zone. This is expressed that there is no free carriers are available, however, this area through the remaining Dotieratome pervaded with fixed charges.
Those fixed charges in the transition area generate an electrical field and thus a potential difference between the layers, which act as a barrier to the further concentration inhibits compensation. Lastly, there is a balance, where the tension between the layers is so large that they are ultimately the cargo distribution constant (for silicon is this tension around 0.7 volts).


 
Figure 10: p-n-transition in the balance

pn transition with Lock-scale voltage in the direction
In this case, you need an extra layer of tension to the sequence, so that the positive pole of the n-doped layer. In this way, the above-discussed potential barrier in the transition area between the layers for further support. Since the load through the outer voltage each of the boundary layer are pulled away, the space charge zone increases. Thus, no cargo transported through the layers. A persistent current flow is disrupted. One speaks from operating in lock direction.





11: p-n-lock transition in the direction

p-n-cut direction in transition
A reverse voltage, however, affects the equilibrium potential difference. This will continue to charge on the boundary layer where they constantly pushed recombine. Therefore flow from the battery electrons in the n-contact and holes in the p-contact, so that a sustained flow. Here we speak of operating in the direction of passage.



Figure 12: p-n-cut direction in transition