There are two basic types of bipolar transistor construction, npn. Mostly npn transistors are used in digital logic families as electrons has less effective mass compared to holes in pnp transistor due to which speed of operation is high in npn transistor compared to pnp transistor. In an npn transistor, the ptype base region is constructed so as to be so narrow that most of the electrons actually pass all of the way through it before this recombination occurs. As a result, currents and voltages have opposite signs when compared to a npn. The former is known as an npn transistor and the latter as a. A transistor consists of two pn junctions is formed by sandwiching either a ptype or ntype semiconductor between a pair of ntypes or ptypes semiconductors respectively. You will able to determine if it is pnp, npn, some other transistor like a mosfet, or a miscellaneous chip. The typical cross section of a planar npn transistor is shown in figure 8. Bjt structure and modes of operation first, lets start with the npn bipolar junction transistor bjt.
Forward biased causes the electron in the ntype emitter to flow toward the base. As long as the transistor is biased in this mode of operation, a. Here the majority charge carriers are the electrons. Most of the power that must be dissipated by the transistor when used as a switch comes from the short time periods between between being fully saturated an fully cutoff. Basic operation transistors 101 adafruit learning system. The principle of operation of the two transistor types. Also notice the base supply battery is quite small, as indicated by the number of cells in the battery, usually 1 volt or less. It has two junctions between ptype and ntype semiconductors.
The operation of these diodes is the subject of this document. Many of these, typically one or two per atom in the metal, are free to move about throughout the metal. The current flow in a npn transistor is through majority charge carriers which are electrons in npn transistor. Npn transistor is one of the bipolar junction transistor bjt types.
The npn transistor consists of two ntype semiconductor materials and they are separated by a thin layer of ptype semiconductor. Conduction in metals metals are filled with electrons. The theory of operation of the npn and pnp transistors will be discussed separately in the next few paragraphs. An npn n egative p ositive n egative type and a pnp positive n egative p ositive type. For correct operation if a transistor, the two pn junctions must be correctly biased with the externally applied dc voltages. Notice that the procedure used earlier to properly bias the npn transistor also applies. The bipolar transistor basic construction consists of two pnjunctions producing three connecting terminals with each terminal being given a name to identify it from the other two.
For the pn transistor to operate, the emitter must be. In short, transistors are the fundamental building block of modern electronic devices basically used for. Bjts have 3 terminals and come in two varieties, as npn and pnp transistors. Operation of the pnp transistor is same as that of a npn transistor, but the change is that the roles of electrons and holes, bias polarities, and current directions are all reversed. They have four distinct modes of operation, which describe the current flowing through them. And, there in the base region these electrons combine with the holes. A bipolar junction transistor is a three layer npn or pnp. Once in the base, the holes combine with base electrons. Extended exposure to stresses above the recommended operating conditions may affect device reliability. The humble transistor q1 emitter e collector c base b transistor basics emitter to base junction is forward biased normally collector to base junction is reverse biased normally transistors are current operated devices, so. Following the procedure above for npn transistor be kvl. Transistor basics emitter to base junction is forward biased normally collector to base junction is reverse biased normally transistors are current operated devices, so kcl should be applied first. The bipolar power transistor is a three layer npn or pnp device within the working range, the collector current ic is a function of the base current ib, a change in the base current giving a corresponding amplified change in the collector current for a given collector emitter voltage vce.
Bjts are currentcontrolled devices which means that a small amount of current flowing through the base of a bipolar junction transistor, results in a large current that flows from emitter to collector. Bjt transistor operation modesactive, saturation, cutoff and reverse active modes of bjt transistor duration. What is a transistor when a third doped element is added to a crystal diode in such a way that two pn junctions are formed, the resulting device is known as a transistor. A pnp transistor operates in a similar manner to a npn bjt, expect that holes instead of electrons from the emitter di. Figure shows the emitterbase junction is forward biased and the collectorbase junction is reversed biased. Difference between npn and pnp transistor elprocus. Electronics tutorial about the bipolar npn transistor, the npn transistor as a. In npn transistor, the direction of movement of an electron is from the emitter to collector region due to which the current constitutes. Pnp, which basically describes the physical arrangement of.
Before the development of modern silicon power rectifier diodes, cuprous oxide and later selenium was used. Bipolar junction transistor npn bjt shown 3 terminals. The power supply polarities are simply reversed for each type. Includes details of bjt device operation which is not covered in this course. The npn transistor amplifies the weak signal enter into the base and produces strong amplify signals at the collector end. Basic npn and pnp transistor operation electronics and. Bipolarjunction bjt transistors a bipolar junction transistor is formed by joining three sections of semiconductors with alternative di.
In typical operation, the baseemitter junction is forward biased and the base collector junction is reverse biased. The ratio of these two currents is of the order of 15 to 100. The transistor in which one ptype material is placed between two ntype materials is known as npn transistor. What is the working principle of an npn transistor. In the pnp transistor the emitter is made from ptype material, the base is ntype material and the collector is ptype material. When an electric field is applied, the electrons move in the direction opposite. F or obvious reasons, the depiction of the transistor in the form of fig. When we talk about current flow through a transistor, we usually mean. The construction of npn transistor is that the collector and emitter regions are doped with ntype material and the base region is doped with small layer of ptype material. The bias batteries in this figure have been labeled v cc for the collector voltage supply, and v bb for the base voltage supply. Transistor npn transistor, pnp transistor definition. All structured data from the file and property namespaces is available under the creative commons cc0 license. Lecture 7 bipolar junction transistors bjts guyeon wei division of engineering and applied sciences.
Npn and pnp transistors function in essentially the same way. Typical hfe values for small signal transistors range from 10 to 500, with maximum ic ratings from about 80 to 600ma. The most commonly used transistor configuration is the npn transistor. Take, for example, a transistor that is listed on a manufacturer s data sheet as. Mostly the common type of transistor used are silicon type because silicon is the most abundant element on the earth after oxygen. Junction field effect transistor jfet the single channel junction fieldeffect transistor jfet is probably the simplest transistor available. However, since the emitter, base, and collector in the pnp transistor are made of materials that are different from those used in the npn transistor, different current carriers flow in the pnp unit.
The major difference between npn and pnp transistor is, an npn transistor gets the power when the flow of current runs through the base terminal of the transistor. The flowing of these electrons from emitter to collector forms the current flow in the. The middle section base is narrow and one of the other two regions emitter is heavily doped. Files are available under licenses specified on their description page. Theory of transistors and other semiconductor devices 1.
These extra holes will combine with the electrons traveling from emitter to collector, reducing the. Each of the three silicon regions has one terminal electrode connected to it, and thus the npn bjt is a three terminal. The pnp transistor works essentially the same as the npn transistor. The central idea to the operation of a transistor is that a little bit of current flowing between the base and emitter causes a larger current to flow between the collector and emitter. Maximum operating frequencies range from about 1 to 300 mhz. This page was last edited on 19 december 2019, at 15. However, since the emitter, base, and collector in the pnp transistor are made of materials that are different from those used in the. A block representation of a layer of ptype material between two layers of ntype is shown in fig. Npn transistor circuit working, characteristics, applications. Usually made of doped silicon or, more rarely, germanium. Pnp, is exactly the same the only difference being in their. Bipolar transistors are current regulating devices that control the amount of current flowing through them in proportion to the amount of biasing voltage applied to their base terminal acting like a currentcontrolled switch. A transistor in saturation mode acts like a short circuit between collector and emitter.
Small signal transistors are transistors that are used to amplify lowlevel signals but can also can be used as a switch. Permission is granted to copy, distribute andor modify this document under the terms of the gnu free documentation license, version 1. Bipolar junction transistors bjt and fieldeffect transistorsfet. Transistor working, construction and symbols analyse a meter.
A properly biased pnp transistor the pnp transistor works essentially the same as the npn transistor. A copy of the license is included in the section entitled gnu free documentation license. As the name implies, the npn bjt is simply an hunk of ptype silicon sandwiched between two slices of ntype material. In npn transistor, the flow of current runs from the collector terminal to the emitter terminal.
Functional operation above the recommended operating conditions is not implied. To understand how a pnp transistor works, simply flip the polarity or and transistor. Pnp transistor and npn transistor are one of the most asked queries by students in this post, we are going to explain transistor working and its operation. Transistors peter mathys ecen 1400 transistor families there are two major families of transistors. Any additional information about the pn junction will be given as the theory of transistor operation is developed. Npn transistors are threeterminal, threelayer devices that can function as either. Transistor operation npn if the collector, emitter, and base of an npn transistor are shorted together as shown in figure 5. Npn transistor tutorial the bipolar npn transistor. Make sure the bjt current equations and region of operation match. Earlier we have covered its basics in the previous post of transistors where i have explained about what it is and its types.
As there are few holes in the base for the electrons to combine with. An npn transistor can be viewed as two pn junction diodes with a very thin shared anode, p layer. The electrons enter into the ptype material and combine with the holes. In the previous tutorial we saw that the standard bipolar transistor or bjt, comes in two basic forms. However, since the emitter, base, and collector in the pnp transistor are made of. It is in this region that the transistor can act as a fairly linear ampli.
Three terminals of transistor are emittere, baseb, and collector c. N stands for ntype material and p stands for ptype material. Bipolar operation 16 page 23 transistor action the term transistor action refers to the control of the large collectoremitter linking current by the smaller base back injection current in forward active operation, the origin of current gain in a bjt two features of. The only major difference between the two types is that the npn transistor has a higher frequency response than does the pnp because. If you want the npn transistor to work as a switch, then the base is. Bipolar junction transistor operationtypesapplications.
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