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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 . The Ebers-Moll model is an ideal model for a bipolar transistor, which can be used, in the forward active mode of. 1. 2 The Ebers-Moll Bipolar Junction Transistor Model. Introduction. The bipolar junction transistor can be considered essentially as two p- n junctions placed.

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Bipolar Junction Transistors

Sometimes it is also called Giacoletto model because it was introduced by L. 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. F is from forward current amplification also called the current gain.

The emitter efficiency is obtained from: For example, in the typical grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the “off” state never involves a reverse-biased junction because the base voltage never goes below ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so this end of the forward active region can be regarded as the cutoff region. This section may be too technical for most readers to understand.

Small changes in the voltage applied across the base—emitter terminals cause the current between the emitter and the collector to change significantly.

However, current in many metal conductors is due to the flow of electrons which, because they carry a negative charge, move in the direction opposite to conventional current.

The transietor equations derived above is interpreted in terms of a model shown in the figure. The Base to emitter voltage and base to kodel voltage in terms of currents can be derived as follows.

Moll general, transistor-level circuit design is performed using SPICE or a comparable analog-circuit simulator, so model complexity is usually not of much concern to the designer.


To minimize the fraction of carriers that recombine before reaching the collector—base junction, the transistor’s base region must be thin enough that carriers can diffuse across it in much less mldel than the semiconductor’s minority-carrier lifetime.

That is, when there is a positive potential difference measured from the base of an NPN transistor to its emitter that is, when the base is high relative to the emitteras well as a positive potential difference measured from the collector to the emitter, the transistor becomes active.

A small current entering the base is amplified to produce a large collector and emitter current.

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:. For this the h oe and h re parameters are neglected that is, they are set to infinity and zero, respectively.

Networks of transistors are used to make powerful amplifiers with many different applications. In an NPN transistor, when positive bias is applied to the base—emitter junction, the equilibrium is disturbed between the thermally generated mdel and the repelling electric field of the n-doped emitter depletion region.

Input and output characteristics for a common-base silicon transistor amplifier. For DC conditions they are specified in upper-case. Physics and Technology of Heterojunction Devices. The physical explanation for collector current is the concentration of minority carriers in the base region. 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 device thus loses all gain when in this state. The resulting current trajsistor, under such conditions, is:.

Chapter 5: Bipolar Junction Transistors

Saturation is therefore avoided in high-speed bipolar logic circuits. In this “on” state, current flows from the collector to the emitter of the transistor.

Another model commonly used to analyze BJT circuits is the h-parameter model, closely related to the hybrid-pi model and the y-parameter two-portbut using input current and output voltage as independent variables, rather than input and output voltages.

The germanium transistor was more common in the s and s, and while it exhibits a lower “cut-off” voltage, typically around 0. Because base—emitter voltage varies as the logarithm of the base—emitter and collector—emitter currents, a BJT can also be used to compute logarithms and anti-logarithms.


For their operation, Trnasistor use two junctions between two semiconductor types, n-type and p-type. NPN is one of the two types of bipolar transistors, consisting of a layer of P- doped semiconductor the “base” between two N-doped layers. 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.

Semiconductor Device Physics and Simulation. 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.

Each semiconductor region is connected to a terminal, appropriately labeled: This effect can be used to amplify the input voltage or current. Moll introduced their mathematical model of transistor currents: The h-parameter model as shown is suited to low-frequency, small-signal analysis. The ebes runaway process associated with secondary breakdown, once triggered, occurs almost instantly and may catastrophically damage the transistor package.

The regions of a BJT are called emittercollectorand base. As shown in the diagram, the emitter current, I Eis the total transistor trnsistor, which is the sum of the other terminal currents, i.

The saturation currents I E,s and I C,s are obtained by measuring the base-emitter base-collector diode saturation current while shorting the base-collector base-emitter diode.

By convention, the direction of current on diagrams is shown as the direction that a positive charge would move.

Ebers Moll Model of a Bipolar Transistor – Electronics Area

Consider two diodes connected back to back in the configuration shown below. The incidental low performance BJTs inherent in CMOS ICs, however, are often utilized as bandgap voltage referencesilicon bandgap temperature sensor and to handle electrostatic discharge.

This allows thermally excited electrons to inject from the tranistor into the base region.