TRANSISTORS ETC - 3H

This material is nicely presented in Tipler, Section 9-7.

Semiconductors do not have many applications as large blocks. Their importance lies in the devices that can be made out of semiconductor components.

The technological revolution of the last decades has come in three stages.

  1. The development of computers out of vaccum tube diodes and triodes. The first computers took entire rooms to do less than a programmable pocket calculator does today, and were extremely unreliable as the vacuum tubes tended to breakdown all the time.
  2. A transistor out of solid state components was developed first by Bardeen, Brattain and Schockley of Bell Labs in 1947 as the ``point-contact transitor'', followed in 1951 by the pnp transitor of Schokley. These resulted from a research programme aimed at understanding semiconductor physics in terms of atomic physics, and moved into computers by 1959.
  3. In 1959 Kilby, working for Texas Instruments, developed the first integrated circuit. By 1967 ``chips'' had thousands of transistors. and computers as we know them began to be used.
Since then it has been about miniaturization, and of course software, screens and operating systems.

The two important devices built from semiconductors are pn junctions and pnp transistors.

A pn junction is like a diode and was discussed in the semiconductor section. It can be explained only by understanding Fermi Levels and electron and hole transmission in doped seminconductors.

  1. A current can only flow from the p side to the n side, but not from the n side to the p side.
  2. If the potential difference is such that current flows with the positive terminal of a battery connected to the p side, negative to the n side, we say it is forward biased.
  3. If the potential difference is reversed so that the positive terminal is connected to the n side we say it is reverse biased.

A pnp transistor works as follows:

  1. The ``base'' in the middle is n-type and very narrow. The ``emitter'' and ``collector'' are on either side and are p-type.
  2. There are wires connected to each of the three parts.
  3. The emitter/base (EB) junction is forward biased and the collector/base (CB) junction reverse biased.
  4. This means that most current thru EB is positive charges from emitter (holes) and because CB reverse biased positive charges that enter base diffuse across to reach collector except for a few that leave thru an external connection to the base. The current from the collector is much larger than the current from the base so the transistor can be used as an amplifier.
Thus a transistor, which is made of two pn junctions requires atomic physics to understand and apply to new technologies.


RETURN to:

Section Header

PHYSICS 3H - HOMEPAGE