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 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. 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. The physical explanation for collector current is the concentration of minority carriers in the base region.
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. 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 applied voltage causes the lower P-N junction to ‘turn on’, allowing a flow of electrons from the emitter into the base. Networks of transistors are used to make powerful amplifiers with many different applications. Heterojunction transistors have different semiconductors for the elements of the transistor.
Both approaches avoid biasing the transistor in the saturation mode. Microelectronic Circuits, second ed. The heavy doping of the emitter region and light doping of the base region causes many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter.
Retrieved from ” https: The result is that the transistor makes a good switch that is controlled by its base input.
Ebers Moll Equations
It will be obvious that why two diodes connected back to back will not function as a transistor omll the following discussion, as dependent current source term will be missing which is responsible for all the interesting properties of transistor.
The values of the minority carrier densities at the edges of the depletion regions are indicated on the Figure 5. 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.
As shown in the diagram, the emitter current, I Eis the total transistor current, which is the sum of the other terminal currents, i. The thermal runaway process associated with secondary breakdown, once triggered, occurs almost instantly and may catastrophically damage the transistor package.
Ebers Moll Model
Assume there is no recombination in the depletion region. In particular, the thickness of the base must be much less than the diffusion length of the electrons. In active mode, the electric field moll between base and collector caused by V CE will cause the majority modfl these electrons to cross the upper P-N junction into the collector to form the collector current I C. The collector diode is reverse-biased so I CD is virtually zero. This and other similar relations will be used to construct the charge control model of the bipolar junction transistor in section 5.
In a PNP transistor, the emitter—base region is forward biased, so holes are injected into the base as minority carriers.
Bipolar junction transistor
The emitter efficiency defined by equation 5. A small current entering the base is amplified to produce a large collector and emitter current.
The thin shared base and asymmetric collector—emitter doping are what differentiates a bipolar transistor from mode, separate and oppositely biased diodes connected in series. Solution The emitter efficiency is obtained from:.
The base is very thin, and most of the holes cross the reverse-biased base—collector junction to the collector. The BJT is also the choice for demanding analog circuits, especially for very-high-frequency applications, such as radio-frequency circuits for wireless systems. In contrast, unipolar transistors, such as field-effect transistorsonly use one kind of charge carrier.
These electrons diffuse through the ,odel from the region of high concentration near the emitter toward the region of low concentration near the collector.
Poon, “An integral charge control model of bipolar transistors”, Bell Syst. An NPN transistor molo 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.
To allow for greater current and faster operation, most bipolar transistors used today are NPN because electron mpdel is higher than hole mobility. The model can be quite accurate for low-frequency circuits and can easily be adapted for higher-frequency circuits with the addition of appropriate inter-electrode capacitances and other parasitic elements.
Ebers-moll model of transistor
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 ebeers current flows, so this end of the forward active region can be regarded as the cutoff region.
In active mode, the ratio of the collector current to the base current is called the DC current gain. Potentiometer digital Variable capacitor Varicap. 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.
The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. Applying anti log on both sides we get.
Having described the forward active mode morel operation, there remains the saturation mode, egers needs further discussion.
SiGe Heterojunction Bipolar Transistors. Ebers—Moll model for an NPN transistor. Silicon bandgap temperature sensor. The collector—emitter current can be viewed as being controlled by the base—emitter mldel current controlor by the base—emitter voltage voltage control.
Exposure of the transistor to ionizing radiation causes radiation damage.