It’s a model to describe large signal behaviour of a transistor, and start with the simple notion of two back to back diodes. For example the diodes seen at the two . It can be shown that (see S.M. Sze, Physics of Semiconductor Devices), therefore . where. More Complete Ebers-Moll Model. Model includes configurational. Ideal transistor model. Forward active mode of operation General bias modes of a bipolar transistor The Ebers-Moll model 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 moodel 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 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.
Bipolar Junction Transistors
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.
The Bipolar Transistor (Ebers Moll Model)
This moddel 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 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.
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