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A bipolar junction transistor ('''BJT''') is a type of Transistor . It is a three-terminal device constructed of Doped Semiconductor material and may be used in Amplifying or switching applications. Bipolar transistors are so named because their operation involves both Electron s and Hole s. Although a small part of the transistor current is due to the flow of Majority Carrier s, most of the transistor current is due to the flow of Minority Carrier s and so BJTs are classified as 'minority-carrier' devices.
: STRUCTURE A BJT consists of three differently Doped semiconductor regions, the ''emitter'' region, the ''base'' region and the ''collector'' region. These regions are, respectively, ''p'' type, ''n'' type and ''p'' type in a PNP , and ''n'' type, ''p'' type and ''n'' type in a NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: ''emitter'' (E), ''base'' (B) and ''collector'' (C). The ''base'' is physically located between the ''emitter'' and the ''collector'' and is made from lightly doped, high resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. A cross section view of a BJT indicates that the collector–base junction has a much larger area than the emitter–base junction. The bipolar junction transistor, unlike other transistors, is not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. Because the transistor's internal structure is usually optimized to forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those found in forward operation; often the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter. Small changes in the voltage applied across the base–emitter terminals causes the current that flows between the ''emitter'' and the ''collector'' to change significantly. This effect can be used to amplify the input voltage or current. BJTs can be thought of as voltage-controlled Current Source s, but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base. Early transistors were made from Germanium but most modern BJTs are made from Silicon . A significant minority are also now made from Gallium Arsenide , especially for very high speed applications (see HBT, below). NPN NPN is one of the two types of bipolar transistors, in which the letters "N" and "P" refer to the majority Charge Carrier s inside the different regions of the transistor. Most bipolar transistors used today are NPN, because Electron Mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation. NPN transistors consist of a layer of P- Doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the Conventional Current flow when the device is in forward active mode. A convenient mnemonic device for identifying the symbol for the NPN transistor and, by elimination, the PNP transistor is "NPN is Not Pointed iN." PNP The other type of BJT is the PNP with the letters "P" and "N" referring to the majority Charge Carrier s inside the different regions of the transistor. Few transistors used today are PNP, since the NPN type gives better performance in most circumstances. PNP transistors consist of a layer of N- Doped semiconductor between two layers of P-doped material. PNP transistors are commonly operated with the collector at Ground and the emitter connected to a positive Voltage through an Electric Load . A small current flowing from the base allows a much greater current to flow from the emitter to the collector. The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the Conventional Current flow when the device is in forward active mode. Heterojunction bipolar transistor The Heterojunction Bipolar Transistor ( HBT ) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz . It is common nowadays in ultrafast circuits, mostly RF systems. Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. This helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias and increases emitter injection efficiency. The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. With a more traditional BJT, also referred to as homojunction BJT, the efficiency of carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and base. Because the base must be lightly doped to allow the high injection efficiency, its resistance is relatively high. Higher doping in the base can improve figures of merit like the Early Voltage . Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by Epitaxy techniques like MOCVD and MBE . TRANSISTORS IN CIRCUITS The diagram opposite is a schematic representation of an npn transistor connected to two voltage sources. To make the transistor conduct appreciable current (on the order of 1 mA) from C to E, must be above a Threshold Voltage sometimes referred to as the cut-in voltage. The cut-in voltage is usually about 600 mV for silicon BJTs. This applied voltage causes the lower p-n junction to 'turn-on' allowing a flow of electrons from the emitter into the base. Because of the electric field existing between base and collector (caused by ), the majority of these electrons cross the upper p-n junction into the collector to form the collector current, . 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, . As shown in the diagram, the emitter current, , is the total transistor current which is the sum of the other terminal currents. That is: : In the diagram, the arrows representing current point in the direction of the electric or Conventional Current —the flow of electrons is in the opposite direction of the arrows since electrons carry negative Electric Charge . The ratio of the collector current to the base current is called the ''DC current gain''. This gain is usually quite large and is often 100 or more. It should also be noted that the emitter current is related to exponentially. At room temperature, increasing by about 60 mV increases the emitter current by a factor of 10. The base current is approximately proportional to the emitter current, so it varies the same way. REGIONS OF OPERATION Bipolar transistors have five distinct regions of operation, defined mostly by applied bias:
While these regions are well defined for sufficiently large applied voltage, they overlap somewhat for small (less than a few hundred millivolts) biases. 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. HISTORY The bipolar (point-contact) transistor was invented in December 1947 at the Bell Telephone Laboratories by John Bardeen and Walter Brattain under the direction of William Shockley . The junction version, invented by Shockley in 1948, enjoyed three decades as the device of choice in the design of discrete and Integrated Circuits . Nowadays, the use of the BJT has declined in favour of CMOS technology in the design of digital integrated circuits. Germanium transistors The Germanium transistor was more common in the 1950s and 1960s , and while it exhibits a lower "cut off" voltage, making it more suitable for some applications, it also has a greater tendency to exhibit thermal runaway. Early manufacturing techniques Various methods of manufacturing bipolar junction transistors were developed Third case study – the solid state advent (PDF). . Patent filed on June 26 , 1948.
THEORY AND MODELING Large-signal models Ebers–Moll model The DC emitter and collector currents in normal operation are well modeled by the Ebers–Moll model: : : The base internal current is mainly by diffusion and : Where
The collector current is slightly less than the emitter current, since the value of is very close to 1.0. In the BJT a small amount of base–emitter current causes a larger amount of collector–emitter current. The ratio of the allowed collector–emitter current to the base–emitter current is called ''current gain'', β or . A β value of 100 is typical for small bipolar transistors. In a typical configuration, a very small signal current flows through the base–emitter junction to control the emitter–collector current. β is related to α through the following relations: : : : Emitter Efficiency : Another set of equations used to describe the three currents in the any operating region are given below. These equations are based on the transport model for a Bipolar Junction Transistor. Where
=Base-width modulation As the applied collector–base voltage () varies, the collector–base depletion region varies in size. This is often called the " Early Effect " after its discoverer James M. Early . This effectively means a variation in the width of the base region of the BJT. An increase in the collector–base voltage, for example, causes a greater reverse bias across the collector–base junction, increasing the collector–base depletion region width, decreasing the width of the base. This has two consequences :
Both factors increase the collector or "output" current of the transistor due to an increase in the collector–base voltage. In the forward active region the Early effect modifies the collector current () and the forward common emitter current gain () to the following equations. Where
=Punchthrough When the base–collector voltage reaches a certain (device specific) value, the base–collector depletion region boundary meets the base–emitter depletion region boundary. When in this state the transistor effectively has no base. The device thus loses all gain when in this state. Gummel–Poon charge-control model The Gummel–Poon Model H. K. Gummel and R. C. Poon, "An integral charge control model of bipolar transistors," ''Bell Syst. Tech. J.'', vol. 49, pp. 827--852, May-June 1970 is a detailed charge-controlled model of BJT dynamics, which has been adopted and elaborated by others to explain transistor dynamics in greater detail than the terminal-based models typically do {Link without Title} . Small-signal models h-parameter model Another model commonly used to analyse BJT circuits is the ''h-parameter'' model. This model is a 2-port network particularly suited to BJTs as it lends itself easily to the analysis of circuit behaviour, and may be used to develop further accurate models. As shown, the term ''"x"'' in the model represents the BJT lead depending on the topology used. For common-emitter mode the various symbols take on the specific values as –
and the h-parameters are given by –
As shown, the h-parameters have lower-case subscripts and hence signify AC conditions or analyses. For DC conditions they are specified in upper-case. For the CE topology, an approximate h-parameter model is commonly used which further simplifies the circuit analysis. For this the ''h''oe and ''h''re parameters are ignored (rather, they are set to infinity and zero, respectively). It should also be noted that the h-parameter model is suited to low-frequency, small-signal analysis. For high-frequency analyses this model is not used since it ignores the inter-electrode capacitances which come into effect at high frequencies. APPLICATIONS OF TRANSISTORS The BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide selection of BJT types available and because of knowledge about the bipolar transistor characteristics. The BJT is also the choice for demanding analog circuits, both integrated and discrete. This is especially true in Very-high-frequency applications, such as Radio-frequency circuits for wireless systems. The bipolar transistors can be combined with MOSFET 's in an integrated circuit by using a BiCMOS process to create innovative circuits that take advantage of the best characteristics of both types of transistor. Temperature sensors Because of the known temperature and current dependence of the forward-biased base–emitter junction voltage, the BJT can be used to measure temperature by subtracting two voltages at two different bias currents in a known ratio {Link without Title} . Logarithmic converters Since base–emitter voltage varies as the log of the base–emitter and collector–emitter currents, a BJT can also be used to compute Logarithm s and anti-logarithms. A diode can also perform these nonlinear functions, but the transistor provides more circuit flexibility. VULNERABILITIES OF TRANSISTORS Exposure of the transistor to Ionizing Radiation causes Radiation Damage . Radiation causes a buildup of 'defects' in the base region that act as Recombination Centers . The resulting reduction in minority carrier lifetime causes gradual loss of gain of the transistor. Power BJT's are subject to a failure mode called Secondary Breakdown . In this failure mode, certain parts of the die (the actual piece of silicon inside the device) get hotter than the others. As a result, the hottest part of the die conducts the most current causing it to get hotter still until the device short-circuits internally. SEE ALSO
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