The ideal transistor model is based on the ideal p-n diode model and provides a first-order calculation of the dc parameters of a bipolar junction transistor. To further simplify this model, we will assume that all quasi-neutral regions in the device are much smaller than the minority-carrier diffusion lengths in these regions, so that the "short" diode expressions apply. The use of the ideal p-n diode model implies that no recombination within the depletion regions is taken into account. Such recombination current will be discussed in section 5. The discussion of the ideal transistor starts with a discussion of the forward active mode of operation, followed by a general description of the four different bias modes, the corresponding Ebers-Moll model and a calculation of the collector-emitter voltage when the device is biased in saturation.

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This model is based on assumption that base spreading resistance can be neglected. It will be obvious that why two diodes connected back to back will not function as a transistor from the following discussion, as dependent current source term will be missing which is responsible for all the interesting properties of transistor.

The normal mode of operation corresponds to the use of emitter as source of collector current and inverted mode of operation corresponds to the use of collector as source of emitter current which is the case when BJT is operated in inverse active region.

The BJT when operated in normal mode and inverse mode is shown in the figure below. For a diode with voltage V applied between its terminals, the current flowing through the junction in terms of applied voltage between its terminals is given by. The collector current in a BJT when operated in normal mode is given as.

The current equations derived above is interpreted in terms of a model shown in the figure. This model of transistor is known as Ebers Moll model of transistor. The above equations are derived based on the assumption of low level minority carrier injection the hole concentration injected into the base is very much less compared to the intrinsic electron concentration in base , in such a case emitter or collector current is mainly dominated by diffusion currents, drift current is negligible compared to drift currents.

The Base to emitter voltage and base to collector voltage in terms of currents can be derived as follows. Applying anti log on both sides we get. Now coming to important question of Why two back to back diodes cannot function as a transistor? Consider two diodes connected back to back in the configuration shown below.

It is obvious that if one junction is forward biased then other junction will be reverse biased consider for example diode D1 is forward biased and diode D2 is reverse biased much like a NPN transistor in active region according to the junction voltages only current order of reverse saturation current flows through the series junctions.

This can be explained as follows: the reverse biased diode D2 at most will allow only currents order of reverse saturation currents. Since D1 and D2 are in series same current should flow through both of them then only currents order of reverse saturation currents flow through their junctions.

It is obvious that this is not the case with the transistor in active region because of the internal design of transistor. The forward current entering the base is sweeped across into collector by the electric filed generated by the reverse bias voltage applied across the base collector junction. Your email address will not be published. Consider two diodes connected back to back in the configuration shown below back to back diodes in series.

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Bipolar junction transistor

A bipolar junction transistor BJT is a type of transistor that uses both electrons and holes as charge carriers. Unipolar transistors, such as field-effect transistors , use only one kind of charge carrier. A bipolar transistor allows a small current injected at one of its terminals to control a much larger current flowing between two other terminals, making the device capable of amplification or switching. BJTs use two junctions between two semiconductor types, n-type and p-type, which are regions in a single crystal of material. The junctions can be made in several different ways, such as changing the Doping of the semiconductor material as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n -type and p-type doping substances into the crystal.


Chapter 5: Bipolar Junction Transistors

If transistor circuits are to be of any use or amenable to diagnostic procedures, we must be able to model them. Transistors characteristically have multiple modes of conduction. We can view these phenomena in the two-diode model of a bipolar junction transistor BJT. Two diodes whose anodes join to form a center tap are analogous to an NPN transistor insofar as ohmmeter readings accurately represent the real device. Two diodes with cathodes connected to a common node are analogous to a PNP transistor.

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