# Physics Photo of the Week

## Transistor

 Photo by Aaron Keegan, WWC class of 2005 The transistor is perhaps the most significant technological development of the 20th century.  The transistor was invented in 1948 by John Bardeen and William Shockley, and Walter Brattain who shared the Nobel Prize in Physics only 8 years later in 1956. The transistor is the black "D-shaped" object in the center of the photo.  The transistor in the photo is connected into a circuit which is used by the physics class at Warren Wilson College to study the properties of this device.  The transistor consists of three parts: emitter, base, and collector.  These three regions are all parts of a tiny piece of solid silicon, but have impurities to make them N-type, P-type, and N-type respectively.

This tiny piece of solid silicon has the ability to serve as an amplifier, or a switch, with no moving parts.  The physics students measure the current into the base and the current through the collector.  What is found is that that the collector current (or output current) is controlled completely by means of the base current with no moving parts.  The graph below shows the effect:  a small change in base current leads to a large change in collector current by a factor of over 100.  This allows the transistor to function as an amplifier.  A typical application is to amplify the small currents generated by a microphone into large currents to drive a speaker.

The graph at right is the graph made by physics students Lily Doyle and Valerie Bartell in measuring the effects of a transistor on electric current.  The horizontal axis is the current into the base of the transistor.  That is the current through the resistor called Rb in the photo.  The collector current, the current that goes through the resistor labeled Rc in the photo, is plotted on the vertical axis.  Up to a point, the collector current is over 100 times the magnitude of the base current, and is completely controlled by the base current with no moving parts (except electrons).  The transist

Of course the largest application of transistors is in computers.  Computers, calculators, digital cameras, TV's radios, etc, use integrated circuits - tens of thousands of transistors on a single piece or "chip" of silicon.  In computers, transistors function almost entirely as On/Off switches rather than as amplifiers, in order to perform binary calculations
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There will be no Physics Photo of the Week next week (Nov. 25, 2005) due to Thanksgiving break.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to dcollins@warren-wilson.edu.

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.