Let us consider a diode without bias voltage. In that diode, diffusion current and drift current exist. Are they equal and hence cancel each other?
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A pn-junction without external bias is in thermal equilibrium. Therefore no net current exist. This means that the sum of drift and diffusion current must be zero everywhere. In the pn-junction in equilibrium a built-in electric field exists in the depletion zone. There the drift current due to the elctric field is exactly canceled by the diffusion current so that the total current according to the drift-diffusion equation is zero.
answered Mar 19, 2017 at 22:59
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$\begingroup$ How drift current equals the diffusion current? Because drift current is due to minority carriers and diffusion current is due to majority carriers which is more in number than the minority carriers. $\endgroup$– AyyanarCommented Mar 20, 2017 at 2:17
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$\begingroup$ @Ayyanar - The drift and diffusion currents are equal for both the majority and the minority carriers. There is no net recombination or generation in thermal equilibrium. $\endgroup$ Commented Mar 20, 2017 at 11:41
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$\begingroup$ Sorry for disturbing you. After a diode formed, do these two type of current exist all the time till it is used in a circuit? Because I read in a book that this equilibrium is dynamic. Am I right? $\endgroup$– AyyanarCommented Mar 20, 2017 at 12:58
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$\begingroup$ @Ayyanar - These drift and diffusion currents summing up to zero net current for holes and for electrons at any location are always there in the unbiased diode. $\endgroup$ Commented Mar 20, 2017 at 16:03
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$\begingroup$ In forward bias, does the depletion region vanish completely when the external voltage is more than the barrier potential? $\endgroup$– AyyanarCommented Mar 22, 2017 at 2:17