How transistor works? | Depletion layer | Diffusion | Drift

How transistor works NPN TRANSISTOR Working UNBIASED TRANSISTOR When no battery is connected between different terminals of transistor, then the it is said to be unbiased or open circuit state In NPN transistor diffusion of electrons from N side to P side and diffusion of holes from P side to N side takes place, as a result of which depletion layer is formed as shown in figure. The width of the depletion layer depends on the doping level in the semiconductor. It penetrates deeply into lightly doped regions and penetrate less in collector and emitter regions. Due to formation of depletion layer barrier potentials are formed. Barrier potentials are negative on base and positive on emitter and collector in NPN transistor. Barrier potentials are positive on base and negative on emitter and collector in PNP transistor. Barrier potential is 0.7 V for Si and 0.3 V for Ge transistors WORKING OF TRANSISTOR For normal operation of transistor, Emitter base junction is forward biased and collector base junction is reverse biased. Forward bias at E-B junction narrows the depletion layer and Reverse bias at C-B junction widens the depletion layer. And effective base width is narrow. Forward bias at E-B junction narrows the depletion layer and Reverse bias at C-B junction widens the depletion layer. And effective base width is narrow. Electrons are injected into emitter region by emitter base supply VEB ,these are the conduction band electrons which have enough energy to overcome the narrow potential barrier Then the electrons enter into the very thin and lightly doped base region. Because base is lightly doped few electrons recombine with the base holes and most of the electrons cross into collector region. Collector is reverse biased and creates a strong electrostatic field between base and collector .This field immediately collects the diffused electrons which enter the collector region. To maintain the base neutrality, the base electrodes provide equal no of electrons which have combined with holes and results in base current IB Due to injected electrons by forward bias junction , emitter current IE flows and due to collected electrons by collector electrode, collector current IC flows. ENERGY BAND DIAGRAM OF NPN TRANSISTOR IN ACTIVE MODE • Electrons must loose energy in moving form emitter to base and base to collector. • Only a few electrons loose energy while recombination with holes in base region. • It is seen from the figure that the potential hill bend in the Fermi level between base and collector region is very steep. • Thus, electrons loose considerable amount of energy as they move from base region to collector region. • Electrons give up their energy in the form of heat and collector region must be able to dissipate this heat energy. • At EB junction potential barriers lowers and some of the electrons drift into base and they are swept by electric field from collector and fall down the potential hill into collector • A few holes also diffuse from base to emitter region.