BIPOLAR JUNCTION TRANSISTOR

Introduction:
The bipolar junction transistor was invented in the United States by John Bardeen, Walter Brattain and William Shockley in 1949. A bipolar junction transistor (BJT) is a type of transistor (device) that has three terminals connected to three doped semiconductor regions. The emitter region, the base region and the collector region. There are two types of bipolar junction transistor: NPN transistor and PNP transistor. In an NPN type, a thin and lightly doped P-type material is sandwiched between two thicker N-type materials; while in a PNP type, a thin and lightly doped N-type material is sandwiched between two thicker P-type materials. BJTs can also be used as amplifiers, switches, or in oscillators.

(BJTs) can be characterized by the single relationship between the current going through and the voltage across the two leads. And it can be considered as a two-port network with an input-port and an output-port, each port formed by two of the three terminals, and characterized by the relationships of both input and output currents and voltages. There are three possible configurations for the two-port network formed by a transistor, depending on which of the three terminals is used as common terminal: common emitter (CE), common base (CB), and common collector (CC).

Common-base configuration is a transistor operation mode that base is common on both collector and emitter of the BJT.

Common-base configuration diagram

Common-collector configuration is a transistor operation mode that collector is common on both base and emitter of the BJT. It has high input impedance and low output impedance.

Common-collector configuration diagram

Common-emitter configuration is a transistor operation mode that the emitter is common on both collector and base of the BJT.

Common-emitter configuration diagram

Structure

The bipolar junction transistors are made from the semiconductor materials germanium (Ge) and the Silicon (Si). A bipolar junction transistor (BJT) constructed with three doped semiconductor regions, in the NPN type of BJT the P layer which has hole in excess is sandwiched between the two N type layers which have high electron density. In the PNP type of BJT there is an N type layer is sandwiched between the two P type layers. These are the three doped semiconductor regions. The three regions are, collector (C) – moderately doped region and it has highest thickness then the emitter and then base regions, emitter (E) – heavily doped region and it has the highest density then collector and base, base (B) - lightly doped and very thin region. The positive (+) sign are used to represent the holes and the negative (–) sign are used to represent the electrons.

Operation:

The bipolar transistor has four distinct regions of operation; Forward Active, Reverse Active, Saturation, Cut-off.
Active region: Base-emitter junction forward biased, Collector-base junction is reverse biased.
Cut-off region: Base-emitter junction is reverse biased. No current flow
Saturation region: Base-emitter junction forward biased, Collector-base junction is forward biased Ic reaches a maximum which is independent of IB and β. No control. VCE<VBE
Breakdown region: IC and VCE exceed specifications, damage to the transistor.

NPN Transistor operation
The forward bias from base to emitter narrow the BE depletion region, and the reverse bias from base to collector widens the BC depletion region. The heavily doped n-type emitter region is teeming with conduction-band (free) electrons that easily diffuse through BE junction into the p-type region where they become minority carriers. The base region is lightly doped and very thin so that it has a very limited number of holes. Thus, only a small percentage of all electrons flowing through the BE junction can combine with the available holes in the base. A few recombined electrons flow out of the base lead as valence electron, forming the small base electron current. Most of electrons from the emitter diffuse into the BC depletion region. Once in this region they are pulled through the reverse-biased BC junction by the electric field set up by the force of attraction between the positive and the negative ions. The electron now moves through the collector region out through the collector lead into the positive terminal of the collector voltage source.

* The operation of PNP transistor is the same as for the NPN except that the roles of the electrons and holes, the bias voltage polarities and the current directions are all reversed.

Application:
The bipolar junction transistors (BJTs) can be used as amplifiers and switches.

BJT as a switch
An NPN bipolar junction transistor could be wired to function as a switch for a light-emitting diode, or LED. Closing the switch will put a small input from the 9-volt supply on the transmitter's base. This input will turn on the BJT, allowing the LED to light when it receives the full input from the power supply.

BJT as a switch

BJT as an amplifier
The circuit shown in figure1 what is known as a Darlington pair of transistors. This configuration functions as one big amplifier--although the signal is increased by the first transistor, which then feeds this to the second transistor, which in turn amplifies the signal even more. This circuit illustrates just how well a Darlington pair can work. Complete the circuit by touching the S1 contacts with your finger, and the Darlington pair will amplify the small current enough to turn on the LED.

Figure1
References:

R S Muller, Kamins TI & Chan M (2003). Device electronics for integrated circuits (Third Edition ed.). New York: Wiley http://en.wikipedia.org/wiki/Bipolar_junction_transistor http://www.mpoweruk.com/semiconductors.htm
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