<P> The p--n junction possesses essential properties for modern electronics . A p - doped semiconductor is relatively conductive . The same is true of an n - doped semiconductor, but the junction between them can become depleted of charge carriers, and hence non-conductive, depending on the relative voltages of the two semiconductor regions . By manipulating this non-conductive layer, p--n junctions are commonly used as diodes: circuit elements that allow a flow of electricity in one direction but not in the other (opposite) direction . Bias is the application of a voltage across ap--n junction; forward bias is in the direction of easy current flow, and reverse bias is in the direction of little or no current flow . </P> <P> The forward - bias and the reverse - bias properties of the p--n junction imply that it can be used as a diode . A p--n junction diode allows electric charges to flow in one direction, but not in the opposite direction; negative charges (electrons) can easily flow through the junction from n to p but not from p to n, and the reverse is true for holes . When the p--n junction is forward - biased, electric charge flows freely due to reduced resistance of the p--n junction . When the p--n junction is reverse - biased, however, the junction barrier (and therefore resistance) becomes greater and charge flow is minimal . </P> <P> In ap--n junction, without an external applied voltage, an equilibrium condition is reached in which a potential difference is formed across the junction . This potential difference is called built - in potential V b i (\ displaystyle V_ (\ rm (bi))). </P> <P> After joining p - type and n - type semiconductors, electrons from the n region near the p--n interface tend to diffuse into the p region leaving behind positively charged ions in the n region and being recombined with holes, forming negatively charged ions in the p region . Likewise, holes from the p - type region near the p--n interface begin to diffuse into the n - type region, leaving behind negatively charged ions in the p region and recombining with electrons, forming positive ions in the n region . The regions near the p--n interface lose their neutrality and most of their mobile carriers, forming the space charge region or depletion layer (see figure A). </P>

What is built in potential of pn junction