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 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 BJT also dbers a mooll amplifier, since it can multiply a weak input signal to about times its original strength.

Ebers Moll Model of a Bipolar Transistor – Electronics Area

The collector—emitter current can be viewed as being controlled by the base—emitter current current controlor by the base—emitter voltage voltage control. When a transistor is mode, at higher frequencies, the fundamental limitation is the time it takes the carriers to diffuse across the base region The current equations derived above is interpreted in terms of a model shown in the figure.

Exposure of the transistor to ionizing radiation causes radiation damage. These have been modeo in various more advanced models: By convention, the direction of current on diagrams is shown as the direction that a positive charge would move. A bipolar junction transistor bipolar transistor or BJT is a type of transistor that uses both electron and hole charge carriers.

Chapter 5: Bipolar Junction Transistors

In this article, current arrows are shown in the conventional direction, but labels for the movement of holes and electrons show their actual direction inside the transistor. The saturation voltage equals: The BJT is also the choice for demanding analog circuits, especially for very-high-frequency applications, such as radio-frequency circuits for wireless systems.


Such recombination current will be discussed in section 5. The values of the minority carrier densities at the edges of the depletion regions are indicated on the Figure 5. A typical current gain for a silicon bipolar transistor is 50 – The device thus loses all gain when in this state. Detailed transistor models of transistor action, such as the Gummel—Poon modelaccount for the distribution of this charge explicitly to explain transistor behaviour more exactly.

In the active mode of operation, electrons are injected from the forward biased n-type emitter region into the p-type base where they diffuse as minority carriers to the reverse-biased n-type collector and are swept away by the electric field in the reverse-biased collector—base junction.

As a transistor is switched from saturation to cut-off, this charge initially remains in the base and a collector current will remain until this charge is removed by recombination.

A BJT consists of three differently doped semiconductor regions: These electrons diffuse through the base from the region of high concentration near the emitter toward the region of low concentration near the collector.

NPN base width for low collector—base reverse bias; Bottom: The base transport factor, as defined in equation 5. The two diodes represent the base-emitter and base-collector diodes. They are the forward active mode of operation, the reverse active mode of operation, the saturation mode and the cut-off mode.

Bipolar junction transistor

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. 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, A E and A Cdiffer.

These views are related by the current—voltage relation of the base—emitter junction, which is the usual exponential current—voltage curve of a p—n junction diode. 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.


This and other similar relations will be used to construct the charge control model of the bipolar junction transistor in section 5.

In analog circuit design, the current-control view is sometimes used because it is approximately linear. Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way. In particular, the thickness of the base must be much less than the diffusion length of the electrons. 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.

Networks of transistors are used to make powerful amplifiers with many different applications.

Ebers Moll Model

Darlington transistor Sziklai pair Cascode Long-tailed pair. 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. This model is based on assumption that base spreading omdel can be neglected.

Radiation causes a buildup of ‘defects’ in the base region that act as recombination centers. From Wikipedia, the free encyclopedia. Thus, the hottest part of the die conducts the most current, causing its conductivity to increase, which then causes it to morel progressively hotter modeo, until the device fails internally. 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.

Now coming to important question of Why two back to back diodes cannot function as a transistor?