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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The BJT is also the choice for demanding analog circuits, especially for very-high-frequency applications, such as radio-frequency circuits for wireless systems. Cut-off requires little further analysis, while the reverse active mode of operation is analogous to the forward active mode with the added complication eebers the areas of the base-emitter and base-collector junction, A E and A Cdiffer.

Next, we need to find the emitter efficiency and base transport factor.

Ebers Moll Equations

For the specific case where the base-emitter and base-collector voltage are the same and the base doping is uniform, there can be no minority carrier diffusion in the base so that: BJTs can be thought of as voltage-controlled current sourcesbut are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base.

Leave a Reply Cancel reply Your email address will not be published. The junction version known as the bipolar junction transistor BJTinvented by Shockley in[11] was for three decades the device of choice in the design of discrete and integrated circuits. The use of the ideal p-n diode model implies that no recombination within the depletion regions is taken into account.

Common source Common drain Common gate. From this equation, we conclude that the current gain can be larger than one if the emitter doping is much larger than the base doping. Transistors can be thought of as two diodes P—N junctions sharing a common region that minority carriers can move through. The current sources quantify the transport of minority carriers through the base region.


Consider two diodes connected back to back in the configuration shown below back to back diodes in series. When a transistor is used at higher frequencies, the fundamental limitation is the time it takes the carriers to diffuse across the base region Silicon bandgap temperature sensor. 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.

Bipolar transistors can be combined with MOSFETs in an integrated circuit by using a BiCMOS process of wafer fabrication to create circuits that take advantage of the application strengths of both types of transistor. 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.

Charge flow in a BJT is due to diffusion of charge carriers across a junction between two regions of different charge concentrations.

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. These have moedl addressed in various more advanced models: This model of transistor is known as Ebers Moll model of transistor.

For high current gain, most of the carriers injected into the emitter—base junction must come from the emitter.

Bipolar junction transistor

The remainder of the electrons recombine with holes, the majority carriers in the base, making a current through the base connection to form the base current, I B. This and other similar relations will be used to construct the charge control model of the bipolar junction transistor in section 5.

The diagram shows a schematic representation of an NPN transistor connected to two voltage sources. The germanium transistor was more common in the s and s, and while it exhibits a lower “cut-off” voltage, typically around 0. The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. An NPN transistor comprises two semiconductor junctions that share a thin p-doped region, and a PNP transistor comprises two semiconductor junctions that share a thin n-doped region.


The modes of operation can be described in terms of the applied voltages this description applies to NPN transistors; polarities are reversed for PNP transistors:.

Physics and Technology of Heterojunction Devices. The base transport factor, as defined in equation 5. It is obvious that this is not the case with the transistor in active region because of the internal design of transistor. Simplified cross section of a planar NPN bipolar junction transistor. That drift component of transport aids the normal diffusive transport, increasing the frequency response of the transistor by shortening the transit time across the base. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way.

The hybrid-pi model is a popular circuit model used for analyzing the small signal behavior of bipolar junction and field effect transistors. The physical explanation for collector current is the concentration of minority carriers in the base region. Microelectronic Circuits 5th ed. In particular, the thickness of the base must be much less than the diffusion length of the electrons.

Chapter 5: Bipolar Junction Transistors

Both approaches avoid biasing the transistor in the saturation mode. 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 resulting reduction in minority carrier lifetime causes gradual loss of gain of the transistor. Views Read Edit View history. This relation ship is also referred as the reciprocity relation and can be derived by examining the minority carrier current through the base.

This applied voltage causes the lower P-N junction to ‘turn on’, allowing a flow of electrons from the emitter into the base. The minority-carrier distribution in the quasi-neutral regions of the bipolar transistor, as shown in Figure 5. NPN base width for low collector—base reverse bias; Bottom: