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Bipolar junction transistor
Bipolar junction transistor
Bipolar junction transistor (BJT) is a type of transistor using both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, like field-effect transistors (FETs), uses 1 kind of charge carrier. A bipolar transistor lets small current to inject at 1 of its terminals to control much higher currents between the other 2 terminals, making the device able of amplification/switching.
bipolar: device with current carried by 2 charge carrier types (both electrons and holes)
junction: semiconductor transition part between where conduction is mainly by electorns and where mainly by holes
transistor: semiconductor device acting as a switch/amplifier, controlling electrical current flow via a small input signal
semiconductor: solid substance with an insulator and metal's conductivities
insulator: a substance that won't readily let heat/sound flow
electron hole/hole: positive charge carriers, representing an electron's absence in the semiconductor material
charge carrier: particle carrying electric charge and let current flow in a material (e,g, electrons, holes)
unipolar: device with current carried only by 1 charge carrier type (e.g., elecrons or holes)
[6] Figure 11
[3] Figure 2
Components
Components
A BJT has 3 parts: the emitter, base, and collector, which makes 2 p–n junctions between 2 semiconductor types: the emitter–base and base–collector junction.
p-n junction: boundary where p and n-type semiconductor materials meet
boundary: dividing line between 2 regions of different doping
In both NPN and PNP BJTs:
The emitter–base junction is often forward-biased (i.e. to let to reduce the junction's barrier) to let charge carriers flow from emitter to base.
The base–collector junction is often reverse-biased (i.e. to increase the junction's barrier) to block current from base to collector and collect the carriers.
n-type and p-type properties
n-type and p-type properties
A n-type (semiconductor) is a material (e.g., silicon) doped with impurities (e.g., phosphorus, arsenic) with extra valence electron (crucial they can be excited to move through the material, allowing it to conduct electricity.), creating a surplus of free electrons acting as majority negative charge carriers, hence "n"
to dope (in electronnics): to intentionally introduce impurities into a pure (intrinsic) semiconductor to modify its electrical properties and increase its conductivity
intrinsic: material in its purest natural form, prior impurities are added
surplus: amount left over if requirements are met
A p-type (semiconductor) is the same material dope with other impurities (e.g., boron, aluminum) but with excess holes (missing electrons), acting as majority positive charge carriers, hence "p"
Note: Negative carriers refer to electrons but, positive carriers don't refer to protons (which are positive subatomic particles), but holes (missing electrons) instead.
The charge carriers (holes/electrons) of the semiconductors don't work the same way as the convential current that flow on a circuit. Thus,
A BJT's terminals' names originate from the type of doping, not the direction of current flow:
In a PNP BJT, true to their names:
Emitter (p-type) “emits” holes (charge carriers not conventional current flow) out of the BJT.
Base (n-type) is called that as it's the “base layer” between the emitter and collector. Small base current in it controls the larger current from emitter to collector.
Collector (p-type) “collects” the holes into the BJT to the base.
Similarly in a NPN BJT:
Collector (n-type) collects the electrons flowing from the emitter and sends it out (Fig. 11).
Small current flows in the base (p-type) controls the larger current from emitter to controller.
Emitter (n-type) emits electrons instead (not holes) into the BJT to the base.
Figure 11 conventional current flow (in red) and charge carrier (in cyan) flow on an NPN BJT circuit. Same direction on a PNP BJT.
The junctions can be made in many ways, like changing the semiconductor material's doping as it is grown, by depositing metal pellets to form alloy junctions, or by such methods as diffusion of n-type and p-type doping substances into the crystal. The superior predictability and performance of junction transistors quickly displaced the original point-contact transistor. Diffused transistors, along with other components, are elements of integrated circuits for analog and digital functions. Hundreds of bipolar junction transistors can be made in one circuit at a very low cost.
Bipolar transistor integrated circuits were the main active devices of a generation of mainframe and minicomputers, but computer systems now use complementary metal–oxide–semiconductor (CMOS) integrated circuits relying on the field-effect transistor (FET). Bipolar transistors are still used for amplification of signals, switching, and in mixed-signal integrated circuits using BiCMOS. Specialized types are used for high voltage and high current switches, or for radio-frequency (RF) amplifiers.
Operation
Operation
Figure 3
Figure 4
NPN BJT
NPN BJT
[3] Water pipe analogy (for NPN BJT) In the vertical part (BJT's collector), a horizontal red door controls if water (current) flows down into the lower part (BJT's emitter). A vertical yellow door that controls its own water floor there in the horizontal part (BJT's base) of the pipe also controls the red door.
Without pressure on the horizontal red door, it holds water.
Like on a BJT: If voltage is used on the emitter and collector and if the base isn't biased, current won't flow from the BJT.
Figure 5
Figure 6
[3] If a bit of water flows into horizontal tube, the vertical door opens, if it's high enough. A certain lower water amount won't open door. If the vertical door is open, this is considered as the transistor's on mode (VBE). As the vertical door opens, so will the horizontal door to let water flow.
Same as we apply the BJT's base to emitter voltage, current flows throught the collector to the emitter. This can occurs if voltages as low as 0.7 V and 1 mA current, it's enough to turn the BJT on.
Note: The (conventional) current shown moving (in red in Fig. 6) in the circuit doesn't work like the charge carriers (electrons/holes). The charge carriers flow in the opposite way of both PNP and NPN BJTs.
Thus, when active, a NPN's conventional current flows into the collector and out of the emitter, but vice versa for its charge carriers, which flow into the emitter and out of the collector
Figure 11
Overally, ideally, regardless how much current the collector gets (tube's upper part), if the base receives under the needed current amount, the BJT is stays off (neither the collector or base opens).
The base controls the collector, both for PNP and NPN.
Figure 7
Figure 8
Figure 9
PNP BJT
PNP BJT
[3] Water pipe analogy (for PNP BJT) Consider a NPN transistor BC547 (Fig. 10). The tube's upper part for this is the emitter. Its collector is the lower part. The base is the horizontal part. If the tube's horizontal part receives water, it stays closed.
For as a PNP BJT's base connected to positive (Fig. 8), as the base gets positive voltage, it turns off/stays off. Without water at the horizontal part opens the part to let water flow.
Connecting the base to negative voltage (Fig. 9) (causing current flow into the emitter) turns on the BJT as long as the base has lower current (less positive) than the emitter or the emitter has more positive voltage than it (is more positive),
Figure 10
Note: Again, the (conventional) current and charge carrier of a PNP BJT move move in the same direction as in a NPN. If active, a PNP's (conventional) current flows into the emitter and out of the collector, but vice versa for its charge carriers.
Overall, a PNP BJT's emitter connects to positive voltage, and gets the voltage and its collector outputs the current.
But a NPN BJT's emitter connects to positive voltage, and gets the voltage and its collector outputs the current.
History
History
[1] 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 known as the bipolar junction transistor (BJT), invented by Shockley in 1948,[2] was for three decades the device of choice in the design of discrete and integrated circuits. Nowadays, the use of the BJT has declined in favor of CMOS technology in the design of digital integrated circuits. The incidental low performance BJTs inherent in CMOS ICs, however, are often utilized as bandgap voltage reference, silicon bandgap temperature sensor and to handle electrostatic discharge.
Types
Types
A BJT
Quiz
Quiz
[1] (Knowt)
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