THE TRANSISTOR
The junction diode is component, which can only rectify.A transistor is a more useful component; it is a current amplifier.A transistor consists of a single crystal of germanium or silicon that is grown in such a manner that it has three regions.

In first figure the central region is p type which is sandwiched between two n-type regions.and it is known as n-p-n transistor.

in second figure the n-type is sandwiched between two p-type regions.it form a p-n-p transistor.

the central region is known as base and the other two regions are called emitter and collector.So transistor is a three terminal device.usually the base is very thin and collector is comparatively larger than the emitter.The emitter has a greater concentration of impurity as compared to the collector.you can see in the figures that transistor is made up of two p-n junctions back to back.here arrow shows the conventional current direction for the normal operation of the transistor, batteries Vbb and Vcc are connected in such a way that its emitter-base junction is forward and its collector-base junction is reversed biased.note that in this saturation Vcc will always remain greater than Vbb.