Adonis Antoniades

 

Important things about the physics of transistors/diodes

(Rule for the exam: If it was in your lecture notes, or covered in section, it may be on the exam. What I am giving you here is a highlight of what you should focus on concerning the physics and fabrication of transistors and diodes)

  1. What is a p-type region? It is a region with an excess of holes.
  2. What is an n-type region? Region with an excess of electrons.
  3. How do we fabricate p or n-type regions? By implanting specific impurities (e.g. As) on a piece of silicon.
  4. In the lecture notes you have seen a diagram for the FET several times. It shows the gate, then a layer of insulator, the drain, the source etc. If you see that diagram, you should know where the insulator is, where the p-region is etc. You should pretty much know how to label things on that diagram.(That is in pages 7 and 10 in the notes on April 1st).
  5.  For FETs, what is the role of the gate electrode? Well, it controls how much current can flow between the drain and the source. A way to visualize this: The drain and source seem to be unconnected. The only thing that can connect the two is the gate(because it is between them). The gate will connect the two if it is given sufficient voltage, and once they are connected, you will have current flowing through. Page 9 in the notes of April 1st has a nice diagram showing this.
  6. PN junction. The PN junction is a diode. When it is forward biased by a source, holes try to go from the p-region to then n-region and electrons try to go from the n-region to the p-region. On the boundary of the two regions, the holes “meet” the electrons and they “destroy” each other. Well, in order to account for these losses, the PN junction draws electrons and holes from the source, and that is how the current is created in the circuit.

  7. Forward biased diode
     
    In the reversed biased case, the source tries to attract electrons from the n-region and holes from the p-region. If the diode allowed this, however, then it would lose all its free electrons and holes. So, the diode refuses to allow any electrons or holes to flow, so it does not conduct. (The diagram for this would have the source placed the other way around).