Formation of a PN Junction Diode and its Band Diagram !


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A click on a button will show an animated formation of PN junction diode, a snapshot of which is:










The red rectangle represents a p-type semiconductor and the blue rectangle an n-type semiconductor. At the moment, three semiconductors, Si, Ge and GaAs, have correct materials parameters. You can change the doping level by typing in the value and clicking the 'setNewValue' button.

Equilibrium band diagrams appear below the semiconductor. The green horizontal line is the Fermi level. Initiate the pn junction formation by clicking the 'FormJunction' button or using mouse drag and watch the physical system approach a new (electro-thermal) equilibrium which is characterized by a constant Fermi level throughout the material. The white region between the red (p-type) and blue (n-type) regions is the transition (or depletion) region where most of the free carriers are depleted. This white region is the space charge region with uncompensated ionized impurity charges which produce electric field in this region.

Repeat formation and separation of the PN junction several times trying out different doping values. When you become sufficietly familiar with the equilibrium band diagram and its formation principles, do the following quiz.

Quiz:

  1. Depletion thickness
  2. Band diagram


PROBLEMS:

  • The pn junction band diagram :  This applet demonstrates how the band diagram of a pn junction diode is formed.  Answer the following questions.
    1. By mouse drag of the rectangles toward each other or clicking on the "formTheJunction" button, you can bring the two 'bulk' semiconductors to form the junction.  This applet visualizes the hypothetical, initial transient process during junction formation, and the band stabilizes when the junction arrives at a new thermal equilibrium (for the combined physical system).  
    2. Comment on how this problem can be better formulated to assist you to learn better, conceptually and mathematically.
    3. Comment on what additions (ie, graphical presentations) to this applet will be helpful for learning the space charge (and total charge = 0), electric field, and potential drop on each side of the junction.
    4. Any other related applet programs do you desire ??


    Copyright (c) C.R.Wie, SUNY-Buffalo, 1996-1997


    Space charge and electric field

    Diffusion and drift currents

    Applied bias and current flow

    Diffusion currents and the recombination in the neutral regions

    The SPICE device model for junction diode











    Answer to the quiz:

    1.