The P type has majority holes and N type majoritiy electrons. Bring the two side by side , the electrons at the edge touch the P type and fall into the hole . When that happens the positive ions are found where the electrons left ,similarly negative ions are formed where the holes are filled. This is because initially p and n are neutral eventhough they have holes and electrons.
This region where where join is call a junction and depleted of charge carrier and therefore referred to as depletion region. The width of this region is limited. This diffusion process will stop when internal electric field is set up (dir from +ions to -ve ions) that prevent electrons and holes from entering this region. Draw the dir of E field , you find the force on holes and electrons is away from this region. Try electron , force is right (E is to the left in depletion region)so it stays in the n type. For holes , the ssme explanation.
If an external E field is applied , using a battery with positive terminal to the p type , the direction of E ext is the right. This E ext will cancel the E int (overcome) and electrons will move into the depletion region thereby narrowing or removing the depletion region . The flow of electrons and holes through the junction means that the pn junction is now forward biased allowing current to flow. Hence conduction is possible and resistance low.
If an external E field is applied , using battery with positive terminal to the ntype, the dir of E ext is the the left .This E ext will increase the depletion region as the electrons in the n type is forced to the right away from the junction . It does the same to the holes therby widening the depletion region. No flow of charge is possible as the depletion region does not have any mobile charge carriers. Hence , the diode is connected in reverse biased and resistance is very high.
SUmmary : forward biased-battery positive terminal to p type-depletion region narrows (dissapear) , resistance low.
reverse biased- battery positive terminal to n type - depletion region widens , resistance high.
Depletion region - no mobile charge carrier - ions set up internal E field that prevents further diffusion of mobile carriers into this region.
google : hyperphysics
See other older entries at semiconductor (13)
