Applet Tutorial

The convention:

• Moving dots: red = hole, blue = electron.
• Colored regions: red = p-type, blue = n-type.
• dp = excess hole concentration at the depletion boundary
• dn=excess electron concentration at the depletion boundary.
• Lp = hole diffusion length
• Ln = electron diffusion length.
• I = total current
• In(x) = electron current (blue curve)
• Ip(x) = hole current (red curve).
• The diagram in the middle may be viewed as a band diagram, but the band banding across the depletion region is not shown here. The numerical values are for a Si pn junction diode.

• The diffusion process and the recombination process together produce an exponential concentration profile for the minority holes, as can be seen in the second diagram (lower-right, red). At a constant forward-bias, the number of holes injected across the junction per unit time equals the number of holes lost by recombination, thus establishing a steady state (i.e., constant in time).
• The hole component of the total current is shown by the red curve in the third figure. Going from left to right, the hole current in p-type region is drift current (due to small electric field in the neutral p-region, red), drift current in the depletion region (due to the built-in field), and diffusion current in the n-side (blue) due to the concentration gradient. The excess minority hole concentration reaches zero eventually.
• Electrons (blue) are injected from n-side (blue) to the p-side (red) across the junction (or depletion region, gray). They go through the same process as holes.

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