EBERS MOLL MODEL OF BJT PDF

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. This model of transistor is known as Ebers Moll model of transistor. 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 back to back diodes in series 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.

Author:Zutilar Gardashicage
Country:Albania
Language:English (Spanish)
Genre:Personal Growth
Published (Last):22 November 2018
Pages:335
PDF File Size:6.71 Mb
ePub File Size:13.34 Mb
ISBN:643-8-66045-616-1
Downloads:33120
Price:Free* [*Free Regsitration Required]
Uploader:Gakasa



Saturation 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.

Forward active mode of operation The forward active mode is obtained by forward-biasing the base-emitter junction. The minority-carrier distribution in the quasi-neutral regions of the bipolar transistor, as shown in Figure 5. Figure 5. The values of the minority carrier densities at the edges of the depletion regions are indicated on the Figure 5. The carrier densities vary linearly between the boundary values as expected when using the assumption that no significant recombination takes place in the quasi-neutral regions.

The minority carrier densities on both sides of the base-collector depletion region equal the thermal equilibrium values since VBC was set to zero. While this boundary condition is mathematically equivalent to that of an ideal contact, there is an important difference. Instead, they drift through the base-collector depletion region and end up as majority carriers in the collector region. The emitter current due to electrons and holes are obtained using the "short" diode expressions derived in section 4.

This charge is proportional to the triangular area in the quasi-neutral base as shown in Figure 5. The emitter current therefore equals the excess minority carrier charge present in the base region, divided by the time this charge spends in the base. This and other similar relations will be used to construct the charge control model of the bipolar junction transistor in section 5.

A combination of equations 5. The emitter efficiency defined by equation 5. The long minority-carrier lifetime and the long diffusion lengths in those materials justify the exclusion of recombination in the base or the depletion layer.

The resulting current gain, under such conditions, is: 5. A typical current gain for a silicon bipolar transistor is 50 - The base transport factor, as defined in equation 5. This base transport factor can also be expressed in function of the diffusion length in the base: 5.

The quasi-neutral region width in the emitter is 1 mm and 0. The minority carrier lifetime in the base is 10 ns. Calculate the emitter efficiency, the base transport factor, and the current gain of the transistor biased in the forward active mode. Assume there is no recombination in the depletion region.

Solution The emitter efficiency is obtained from: The base transport factor equals: The current gain then becomes: where the transport factor, a, was calculated as the product of the emitter efficiency and the base transport factor: 5. General bias modes of a bipolar transistor While the forward active mode of operation is the most useful bias mode when using a bipolar junction transistor as an amplifier, one cannot ignore the other bias modes especially when using the device as a digital switch.

All possible bias modes are illustrated with Figure 5. They are the forward active mode of operation, the reverse active mode of operation, the saturation mode and the cut-off mode. This mode, as discussed in section 5. Finally, there is the reverse active mode of operation. In the reverse active mode, we reverse the function of the emitter and the collector. In this mode, the transistor has an emitter efficiency and base transport factor as described by equations 5.

Most transistors, however, have poor emitter efficiency under reverse active bias since the collector doping density is typically much less than the base doping density to ensure high base-collector breakdown voltages. In addition, the collector-base area is typically larger than the emitter-base area, so that even fewer electrons make it from the collector into the emitter. Having described the forward active mode of operation, there remains the saturation mode, which needs further discussion.

Cut-off requires little further analysis, while the reverse active mode of operation is analogous to the forward active mode with the added complication that the areas of the base-emitter and base-collector junction, AE and AC, differ. The Ebers-Moll model describes all of these bias modes. 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 operation, in the reverse active mode, in saturation and in cut-off.

The model contains two diodes and two current sources as shown in Figure 5. The two diodes represent the base-emitter and base-collector diodes. The current sources quantify the transport of minority carriers through the base region.

These current sources depend on the current through each diode. The parameters IE,s, IC,s, aF and aR are the saturation currents of the base-emitter and base collector diode and the forward and reverse transport factors.

LA CIUDAD ANTIGUA FUSTEL PDF

EBERS MOLL MODEL OF BJT PDF

Saturation 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. Forward active mode of operation The forward active mode is obtained by forward-biasing the base-emitter junction. The minority-carrier distribution in the quasi-neutral regions of the bipolar transistor, as shown in Figure 5.

CHARTULARIUM UNIVERSITATIS PARISIENSIS PDF

Measuring Ebers-Moll model parameters in transistors

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. NPN transistors are preferred due to increased mobility of electrons compared to holes and also because they are compatible with a negative ground system. Because two diodes are separate components and cannot share in common a semiconducting layer, they do not function as an amplifier, go into oscillation or perform switching action in the manner of actual transistors. When in forward-active mode, the collector diode is reverse-biased so ICD is virtually zero.

Related Articles