Ebers–Moll model for an NPN transistor. mode are well modeled by an approximation to the Ebers–Moll model. Ebers and Moll created a model between the current and voltages in the transistor terminals. This model is knowned as the Ebers Moll model. The Ebers-Moll model is an ideal model for a bipolar transistor, which can be used, in the forward active mode of.

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This model is based on assumption that base spreading resistance can be neglected.

Bipolar junction transistor – Wikipedia

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.

Bipolar Junction Transistors

The above trwnsistor are derived based on the assumption of low level minority carrier injection the hole concentration hransistor into the base is very much less compared to the intrinsic electron concentration in basein 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 ebeers order of reverse saturation current flows through the series junctions. This can be explained as follows: 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 mo,l.


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.

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Consider two diodes connected back to back in the configuration shown below back to back diodes in series. Leave a Reply Cancel reply Your email address will not be published.