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bipolar transistor - technical definition
Also called a "bipolar junction transistor" (BJT), it is one of two major transistor categories; the other is "field effect transistor" (FET). Although the first transistors and first silicon chips used bipolar transistors, most chips today are field effect transistors wired as CMOS logic, which consume less power (see FET and MOSFET).
High Power, High Frequency
Although the overall market for bipolar transistors has decreased, they are still used for high power applications and high radio frequency (RF) applications that reach into the gigahertz range. For example, from 1997 to 2002, worldwide sales of bipolar chips dropped from $1.5 billion to $226 million, the latter out of a total semiconductor market of $136 billion. See transistor, IGBT and BiCMOS.
The First Transistor Was Bipolar
In 1954, Texas Instruments pioneered the bipolar transistor. Although BJTs are fabricated in microscopic proportions on chips, individual discrete BJTs similar to this one are still in common use. (Image courtesy of Texas Instruments, Inc.)
NPN Bipolar Transistor
BJTs are either a sandwich of p-type silicon surrounded by n-type regions or n-type surrounded by p-type. To turn an NPN BJT on (example above), a negative potential is applied to the emitter terminal and a positive potential to the collector. When a positive voltage is applied to the base, it allows electrons to flow from the emitter to collector. For the PNP BJT, the polarities are reversed. The "bipolar" name comes from using both mobile carriers (electrons and holes).
Bipolars Use More Power
BJTs require continuous voltage at the base to keep the transistor closed (on) and current flowing from emitter to collector. In the field effect transistors (FETs) used in CMOS chips, the transistor is closed (turned on) by charging the gate, and current is only used during the gate charging period (see FET and MOSFET).
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