bipolar

(1) See bipolar transmission.

(2) One of two major categories of transistor; the other is "field effect transistor" (FET). Although the first transistors and first silicon chips were bipolar, most chips today are field effect transistors wired as CMOS logic, which consume significantly less power (see FET and MOSFET).

Bipolar transistors are available as individually packaged discrete components as well as by the hundreds of thousands on a single chip.

Injecting/Pulling Electrons
The bipolar transistor works by injecting electrons into the "base," which is the trigger that turns the transistor on or off. Depending on the type, it may pull electrons out. In either case, a non-destructive breakdown occurs in the silicon channel between "emitter" and "collector" ("in" and "out" in the illustrations below).

High Power, High Frequency
Although the overall market for bipolar junction transistors (BJTs) has decreased, they are still used for high power applications and high radio frequency (RF) applications that reach into the gigahertz range. For example, in the five year span 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 and BiCMOS.

Bipolar Vs. FETs

Bipolar transistors (BJTs) require current to flow in order to keep the transistor on (closed), whereas the FET charges a gate and the current stops.

The First Transistor Was Bipolar

In 1954, Texas Instruments pioneered the commercial production of silicon transistors, which used the bipolar concept. (Image courtesy of Texas Instruments, Inc.)

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1.	(electronics)	bipolar - See bipolar transistor.
2.	(communications)	bipolar - In digital transmission, an electrical line signalling method where the mark value alternates between positive and negative polarities. See also AMI.