Applet Tutorial

Move up and down the Ef using the scrollbar or the mouse drag. Observe the changes in carrier concentrations n and p. (If you click on the "show parameter" button, you will see the numbers.) Ei is very close to the midgap. Does this applet make it clear that when Ef is closer to Ec than to Ev (ie, when Ef is above Ei), the material is n-type, and when Ef is closer to Ev (ie, when Ef is below Ei) the material is p-type.
Display the donor and acceptor panels by clicking on the appropriate buttons. This time, instead of moving Ef to control n and p, CHANGE THE DOPING LEVEL Na or Nd and observe that Ef, n, and p are controlled by the changing of Nd and/or Na. This is what actually happens in real semiconductors. The Na and Nd values are set by intentionally introducing chemical impurities into semiconductor during the growth of semiconductor crystals.
Compensation: set Na to a certain value like 2E17, and set Nd within a factor of 10 of Na (say, between 0.1Na and 10Na). Find the majority carrier concentration and verify the carrier compensation (ie, n = Nd - Na or p = Na - Nd). Explain physically why this happens.