Electronic Devices and Circuits
Engineering Sciences 154

The pn Junction


For reference -- not applets: Two graphs of carrier concentration vs.electron energy: linear plot and logarithmic plot (see below)

Junction at Equilibrium - Junction Formation
Applet: A graphical illustration of the formation of a PN junction diode and its band diagram.  It shows both the band diagram and a schematic cross-section of the device as the junction approaches its thermal equilibrium. 
See also the non applet: Energy Diagrams of p-type and n-type semiconductors

and the two non applet graphs of carrier concentration vs.electron energy: linear plot and logarithmic plot

Point to note: (see Math. Analysis)
    Barrier height        VBH = (kT/q) ln(NaNd/ni2)   { "built-in voltage" }
        Overview slide (source)

Space Charge
Applet: PN junction diode, space charge profile and electric field which shows a cross-section of the sample, space charge and the electric field in depletion layer 
Points to note: (see Math. Analysis) about the depletion width
  • Note how the electric field varies with position in the depletion layer.
    •   xn Nd = xp Na    {  q A xn Nd = q Axp Na charge-equality condition}
    Applet: Space charge and electric field in biased PN junction.
    Note, in particular, how depletion layer stretches with increasing back bias.
    Junction with Bias
    Applet: Biased PN Junction: band diagram, diffusion and drift currents, etc.  It shows both the energy band under bias and the animated carriers flowing across the junction.
    Note, in particular, the importance of accounting for the recombination of carriers.
    See also the non applet: Energy diagrams of biased pn homojunctions
    Applet: Current flow in forward-biased pn junction  An applet that shows an animated flow of carriers in a simplified band model.



    This page was prepared and is maintained by R. Victor Jones
    Comments to: jones@deas.harvard.edu.

    Last updated October 18,  2001