Product data sheet. Supersedes data of Apr Oct DISCRETE SEMICONDUCTORS. BC; BC PNP general purpose. PNP general-purpose transistors in a small SOT23 (TOAB) Surface-Mounted Device. (SMD) plastic package. Table 1. Product overview. Low noise input stages of audio frequency equipment. DESCRIPTION. PNP transistor in a SOT23 plastic package. NPN complements: BC
|Published (Last):||10 June 2006|
|PDF File Size:||10.38 Mb|
|ePub File Size:||13.47 Mb|
|Price:||Free* [*Free Regsitration Required]|
A bipolar junction transistor bipolar transistor or BJT is a type of transistor that uses both electron and hole charge carriers.
In contrast, unipolar transistors, such as field-effect transistorsonly use one kind of skt carrier. For their operation, BJTs use two junctions between two semiconductor types, n-type and p-type.
Facebook Official Page
BJTs are manufactured in two types, NPN and PNP, and are available as individual components, or fabricated in integrated circuitsoften in large numbers. The basic function of a BJT is to amplify current. This allows BJTs to be used as amplifiers or switches, giving them wide applicability in electronic equipment, including computers, televisions, mobile phones, audio amplifiers, industrial control, and radio transmitters.
By convention, the direction of current on diagrams is shown as the direction that a positive charge would move. This is called conventional current. 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. 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 arrow on the symbol for bipolar transistors indicates the PN junction between base and emitter and points in the direction conventional current travels. An NPN transistor comprises 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. Charge flow in a BJT is due to diffusion of charge carriers across a junction between two regions of different charge concentrations.
The regions of a BJT are called emittercollectorand base. Typically, the emitter region is heavily doped compared to the other two layers, whereas the majority charge carrier concentrations in base and collector layers are about the same collector doping is typically ten times lighter than base doping . By design, most of the BJT collector current is due to the flow of charge carriers electrons or holes injected from a high-concentration emitter into the base where they are minority carriers that diffuse toward the collector, and so BJTs are classified as minority-carrier devices.
In typical operation, the base—emitter junction is forward-biasedwhich means that the p-doped side of the junction is at a more positive potential than the n-doped side, and the base—collector junction is reverse-biased. In an NPN transistor, when positive bias is applied to the base—emitter junction, the equilibrium is disturbed between the thermally generated carriers and the repelling electric field of the n-doped emitter depletion region.
This allows thermally excited electrons to inject from the emitter into 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 electrons in the base are called minority carriers because the base is doped p-type, which makes holes the majority carrier in the base.
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 time than the semiconductor’s minority-carrier lifetime.
As well, as the base is lightly doped in comparison to the emitter and collector regionsrecombination rates are low, permitting more carriers to diffuse across the base region. In particular, the thickness of the base must be much less than the diffusion length of the electrons. The collector—base junction is reverse-biased, and so little electron injection so 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 thin shared base and asymmetric collector—emitter doping are what differentiates a bipolar transistor bc57 two separate and oppositely biased diodes connected in series. The collector—emitter current can be viewed as being controlled by the base—emitter current current controlor by the base—emitter voltage voltage control.
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 physical explanation for collector current is the concentration of minority carriers in the base region.
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. However, because base charge is not a signal that is wot at the terminals, the current- and voltage-control views are generally used in circuit design and analysis. In analog circuit design, the current-control view is sometimes used because it is approximately linear.
However, to accurately and reliably design production BJT circuits, the voltage-control for example, Ebers—Moll model is required. For translinear circuitsin which the exponential I—V curve is key to the operation, the transistors are usually modeled as voltage-controlled current sources whose transconductance is proportional to their collector current.
In general, transistor-level circuit design is performed using SPICE or a comparable analog-circuit simulator, so model complexity is usually not of much 223 to the designer. Most bipolar transistors, and especially power transistors, have long base-storage times when they are driven into saturation; the base storage limits turn-off time in switching applications.
A Baker clamp can prevent the transistor from heavily saturating, which reduces the amount of charge stored in the base and thus improves switching time. The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. 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.
It is typically greater than 50 for small-signal transistors, but can be smaller in transistors designed for high-power applications.
The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.
It is less than unity due to recombination of charge carriers as they cross the base region. A BJT consists of three differently doped semiconductor regions: These regions are, respectively, p type, n type and p type in a PNP transistor, and n type, p type and n type in an NPN transistor. Each semiconductor region is connected to a terminal, 223 labeled: The base is physically located between the emitter and the collector and is made from lightly doped, high-resistivity material.
A cross-section view of a BJT indicates zot the collector—base junction has a much larger area than the emitter—base junction. The bipolar junction transistor, unlike other transistors, is usually 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. 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: For high current gain, most of the carriers injected into the emitter—base junction must come from the emitter.
The low-performance “lateral” bipolar transistors sometimes used in CMOS processes are sometimes designed symmetrically, that is, with no difference between forward and backward operation. Small changes in the voltage applied across the base—emitter terminals cause the current 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 sourcesbut are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base. Early transistors were made dot germanium but most modern BJTs are made from silicon.
A significant minority are also now made from gallium arsenideespecially for very high speed applications see HBT, below. NPN is one of the two types of bipolar transistors, consisting of a layer of P- doped semiconductor the b557 between two N-doped layers. A small current entering the base is amplified to produce a large collector and emitter current.
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, bc557 transistor becomes active. In this “on” state, current flows from the collector to the emitter of the transistor.
Most of the current is carried by electrons moving from emitter to collector as minority carriers in the P-type base region. To allow for greater current and faster operation, most bipolar transistors used today are NPN because electron mobility is higher than hole mobility.
A small current leaving the base is amplified in the collector output. That is, a PNP transistor is “on” when its base is sof low relative to the emitter. In a PNP transistor, the emitter—base region is forward biased, so holes are injected into the base as st carriers. The base is very thin, and most of the holes cross the reverse-biased base—collector junction to the collector.
When 32 device is in forward active or forward saturated mode, the arrow, placed on the emitter leg, points in the direction of the conventional current.
It is common in modern 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 barrier arrangement helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias, and thus reduces base current 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.
In the 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, which means the base must be lightly doped to obtain high injection efficiency, making its resistance relatively high. In addition, higher doping in the base can improve figures of merit like the Early voltage by lessening base narrowing. That drift component of transport aids the normal diffusive transport, increasing the frequency response of the transistor by shortening the transit time across the base.
Two commonly used HBTs are silicon—germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. 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:.
In terms of junction biasing: Although 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.
The diagram shows a schematic representation of an NPN transistor connected to two voltage sources.
Katalog Kodów SMD: Kody na cyfrę 3 – Elektronika Wirtualna
The same description applies to a PNP transistor with reversed directions of current flow and applied voltage. This applied voltage causes the lower P-N junction to ‘turn on’, allowing a flow of electrons from the emitter into the base. In active mode, the electric field existing between base and collector caused by V CE will cause the majority of these electrons to cross the upper P-N junction into the collector to form the collector current I C.
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.
BC857 – (SMD SOT-23 Package) – PNP General Purpose Transistor – 5 Pieces Pack
As shown in the diagram, the emitter current, I Eis the total transistor current, which is the sum of the other terminal currents, i. In active mode, the ratio of the collector current to the base current is called the DC current gain. This gain is usually or more, but robust circuit designs do not depend on the exact value for example see op-amp.
Because the base current is approximately proportional to the collector and emitter currents, they vary in the same way.
The junction version known as the bipolar junction transistor BJTinvented by Shockley in was for three decades the device of choice in the design of discrete and integrated circuits. The incidental low performance BJTs inherent in CMOS ICs, however, are soot utilized as bandgap voltage referencesilicon bandgap temperature sensor and to handle electrostatic discharge. The germanium transistor was more common in xot s and s, and while it exhibits a lower “cut-off” voltage, typically around 0.
Various methods of manufacturing bipolar transistors were developed. Transistors can be thought of as two diodes P—N junctions sharing a common region that minority carriers can move through.