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in the formation of a p-n junction diode, why is there a positive charge on the n-type side and a negative charge on the p-type side? isnt n-type side supposed to have a negative charge since it has more electrons and likewise the p-type side supposed to have a positive charge since it has holes? how does the potential barrier also have a positive and negative side?

6 years ago


Answers : (1)


Dear student,

The recombination of free electrons and holes in the vicinity of the junction leaves a narrow region on either side of the junction that contains no mobile charge. This narrow region which has been depleted of mibile charge is called the depletion layer. It extends into both the p-type and n-type regions as shown in Figure 8(a). Note that the diffusion of holes from the p-type side of the depletion layer leaves behind some uncovered fixed negative charges (the acceptor ions). Similarly, fixed positive charges (donor ions) are uncovered on the n-type side of the depletion layer. There is then a separation of charges: negative fixed charges on the p-type side of the depletion layer and positive fixed charges on the n-type side. This separation of charges causes an electric field to extend across the depletion layer. A potential difference must therefore exist across the depletion layer. The variation of potential with distance is shown in Figure 8(b).

The uncovered charges give rise to a built-in potential of V_i_ volts. For a typical silicon p-n junction, V_i_ ≈ 0.6 to 0.7 volts. It varies with doping levels and temperature. The significance of this built-in potential is that it opposes the flow of holes and electrons across the junction. For this reason, the built-in potential is called a potential barrier or potential hill.

In practice, a p-n junction is formed within a single crystal rather than simply joining two pieces together. Electrical contacts on either side of the crystal enable connection to an external circuit. The resulting device is called a junction diode.


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