bipolar transistorn. A type of transistor used to amplify or switch electrical signals and consisting of three semiconductor layers treated so that the layer in the middle has an inherent electric charge opposite to the charge of the layers on either side of it. Signals applied to the middle layer control the strength of current flow between the outside layers.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.