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Applet Tutorial

1. Depletion thickness
• The depletion thicknesses, xp and xn, represent the distance of the depeletion edge from the junction.
• Change the doping level of one side at steps of a factor ten. Record on paper both depletion thicknesses at each doping level.
• Based on your data, what is the mathematical relationship of xp and xn to Na and Nd ?  Write down a single equation that involves all these four parameters.  Check your answer.
• What does this relation mean physically about the total space charge ?  Check your answer.
2. Band diagram
• Hide the applet page by scrolling down the text or minimizing your web browser.
• Draw on paper the band diagram of PN junction for Na = 1E16 and Nd = 1E15. Make the band diagram correct both quantitatively (in the relative depletion thickness of the two sides) and qualitatively (shape of band diagram).
• Use the applet to check if your diagram is correct.
• Can you draw the band diagram for a PNP structure where Na = 1E18, Nd = 1E17, and Na = 1E15 ? Assume that the N-layer thickness is greater than the sum of the depletion thicknesses from the two junctions.

Answer:

• Nd xn = Na xp.
• Space charge neutrality: that is, the total positive charge ( Nd xn ) due to the uncompensated ionized donors is equal to the total negative charge ( Na xp ) due to the uncompensated ionized acceptors.