- Bipolar transistor

A bipolar transistor consists of a pair of PN junction diodes that are joined back-to-back. This forms a sort of sandwich where one kind of semiconductor is placed in between two others. So there are two kinds of bipolar sandwich, the NPN and PNP types. The three layers of the sandwich are called the collector, base, and emitter.
Some of the basic properties shown by the bipolar transistor are immediately recognisable as being diode-like. However, when the sandwich form's filling is fairly thin some interesting effects become possible. They allow us to use the transistor as an amplifier or as a switch. To see how the bipolar transistor works we can concentrate on the NPN variety.



The first drawing shows the energy levels in an NPN transistor when we aren't externally applying any voltages. We can see that the arrangement looks like a back-to-back pair of PN diode junctions with a thin P-type filling between two N-type slices of 'bread'. In each of the N-type layers conduction can take place by the free movement of electrons in the conduction band. In the P-type (filling) layer conduction can take place by the movement of the free holes in the valence band. However, in the absence of any expernally applied electric field, we find that depletion zones form at both PN-junctions, so no charge wants to move from one layer to another.


Consider now what happens when we apply a moderate voltage between the collector and base parts of the transistor. The polarity of the applied voltage is chosen to increase the force pulling the N-type electrons and P-type holes apart. (i.e. we make the collector positive with respect to the base.) This widens the depletion zone between the collector and base and so no current will flow. In effect we have reverse-biassed the base-collector diode junction. The precise value of the base-collector voltage we choose doesn't really matter to what happens provided we don't make it too big and blow up the transistor! So for the sake of example we can imagine applying a 10 volt base-collector voltage.

 
News:
- CamSemi targets low cost mobile phone chargers

New controller enable manufacturers to develop low cost, energy-efficient BJT-based solutions for mobile phone chargers

News| by CIOL Bureau

ENGLAND, UK: CamSemi has announced recent advances in the company's bipolar transistor (BJT) drive scheme and the first of a new generation of Primary Side Sensing (PSS) controllers to exploit these developments.
The new C2172 PSS controller will enable manufacturers to develop what the company believes will be the industry's lowest cost, most energy-efficient BJT-based solutions for mobile phone chargers and other universal input applications rated to 6.5 W. The new drive scheme will also help designers improve the ruggedness of their designs.
C2172 is the first CamSemi PSS controller to combine this powerful new drive scheme with the company's industry-proven PSS technology to deliver no-load power consumptions well below 30 mW and ‘best in class' load-transient response but with significantly lower system cost and component counts.