Transistor Processing


Concepts Shown:

semiconduction, n-type, p-type, electrons, holes


one transistor transparency setup, overhead projector. Cost: less than $2.


Setup: The transistor transparency can be created on any Macintosh computer, however, if the pictures are attached, simply copy them on to transparency paper.

Presentation: Overlay transparencies on overhead explaining as follows:

  1. Start with layer of n-type silicon. This is silicon that is doped with Phosphor to cause it to have an excess of negative electrons.
  2. Through a process called masking in specific regions, the outer portion of n-type silicon is oxidized to produce a layer of silicon dioxide (SiO2). The process of masking allows you to only oxidize specific areas, allowing gaps in the oxidized layer. The purpose of this layer is to act as an insulator separating certain regions of the silicon.
  3. Using the process of diffusion, two separate areas of the n-type silicon are doped with large quantities of Boron to become p-type silicon.
  4. Aluminum connections are placed above both p-type areas and a third in between above the n-type silicon. The connections are called the base, emitter, and collector. The aluminum connectors allow yo to hook each of the areas up ina circuit.
Note: This demonstration may be more effective when shown in combination with the processed silicon wafer demonstration.

A transistor consists of an emitter, a base and a collector. For the moment, consider only the emitter junction (see "Rectifier" demonstration for the science behind junctions and biasing) which is biased such that the carriers (electrons for n-p-n, holes for p-n-p) move into the base (toward the collector). The number of carriers that cross this junction, and move into the n-type material, is an exponential function of the voltage on the emitter, Ve. Of course, these carriers at once start to recombine with oppositely charged carriers, however, if the base is narrow, or if the recombination time is long, the carriers will keep moving on through the thickness of the base. Once they are at the second junction, the collector junction, the electrons have "Free sailing", because the collector, in this case is a p-type semiconductor, Ic, is controlled by the voltage on the emitter, Ve, as shown by the equation: Ic = Io exp(Ve/B) where Io and B are constants for any given temperature. From this equation, it is evident, that as Ve rises, Ic rises exponentially. To show this in a practical application, think of a transistor as it would be used in a stereo system. If someone is talking into a microphone, in order for this sound to be reproduced by a speaker, it has to be amplified enough to drive the speaker. This is precisely what the transistor does. However, this does not come for free, the transistor has to be powered (Ve) in order for the signal seen by the collector to be amplified. [eq].


Tyler vanHouwelingen

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