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When diode is turned on the holes and electrons gets mixed that is the holes will also appear largely on the n side and electrons also appears on the p side but when turned off how electrons and holes get separated as p and n side with same as when it is not connected. Practicals explanation please

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2 Answers 2

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The key here is the generation/recombination of the carriers (electrons/holes). Electrons and holes are constantly generated and recombine. Only when one of these processes overcomes the other is there an excess or lack of carriers relative to the steady state. This happens for example in the diode when a bias is applied, because the carriers can be swept away before they recombine.

Consider just the case of forward bias. There is an excess of holes in the P region and electrons in the N region. The diffusion force of these local high carrier densities combined with the applied voltage (overcoming the built in voltage preventing migration) sweep the carriers from where they were generated across the depletion region. Here they quickly recombine (since the area is highly favorable for recombination: a lone electron in a sea of holes).

Once the voltage is removed, the majority of the carriers would not 'go back'. Instead those that were in a locally unstable place (now that the voltage is removed) would recombine in that area, after spreading out a bit. Likewise the areas that were missing some carriers, relative to their steady state, would generate them through the normal methods.

However, there would be some carriers that moved away once the voltage is removed. This is due to diffusion. If a region has a locally high number of carriers, those carriers want to spread out. This is called diffusion and is the same reason why if you spray perfume in the corner of a room it eventually spreads out to the whole room. How far these carriers got, however, is related to how pure the nearby silicon is. If this area has very few impurities, the carriers may move a fair bit before finding an opposite carrier or charged dopant ion to recombine with.

Those carriers traveling through the depletion region would suddenly lose the electric field guiding them, and would also recombine.

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  • \$\begingroup\$ Could you elaborate the answer I can't understand \$\endgroup\$ Jul 24, 2016 at 4:01
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    \$\begingroup\$ In steady state, with no applied voltage, both electrons and holes are constantly generated. Their generation rate matches their recombination rates for a net of zero. Now when you apply a voltage you create an electric field which sweeps these carriers away before they can recombine. When the voltage is removed there will be a sudden, local excess of charge in the areas of the PN diode. The carriers causing this will quickly recombine since there is no more electric field to influence them to move. \$\endgroup\$
    – jbord39
    Jul 24, 2016 at 4:03
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A semiconductor has a group of nearly-full electron orbitals which overlap in energy, called the 'valence band'. It has also a group of nearly-empty electron orbitals at a higher energy, called the 'conduction band'. The energy difference between these two bands is called the bandgap, for silicon it is about 1.2 eV per electron.

At any given temperature, some electrons from the valence band will be found in the conduction band, and will only fall down to the lower energy state at a low rate; in equilibrium, the rate of promotion of the many low-energy electrons up to the conduction band (the process called generation) is equal to the rate of those few high-energy electrons (in the conduction band) dropping to the valence band (called recombination).

Electric current in a diode pulls charge carriers out of P regions into N regions, and disturbs the equilibrium. Generation and recombination random events will occur at rates that restore the equilibrium.

This is similar to a liquid causing some vapor pressure when it is below the boiling point: the many molecules in the liquid are thermally excited enough that a few of them will vaporize and leave the liquid at the surface, while the fewer molecules in the vapor will only rarely come into contact with the liquid and have an opportunity to condense. If one exchanges the vapor above the liquid, evaporation and condensation events restore the equilibrium after a time.

The semiconductor's dopants, and any metal contacts, are active participants in the equilibrium-producing generation and recombination events. The time scale of thermalization of holes' and electrons' populations can be nanoseconds (in Si) to microseconds (in Ge) to milliseconds (in CdS).

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