Physics
NPN and PNP Transistor
NPN and PNP transistors are semiconductor devices used in electronic circuits. In an NPN transistor, a positively charged layer is sandwiched between two negatively charged layers, while in a PNP transistor, a negatively charged layer is sandwiched between two positively charged layers. These transistors are fundamental components in amplifiers, switches, and other electronic devices.
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12 Key excerpts on "NPN and PNP Transistor"
eBook - PDF- J. P. Vasseur(Author)
- 2013(Publication Date)
- Pergamon(Publisher)
PHYSICAL PRINCIPLES OF THE TRANSISTOR 51 D. pnpn transistor [8] A pnpn (or npnp) transistor is made up of four alternate layers of which the two intermediate layers are thin in comparison with the diffusion length. The system can be thought of as a combination of a pnp tran-sistor with an npn transistor. In one possible application of this (Fig. 1.40), the outer/7-layer becomes the emitter, the intermediate «-layer becomes the base and the /?-and 6 p r + , ^ t V 1 U_ — r ά ίΛ i Λ i P n — — | · U I I I e <7,1 b 1 -J_+ τ + Τ b ' J ► ■ a a&a 2 - -T-1-a. FIG. 1.40. pnpn transistor and equivalent circuit using a pnp and an npn transistor. P n P + I 1 I 1 -I I I n o i> lh FIG. 1.41. A pnpn transistor used as a two-terminal element is equivalent to a bistable multivibrator made up of a. pnp and an npn transistor. «-layers which follow become the collector. This type ofcollector is known as a/?« hook. The relative values of the currents can be obtained by using the equivalent circuit of a, pnp and an npn transistor. The essential result is that the collector current is greater than, and in the same sense as, the emitter current, and that the base current is greater than that in normal transistors, and in the opposite sense. These characteristics are typical of point-contact transistors which probably have a structure analogous to the pnpn structure. One application envisaged for pnpn transistors is a two-terminal device in which only the two outer layers have connexions (Fig. 1.41). The device can thus be made by diffusion onto small plates of silicon. 52 PROPERTIES AND APPLICATIONS OF TRANSISTORS In the equivalent circuit using two transistors, one pnp and one npn, it can be seen that thepnpn structure is equivalent to a bistable multivibrator with complementary symmetry. Each of the two equivalent transistors has its collector connected to the base of the other, as in a normal multivibra-tor.
eBook - PDFElectricity and Electronics for Renewable Energy Technology
An Introduction
- Ahmad Hemami(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
511 Transistor OBJECTIVES: After studying this chapter, you will be able to • Explain what a transistor is and how it works • Describe the structure of a transistor • Define PNP and NPN transistors • Recognize various transistors of different physical shapes • Name the three elements of a transistor • Explain the terms, common base, common collector, and com-mon emitter • Explain the two primary functions of a transistor • Define what biasing a transistor is • Describe bias conditions for a transistor to work • Define cutoff state of a transistor • Explain how a transistor can be used for switching • Explain how a transistor can be used for amplification • Understand the importance of correct biasing • Describe heat sink 17.1 Introduction A transistor is a semiconductor device with many applications. It is, in fact, the mother of all the other semiconductor electronic devices that we use or see around us today. Before transistors were invented, all elec-tronic devices were made with vacuum tubes. Still one can find devices such as stereo amplifiers and radio receivers that use vacuum tubes, but they are rare and expensive, in addition to being bulky and heavy in comparison to solid-state electronics that use transistors and semicon-ductor technology. There are various types of transistors, and we will study their fundamentals in this chapter. The simplest form of tran-sistor is a junction transistor , also called bipolar junction transis-tor . The reason for calling it junction transistor is because it contains two PN junctions. In other words, from the structure viewpoint it is similar to two diodes put together during the manufacturing process. Nonetheless, note that a transistor is a completely different device than a diode, meaning that one cannot put two diodes together and expect to have a transistor.
eBook - PDFPrinciples of Transistor Circuits
Introduction to the Design of Amplifiers, Receivers and Digital Circuits
- S W Amos(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
CHAPTER 2 Basic Principles of Transistors BIPOLAR TRANSISTORS Introduction Chapter 1 showed that a junction between n-type and p-type materials has asymmetrical conducting properties enabling it to be used for rectification. A bipolar transistor includes two such junctions arranged as shown in Fig. 2.1. Fig. 2.1(a) illustrates one basic type consisting of a layer of n-type material sandwiched between two Π-ΤΥΡΕ MATERIAL p-TYPE MATERIAL p-TYPE MATERIAL n-TYPE MATERIAL Fig. 2.1. Theoretical diagrams illustrating the structure of (a) a pnp and (b) an npn transistor layers of p-type material: such a transistor is referred to as a pnp-type. A second type, illustrated in Fig. 2.1(b) has a layer of p-type material sandwiched between two layers of n-type semiconducting material: such a transistor is referred to as an npn-type. In both types, for successful operation, the central layer must be thin. However, it is not possible to construct bipolar transistors by placing suitably treated layers of semiconducting material in con-22 Basic Principles of Transistors 23 tact. One method which is employed is to start with a single crystal of, say, n-type germanium and to treat it so as to produce regions of p-type conductivity on either side of the remaining region of n-type conductivity. Electrical connections are made to each of the three different regions as suggested in Fig. 2.2. The thin central layer is known as EMITTER --COLLECTOR BASE Fig. 2.2. Electrical connections to a bipolar transistor the base of the transistor, one of the remaining two layers is known as the emitter and the remaining (third) layer is known as the collector. The transistor may be symmetrical and either of the outer layers may then be used as emitter: the operating conditions determine which of the outer layers behaves as emitter, because in normal operation the emitter-base junction is forward-biased whilst the base-collector junction is reverse-biased.
- Dennis L. Eggleston(Author)
- 2011(Publication Date)
- Cambridge University Press(Publisher)
The circuit symbols for these transistors are shown in Fig. 4.1 . We will focus here on the npn transistor; the development for a pnp transistor is similar except the polarities and current directions are reversed. 4.2 Bipolar transistor fundamentals 105 Base Collector Emitter Base Collector Emitter Figure 4.1 Circuit symbols for the npn (left) and pnp (right) transistors. n p n Conduction Band Forbidden Band Valance Band Figure 4.2 An unbiased npn transistor. emitter base collector f e I e f c I c f b I b Figure 4.3 An npn transistor biased for linear active operation. The band structure for our npn sandwich is shown in Fig. 4.2 . The same shift in energy levels charateristic of the p-n diode is seen here. Although the transistor looks symmetric, it is not: as we will see below, the two n-materials are doped differently. We now apply external voltages between the junctions. Although there are various ways to do this, a useful and common situation is shown in Fig. 4.3 . As indicated, the leftmost p-n junction is forward biased and the rightmost junction is strongly reverse biased. The three parts of the transistor are labeled as shown: emitter, base, and collector. The operation of the transistor when biased this way can be described as follows. 106 Bipolar junction transistors 1. Because the emitter-base junction is forward biased, electrons will have a net flow from the emitter to the base. To facilitate this, the emitter is heavily doped so that it has many electrons in the conduction band. 2. Since the collector-base junction is reverse biased, there is very little flow of electrons into the base from the collector. 3. Electrons from the emitter suffer one of two fates. (a) They recombine with a hole in the p-type base. In this case, an electron must flow out of the base electrode to maintain charge neutrality. If this flow out of the base is interrupted, the base will quickly charge up negatively and stop the flow of electrons from the emitter.
eBook - PDFPrinciples of Transistor Circuits
Introduction to the Design of Amplifiers, Receivers and Digital Circuits
- S W Amos(Author)
- 2013(Publication Date)
- Butterworth-Heinemann(Publisher)
C H A P T E R 2 Basic Principles of Transistors B I P O L A R T R A N S I S T O R S Introduction Chapter 1 showed that a junction between η-type and p-type materials has asymmetrical conducting properties enabling it to be used for rectification. A bipolar transistor includes two such junctions arranged as shown in Fig. 2.1. Fig. 2.1(a) illustrates one basic type consisting of a layer of η-type material sandwiched between two Fig. 2.1. Theoretical diagrams illustrating the structure of (a) a pnp and (b) an npn transistor layers of p-type material : such a transistor is referred to as a pnp-type. A second type, illustrated in Fig. 2.1(b) has a layer of p-type material sandwiched between two layers of η-type semiconducting material: such a transistor is referred to as an npn-type. In both types, for successful operation, the central layer must be thin. However, it is not possible to construct bipolar transistors by placing suitably-treated layers of semiconducting material in con-20 Basic Principles of Transistors 21 Fig. 2.2. Electrical connections to a bipolar transistor the base of the transistor and corresponds with the control grid of a triode valve. One of the remaining two layers is known as the emitter and corresponds with the cathode of a triode valve. The remaining (third) layer is known as the collector: it corresponds with the anode of the triode. The transistor may be symmetrical and either of the outer layers may then be used as emitter : the operating conditions determine which of the outer layers behaves as emitter, because in normal operation the emitter-base junction is forward-biased whilst the base-collector junction is reverse-biased. In practice most bipolar transistors are unsymmetrical with the collector junction larger than the emitter junction and it is essential to adhere to the emitter and collector connections prescribed by the manufacturer. The symbols used for bipolar transistors in circuit diagrams are given in Fig.
eBook - PDF- J Engelbrecht(Author)
- 2014(Publication Date)
- Future Managers(Publisher)
A wire lead is connected to each of the three regions to enable external connection. The three leads are labelled b, e and c respectively, for base, emitter and collector. The base region is lightly doped and very thin compared to heavily doped emitter and the moderately doped collector regions. Figure 5.1(a) (b) B B base-collector junction base-emitter junction E E P N N P N P C C N3 Industrial Electronics| Hands-On! 97 5.3 Basic operation In order for a BJT to operate properly as an amplifier or a switch, the two P-N junctions must be correctly biased with DC voltages. In this section, the NPN-transistor will mainly be discussed. The operation of the PNP-transistor is the same as that of the NPN transistor, except that the biasing voltages, majority and minority current carriers and directions of transistor currents are opposite. Figure 5.2 shows the bias arrangement for both PNP and NPN BJTs. Note that in both cases the base-emitter junction is forward biased and the base-collector junction is reverse biased. Figure 5.2 (a) NPN (b) PNP To understand how a transistor operates, examine what happens inside the NPN structure. The heavily doped N-type emitter region has a very high density of conduction-band free electrons. Under the correct bias conditions, these electrons easily diffuse through the forward biased b-e junction into the lightly doped and very thin P-type region. The base has a low density of holes, which are the majority carriers. A small percentage of the free electrons injected into the base region combine with holes and move as valence electrons through the base region as base-current, (I B ), back to the emitter. Most of the free electrons that have entered the base do not combine with holes because the base region is very thin and lightly doped.
eBook - PDFElectronic Concepts
An Introduction
- Jerrold H. Krenz(Author)
- 2000(Publication Date)
- Cambridge University Press(Publisher)
3.6 THE PNP TRANSISTOR: A COMPLEMENTARY DEVICE Up to this point, only NPN bipolar junction transistors, devices with a p-type base sandwiched between an n-type emitter and collector region, have been 190 THE BIPOLAR JUNCTION TRANSISTOR -W-. emitter k base 1 collector + Figure 3.86: A PNP transistor in a common-base circuit. considered. An interchange of the semicon-ductor doping, as indicated in Figure 3.86, results in a PNP transistor, a complementary device with particularly useful circuit prop-erties. Circuits that simultaneously use NPN and PNP Transistors can often accomplish tasks that are not possible with circuits us-ing only NPN or PNP transistors. To forward bias the emitter-base junction of a PNP transistor, a positive emitter-base potential VEB is required. For this biasing, holes of the emitter cross the emitter-base junction and diffuse across the thin base region of the transistor. These holes readily cross a reverse-biased collector-base junction (VCB < 0). For normal op-eration, it is primarily holes originating in the emitter region that account for the behavior of a PNP transistor. The roles of the holes and free electrons in a PNP transistor are reversed from their roles in an NPN transistor (Figure 3.87). For normal operation of a PNP transistor, the emitter current i% is positive, whereas the base and collector currents are negative. Furthermore, all voltages have opposite polarities. Never-theless, the same relationship applies between the collector and emitter current as for an NPN transistor: Rr i c = -a F i E (3.70) The common-base current gain ap remains a positive quantity. As a result of the interchange of the n-and p-type regions, the polarity of the diodes of the equivalent circuit of a PNP transistor are reversed from those of the equivalent circuit of an NPN transistor (Figure 3.88).
eBook - PDFPrinciples of Transistor Circuits
Introduction to the Design of Amplifiers, Receivers and Digital Circuits
- S W Amos, Mike James(Authors)
- 2013(Publication Date)
- Newnes(Publisher)
CHAPTER 2 Basic Principles of Transistors BIPOLAR TRANSISTORS Introduction Chapter 1 showed that ajunction between η-type and p-type materials has asymmetrical conducting properties enabling it to be used for rectification. A bipolar transistor includes two such junctions arranged as shown in Fig. 2.1. Fig. 2.1(a) illustrates one basic type consisting of a layer of η-type material sandwiched between two -MATERIAL -p -T Y P E MATERIAL p -T Y P E MATERIAL^ - η -TYPE MATERIAL u) (b) Fig. 2.1. Theoretical diagrams illustrating the structure of (a) a pnp and (b) an npn transistor layers of p-type material: such a transistor is referred to as a pnp-type. A second type, illustrated in Fig. 2.1(b), has a layer of p-type material sandwiched between two layers of η-type semiconducting material : such a transistor is referred to as an npn-type. In both types, for successful operation, the central layer must be thin. However, it is not possible to construct bipolar transistors by placing suitably treated layers of semiconducting material in con-23 24 Principles of Transistor Circuits tact. One method which is employed is to start with a single crystal of, say, η-type germanium and to treat it so as to produce regions of p-type conductivity on either side of the remaining region of η-type conductivity. Electrical connections are made to each of the three different regions as suggested in Fig. 2.2. The thin central layer is known as EMITTER · • COLLECTOR BASE Fig. 2.2. Electrical connections to a bipolar transistor the base of the transistor, one of the remaining two layers is known as the emitter and the remaining (third) layer is known as the collector. The transistor may be symmetrical and either of the outer layers may then be used as emitter: the operating conditions determine which of the outer layers behaves as emitter, because in normal operation the emitter-base junction is forward-biased whilst the base-collector junction is reverse-biased.
eBook - PDF- Ian Hickman(Author)
- 1999(Publication Date)
- Newnes(Publisher)
Returning for the moment to small-signal am-pli®ers, Figure 3.3c, d and e shows the three possible con®gurations of a single-transistor am-pli®er and indicates the salient performance feat-ures of each. Since the majority of applications nowadays tend to use NPN devices, this type has been illustrated. Most early transistors were PNP types; these required a radical readjustment of the thought processes of electronic engineers brought up on valve circuits, since with PNP transistors the `supply rail' was negative with respect to ground. The confusion was greatest in switching (logic) circuits, where one was used to the anode of a cut-o valve rising to the (positive) HT rail, this Active components 57 Figure 3.5 (a) Darlington connected discrete transistors. (b) Typical monolithic NPN Darlington power transistor (reproduced by courtesy of Philips Components Ltd). being usually the logic 1 state. Almost overnight, engineers had to get used to collectors ¯ying up to 6 V when cut o, and vice versa. Then NPN devices became more and more readily available, and eventually came to predominate. Thus the modern circuit engineer has the great advantage of being able to employ either NPN or PNP devices in a circuit, whichever proves most con-venient ± and not infrequently both types are used together. The modern valve circuit engineer, by contrast, still has to make do without a thermionic equivalent of the PNP transistor. A constant grumble of the circuit designer was for many years that the current gain h FE of power transistors, especially at high currents, was too low. The transistor manufacturers' answer to this complaint was the Darlington , which is now avail-able in a wide variety of case styles and voltage (and current) ratings in both NPN and PNP versions. The circuit designer had already for years been using the emitter current of one tran-sistor to supply the base current of another, as in Figure 3.5a.
- ROBIN PAIN(Author)
- 1996(Publication Date)
- Newnes(Publisher)
PNP transistors are the complement of this. A PNP transitor will only operate when the emitter voltage is more positive (or 'above') both the collector and the base. The b-e junction is still forward biased of course and the b-c is still reversed (Fig. 4.26). Figure 4.26 This silicon PNP transistor is correctly biased with roughly 0.7 V across e-b and with the c-b reverse biased. 67 Diodes and transistors As a memory aid, for the time being, concentrate on NPN types only and forget about PNP. Then when the occasional PNP example crops up, recall the NPN rules and reverse them all. The prevalence of NPN technology has encouraged the conven-tion of negative earth/positive supply rail. In the early days of transistors, when germanium PNP technology prevailed, the convention was positive ground/negative supply rail. In the emitter-follower the output is asymmetrical. When the transistor turns fully on (output goes high), a large current can flow through it from ground to the rail. When the transistor turns off (output low) the maximum current flow from ground to the rail is limited by the emitter resistor (Fig. 4.27). 9V load Figure 4.27 When the output is driven high, the maximum available load current is limited by the transistor. Figure 4.28 When the output is driven low, the maximum available load current is limited by the emitter resistor. As the load demands more and more current, so the output voltage will be 'dragged' upwards by the decreasing impedance of the load. 68 Diodes and transistors In other words the follower can 'pull up' much better than it can 'pull down' because the 'on resistance' between emitter and collector is always much lower than the emitter resistor. If this were not so, if the emitter resistor were chosen to equal the c-e on resistance, then when the transistor was switched on, it would not be able to pull up its own emitter load (more than half way), let alone an external one as well.
eBook - PDF- Behzad Razavi(Author)
- 2021(Publication Date)
- Wiley(Publisher)
Fortunately, as we develop our understanding of the transistor’s operation, we discard some of these current and voltage combinations as irrelevant, thus obtaining a relatively simple model. 4.2 STRUCTURE OF BIPOLAR TRANSISTOR The bipolar transistor consists of three doped regions forming a sandwich. Shown in Fig. 4.4(a) is an example comprising of a p layer sandwiched between two n regions and called an “npn” transistor. The three terminals are called the “base,” the “emitter,” and the “collector.” As explained later, the emitter “emits” charge carriers and the collector “collects” them while the base controls the number of carriers that make this journey. The circuit symbol for the npn transistor is shown in Fig. 4.4(b). We denote the terminal voltages by V E , V B , and V C , and the voltage differences by V BE , V CB , and V CE . The transistor is labeled Q 1 here. 4.3 Operation of Bipolar Transistor in Active Mode 127 n p n Base Collector Emitter Base Collector Emitter (B) (C) (E) V V BE V CE Q 1 (a) (b) CB Figure 4.4 (a) Structure and (b) circuit symbol of bipolar transistor. We readily note from Fig. 4.4(a) that the device contains two pn junction diodes: one between the base and the emitter and another between the base and the collector. For example, if the base is more positive than the emitter, V BE > 0, then this junction is forward-biased. While this simple diagram may suggest that the device is symmetric with respect to the emitter and the collector, in reality, the dimensions and doping levels of these two regions are quite different. In other words, E and C cannot be interchanged. We will also see that proper operation requires a thin base region, e.g., about 100 Å in modern integrated bipolar transistors. As mentioned in the previous section, the possible combinations of voltages and cur- rents for a three-terminal device can prove overwhelming.
eBook - PDFPhysics of Electronic Materials
Principles and Applications
- Jørgen Rammer(Author)
- 2017(Publication Date)
- Cambridge University Press(Publisher)
That the amplification is linear is important in order for an amplified signal not to be distorted. We have only accounted for the main currents, and many other processes, such as additional current decreases due to recombination in depletion layers, etc., may lead to small correc-tions. However, the basic transistor action is explained: the current in one part of the device (the collector current I c ) is controlled by a voltage ( V be or equivalently the current I b ) in another part of the device. The electric circuit symbol for an n–p–n transistor is depicted in Figure 11.19 . An equivalent transistor can be built by swapping the n-and p-type regions, giving a p– n–p transistor. The circuit symbol for the p–n–p transistor has just the arrows reversed, indicating that the injector current is carried by holes. The situation discussed where the base current is amplified is referred to as the common emitter configuration and is depicted as in Figure 11.20 . The bipolar transistor has three modes of operation. (i) In the active mode , which we have just discussed, the emitter–base junction is forward biased and the collector–base junction is reverse biased. (The inverse active mode, where forward and reverse biasing of the junctions are interchanged, is similar but asymmetric due to the asymmetry in doping.) Figure 11.19 Circuit symbol for a bipolar n–p–n transistor. 263 11.6 Field Effect Transistor I in = I b I out = I c Figure 11.20 Common emitter configuration. I b I b = 0 V ce I c Figure 11.21 The I c – V ce characteristic. (ii) In the cut-off mode , both junctions are reverse biased and no current flows through the transistor. The transistor is in the OFF state. (iii) In the saturation mode , both junctions are forward biased and current flows through the transistor with the tiniest voltage across it.
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