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Applet Tutorial
(The pn junction diode under applied bias)

(you may skip this if you are already familiar with pn junction theory)

• By using the checkboxes at top, familiarize with the four basic current components:
• electron leakage : this is part of the reverse saturation current.
• electron injection : once injected (into the p-region, red), electrons move by means of diffusion (due to its concentration gradient in p-region). So, it is also called the diffusion current.
• electron recombination: Whenever the thermal-equilibrium condition of a physical system is distributed, processes exist to restore the system to equilibrium. In pn junction, the band-to band recombination where an electron-hole recombines will occur. This transition of an electron from the conduction band to the valence band is made possible by emission of a photo or by transfer of the energy to another free electron or hole. These electrons form the electron recombination current.
• hole leakage : this is the hole part of the reverse satusation current.
• hole injection : after injection (into the neutral n-region, blue), they move (away from the junction) via diffusion.
• hole recombination: this is the hole part of the recombination current.

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• The sample schematics (top red-white-blue rectangle) and the band diagram are for a given applied bias that is set using the scrollbar. The bias may be changed using the little scrollbar in the upper-left corner. The band diagram shows both majority carriers (electron=blue rectangle; hole=red rectangle) and minority carriers. The heights of the colored rectangles in the conduction(valence) bands in n(p) side are proportional to the log of the majority carrier density.  The heights of the colored rectangles in the conduction(valence) bands in p(n) side changes with the change of the position into the sample. When x=xp(xn), the height of the rectangles is equal to the height of log of the  majority carrier density above the conduction(valence) band in n(p) side. Then the carrier density decreases as it goes deep into the sample, at last it appoaches to the log of the minority carrier density under zero bias and remains as a constant. The length which minority carrier density appoaches to a constant is equal to the length that the  recombination current can occur. The change of the carrier density in this process is according to a decrease exponential function with a parameter named diffusion length. Diffusion length is the average distance a minority carrier travels before it recombines with a majority carrier, it is typically a few microns to a few millimters.

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• You should see the minority carrier drift (i.e., leakage) at all applied bias. Injection current increases under forward bias (positive voltage), and the recombination current is observed only under forward bias.

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• At zero applied bias, the number of electrons moving from the p-side to the n-side is exactly equal to the number moving in the opposite direction (ie, from the n-side to the p-side). This is true for the holes as well. Thus the total current across the junction is zero at zero bias (as it should be).

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• When you apply a positive bias to the p-side with respect to the n-side (forward bias), then more electrons in the n-side lie above the Ec of the p-side (ie, the band edge of the other side of the junction) and these electrons can cross the junction without being objected by the potential barrier. These electrons constitute the electron part of the injection current. (Of course, even at zero bias electrons flow from the n-side to p-side. But these are cancelled by the electrons flowing in the opposite direction.) Those electrons in the n-side which lie below Ec of the opposite side will not be able to cross the junction, and do not contribute to any current.

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• When you apply the reverse bias (ie, p-side negatively biased wrt the n-side), the electron flow from the n-side to the p-side decreases and eventually disappears from certain reverse bias value. Observe that, beyond this reverse bias, the current is constant of the applied reverse bias.

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• The same is true for the injection hole current under the forward or reverse bias.

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• Based on this applet, you can derive the current-voltage relationship for a pn junction diode.

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