is quite large as its the sum of both the base current and collector current amplifiers and which represents the "normal" method of bipolar transistor connection. The only difference between an NPN and PNP is the direction of the arrow on the Don't base your understanding of a transistor's operation on that model (and . Current flows from emitter to collector (not, exactly, the purpose transistors. There are two basic types of bipolar transistor construction, PNP and NPN, which . Since the electrical relationship between these three currents, Ib, Ic and Ie is As the emitter current is the combination of the collector AND the base current.
The regions of a BJT are called emitter, collector, and base. Typically, the emitter region is heavily doped compared to the other two layers, whereas the majority charge carrier concentrations in base and collector layers are about the same collector doping is typically ten times lighter than base doping .
By design, most of the BJT collector current is due to the flow of charge carriers electrons or holes injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices. In typical operation, the base—emitter junction is forward-biasedwhich means that the p-doped side of the junction is at a more positive potential than the n-doped side, and the base—collector junction is reverse-biased.
Bipolar junction transistor - Wikipedia
In an NPN transistor, when positive bias is applied to the base—emitter junction, the equilibrium is disturbed between the thermally generated carriers and the repelling electric field of the n-doped emitter depletion region. This allows thermally excited electrons to inject from the emitter into the base region. These electrons diffuse through the base from the region of high concentration near the emitter toward the region of low concentration near the collector.
The electrons in the base are called minority carriers because the base is doped p-type, which makes holes the majority carrier in the base. To minimize the fraction of carriers that recombine before reaching the collector—base junction, the transistor's base region must be thin enough that carriers can diffuse across it in much less time than the semiconductor's minority-carrier lifetime. As well, as the base is lightly doped in comparison to the emitter and collector regionsrecombination rates are low, permitting more carriers to diffuse across the base region.
In particular, the thickness of the base must be much less than the diffusion length of the electrons. The collector—base junction is reverse-biased, and so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector—base junction.
The thin shared base and asymmetric collector—emitter doping are what differentiates a bipolar transistor from two separate and oppositely biased diodes connected in series. Voltage, current, and charge control[ edit ] The collector—emitter current can be viewed as being controlled by the base—emitter current current controlor by the base—emitter voltage voltage control.
These views are related by the current—voltage relation of the base—emitter junction, which is the usual exponential current—voltage curve of a p—n junction diode.
Detailed transistor models of transistor action, such as the Gummel—Poon modelaccount for the distribution of this charge explicitly to explain transistor behaviour more exactly. However, because base charge is not a signal that is visible at the terminals, the current- and voltage-control views are generally used in circuit design and analysis.
In analog circuit design, the current-control view is sometimes used because it is approximately linear. This gives rise to recombination current. The rest of the electrons exist for more time. Since the collector diode is reverse biased, n is connected to positive supply therefore most of the electrons are pushed into collector layer.
These collector elections can then flow into the external collector lead. Thus, there is a steady stream of electrons leaving the negative source terminal and entering the emitter region.
Bipolar junction transistor
The VEB forward bias forces these emitter electrons to enter the base region. The thin and lightly doped base gives almost all those electrons enough lifetime to diffuse into the depletion layer.Relation of leakage currents ICEO and ICBO of Transistor
The depletion layer field pushes a steady stream of electron into the collector region. These electrons leave the collector and flow into the positive terminal of the voltage source.
But the collector current is less than emitter current. Relation between different currents in a transistor: The total current flowing into the transistor must be equal to the total current flowing out of it.
The total collector current IC is made up of two components. The fraction of emitter electron current which reaches the collector adc IE 2. The normal reverse leakage current ICO adc is known as large signal current gain or dc alpha.