A contraction of trans-resistor, a transistor is a solid-state active device that controls current flow. A transistor comprises a semiconducting material, such as silicon or germanium, in three electrode regions with two junctions.The regions are alternately doped positive-negative-positive or negative-positivenegative in a semiconducting sandwich, so to speak. One outer region serves as the collector, the inner region as the base, and the other outer region as the emitter. The collector circuit collects power from the external power source, the base acts like a control electrode, and the emitter emits the outbound signal. Small signals applied between the base and the emitter control the larger currents and power from the collector, with a small change in the signal applied to the base producing a large and rapid change in the current flowing through the entire component. A transistor can operate linearly, like an audio amplifier, or like a switch, rapidly opening and closing an electronic gate. A transistor can act on a signal to perform a variety of functions such as amplification, rectification, modulation or demodulation, and buffering.The transistor was invented by William Shockley, John Bardeen, and Walter Britain of AT&T Bell Telephone Laboratories, in 1947, and quickly replaced the electron tube, or vacuum tube.As a result of the 1956 Consent Decree, AT&T was forced to license the patented transistor technology to any company for $25,000. General Electric, IBM, Sony, and Texas Instruments are but a few of the companies that wrote a check. Large numbers of transistors are frequently interconnected with microcircuits and baked into a single integrated circuit, many of which can exist on a single circuit board in an electronic device such as a computer, switch, or router. See also amplifier, buffer, Consent Decree, current, modulation, patent, power, rectifier, and signal.