Physics
JFET
A JFET, or Junction Field-Effect Transistor, is a semiconductor device used for amplifying or switching electronic signals. It operates by controlling the current flow between two terminals using an electric field. JFETs are known for their high input impedance and low noise, making them suitable for various applications in electronics.
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10 Key excerpts on "JFET"
eBook - ePub- Dale R. Patrick, Stephen W. Fardo, Ray E. Richardson, Vigyan (Vigs) Chandra(Authors)
- 2023(Publication Date)
- River Publishers(Publisher)
metal-oxide semiconductor field-effect transistor (MOSFET). Both classifications of the FET play an important role in solid-state electronics and are studied in this chapter.Objectives
After studying this chapter, you will be able to:- 8.1 analyze the operation of a junction field-effect transistor;
- 8.2 analyze the operation of E-MOSFET, D-MOSFET, and V-MOSFET transistors;
- 8.3 analyze and troubleshoot field-effect transistors.
Chapter Outline
- 8.1 Junction Field-Effect Transistors
- 8.2 Metal-Oxide Semiconductor FETs
- 8.3 Analysis and Troubleshooting – Field-Effect Transistors
Key Terms
channel depletion metal-oxide semiconductor field-effect transistor (D-MOSFET) drain dynamic transfer curve enhancement metal-oxide semiconductor field-effect transistor (E-MOSFET) gate junction field-effect transistor (JFET) ohmic region pinch-off region source substrate transconductance vertical metal-oxide semiconductor field-effect transistor (V-MOSFET)8.1 Junction Field-Effect Transistors
The junction field-effect transistor (JFET) is a three-element electronic device. Its operation is based on the conduction of current carriers passing through a single piece of semiconductor material instead of a junction. The two major types of JFETs are N-channel and P-channel. JFET circuit operation can be predicted with a family of characteristic curves chart and a dynamic transfer curve. In this section, you will learn about the characteristics and operation of N-channel and P-channel JFETs. You will use a family of characteristic curves and a dynamic transfer curve to predict the operation of a JFET and will learn how to read and interpret a JFET data specifications sheet.
eBook - PDFBasic Electronics
Principles and Applications
- Chinmoy Saha, Arindam Halder, Debarati Ganguly(Authors)
- 2018(Publication Date)
- Cambridge University Press(Publisher)
Chapter 7 Field-Effect Transistors 7.1 INTRODUCTION The field-effect transistor, customarily known as FET, like bipolar junction transistor (BJT), is a three-terminal semiconductor device extensively used in numerous electronic circuits and systems ranging from digital logic circuits to amplifier design. The term ‘ field effect ’ arises due to the fact that applied external electric field creates a conduction path called channel and controls the output current of the device. Even though FET has various structural and operational similarities with BJT, they have some fundamental differences, which are as follows: In BJT, the current is contributed by two types of carriers: electrons and holes (hence the term ‘ bi ’, meaning two, which justifies its name bipolar ). In case of FET, based on its structure, only one type of carrier, i.e. either electron or hole, contributes to the current. Hence, it is known as a unipolar device. Due to this, FETs do not suffer from minority carrier storage effects and consequently have higher switching speeds and higher cut-off frequencies. BJT is a current-control device, where output current I C is solely controlled by the input current I B with a relation I C ≈ βI B . On the other hand, FET is a voltage-controlled device, where the voltage between two terminals V GS controls the output current I D . This is indicated in the schematic block diagram of Figure 7.1. Thus, FET is termed as a voltage-controlled device. 408 Basic Electronics One of the most important features of FET is its very high input impedance, typically much higher than that of BJT. This makes FETs a very good choice for designing linear amplifiers. Another advantageous feature of FETs is their better temperature stability compared to BJT. At high current level, FET offers negative temperature coefficient of current (meaning that current decreases due to increase in temperature). Due to this phenomenon, FETs can prevent thermal runaway.
eBook - PDF- B. Hart(Author)
- 1996(Publication Date)
- Butterworth-Heinemann(Publisher)
5 Field effect transistors (FETs): d.c.-characterization and biasing The FET is the second type of semiconductor control device that has helped to catapult the subject of electronics from a subsidiary to a central role in engineering and scientific disciplines. It operates on an entirely different principle from that of the BJT, leading to some associated circuit properties, e.g. very high input impedance, than can be exploited by the equipment design engineer. In a BJT the flow of minority carriers, across a base region from emitter to collector is dependent upon potential differences (V BE, V CB) that act in the direction of carrier flow. The FET, by contrast, is a semiconductor device in which the flow of majority carriers in a 'channel' between two terminals designated source (8) and drain (D) is dependent upon a transverse field resulting from the application of a potential difference between one of these terminals (normally, S) and a third, control, terminal known as the gate (G). A crude everyday analogy is a garden hose pipe, in which the flow of water can be controlled by a squeezing action in a direction perpendicular to the direction of flow. The particular way in which the electrical control function is achieved, without the requirement of significant d.c. input power, permits a division of FETs into two basic categories each of which can be further sub-divided. This chapter considers the classification of FETs, discusses the characteristics of one popular type in some detail and shows how it can be biased for use in d.c. and in low-frequency small-signal amplifier applications. 5.1 THE FET FAMILY The junction gate FET (Jugfet, JFET) On each side of a PN junction there is a narrow layer devoid of charge carriers The P-side layer has a net negative charge resulting from fixed negatively-charged acceptor impurity atoms that have supplied holes which are elsewhere in the crystal lattice.
eBook - ePubFundamentals of Silicon Carbide Technology
Growth, Characterization, Devices and Applications
- Tsunenobu Kimoto, James A. Cooper(Authors)
- 2014(Publication Date)
- Wiley-IEEE Press(Publisher)
Chapter 8 Unipolar Power Switching Devices 8.1 Junction Field-Effect Transistors (JFETs) We turn now to the first of the three-terminal power devices, the junction field-effect transistor, or JFET. As discussed in Section 7.1, three-terminal power switching devices emulate ideal switches, that is, they attempt to carry high current with minimal voltage drop in the on state and to block high voltages with minimal leakage current in the off state. The basic operation of the JFET can be understood by reference to the schematic cross-section in Figure 8.1. The prototype n-channel JFET consists of two gate regions on either side of an n-type channel region. Each end of the channel is connected to an ohmic contact, designated the source and the drain. A depletion region exists at each gate-channel junction, and the width of this depletion region increases as the square root of the gate-to-channel voltage. Assuming a one-sided step junction, the depletion width can be written 8.1 where is the voltage difference (or Fermi level splitting) between the gate and the channel, is the channel doping, and is the built-in potential (or band bending) of the gate-to-channel junction, given by 8.2 where and are the ionized doping concentrations of the gate and the n-type channel, respectively (see Appendix A for a discussion of incomplete ionization in 4H-SiC. Note that in Equation 8.1 is the total dopant concentration in the channel.) If a non-zero drain voltage is applied and the channel is not pinched-off by the depletion regions from the gate, an electron current will flow from the grounded source to the positive drain, and a voltage drop will exist along the channel from source to drain. Let us designate the voltage at point in the channel as. In this case, and in Equation 8.1 become functions of, and can be written 8.3 Figure 8.1 Schematic cross-section of a basic JFET. The width in the direction into the paper is
eBook - PDFFundamentals of Electronics
Book 1 Electronic Devices and Circuit Applications
- Thomas F. Schubert, Ernest M. Kim(Authors)
- 2022(Publication Date)
- Springer(Publisher)
229 C H A P T E R 4 Field Effect Transistor Characteristics In Chapter 3 Bipolar Junction Transistors were shown to be semiconductor devices that operate on carrier flow from the emitter to the base and then through to the collector. For example, npn BJTs are devices where the current flow from the collector to the emitter is regulated by the current injected into the base. erefore, the BJT is a current controlled three-terminal semiconductor device. Field Effect Transistors (FETs) are semiconductor devices that employ a channel between the drain and the source to transport carriers. An adjacent controlling surface, called the gate, regulates the current flow through the drain-source channel. is channel is controlled by a volt- age applied to the gate of the FET. erefore, the FET can be described as a voltage controlled three-terminal semiconductor device (see Figure 4.1). e physical properties of FETs make them suitable for amplification, switching, and other electronic applications. '&5 %SBJO % (BUF ( 4PVSDF 4 $VSSFOU I Figure 4.1: FET represented as a three terminal device. e terminal characteristics of Junction Field Effect Transistors ( JFETs) and Metal-Oxide- Semiconductor FETs (MOSFETs) are described in this chapter. Other types of FETs exist, but JFETs and MOSFET are the predominate FET types.¹ MOSFETs are used extensively in inte- grated circuits for digital applications: JFETs are most commonly found in analog applications. ¹Other FET types include the Metal-Semiconductor FET (MESFET), Modulation-doped FET (MODFET), and Vertical MOSFET (VMOSFET). Analysis of JFETs and MOSFETs will allow for a general understanding of FET behavior that can be used with other FET devices. 230 4. FIELD EFFECT TRANSISTOR CHARACTERISTICS Terminal characterization of FETs is sufficient for electronic analysis and design: for discussions on the device physics of FETs, the reader is referred to the references.
- Earl Boysen, Harry Kybett(Authors)
- 2012(Publication Date)
- Wiley(Publisher)
Note The output voltage in this problem is half of the supply voltage. This condition is important in AC electronics and is covered in Chapter 8.The Junction Field Effect Transistor (JFET)
28 Up to now, the only transistor described has been the BJT. Another common transistor type is the JFET. Like the BJT, the JFET is used in many switching and amplification applications. The JFET is preferred when a high input impedance circuit is needed. The BJT has a relatively low input impedance as compared to the JFET. Like the BJT, the JFET is a three-terminal device. The terminals are called the source, drain, and gate. They are similar in function to the emitter, collector, and base, respectively.QuestionsA. How many terminals does a JFET have, and what are these terminals called? _____B. Which terminal has a function similar to the base of a BJT? _____AnswersA. Three, called the source, drain, and gate.B. The gate has a control function similar to that of the base of a BJT.29 The basic design of a JFET consists of one type of semiconductor material with a channel made of the opposite type of semiconductor material running through it. If the channel is N material, it is called an N-channel JFET; if it is P material, it is called a P-channel.Figure 3.28 shows the basic layout of N and P materials, along with their circuit symbols. Voltage on the gate controls the current flow through the drain and source by controlling the effective width of the channel, allowing more or less current to flow. Thus, the voltage on the gate acts to control the drain current, just as the voltage on the base of a BJT acts to control the collector current.QuestionsA. Which JFET would use electrons as the primary charge carrier for the drain current? _____B. What effect does changing the voltage on the gate have on the operation of the JFET? _____Figure 3.28AnswersA. N-channel because N material uses electrons as the majority carrier.B.
eBook - ePubFundamentals of Electronics 1
Electronic Components and Elementary Functions
- Pierre Muret(Author)
- 2017(Publication Date)
- Wiley-ISTE(Publisher)
3 Field Effect Transistors and Applications3.1. Operating principle of junction field effect transistors (JFET and MESFET types)
The channel (n-type) represented below (Figure 3.1 ) by the non shaded zone has a width depending on abscissa x and voltages VGS and VDS and is traversed by drain current ID , as shown in Figure 3.1 .Figure 3.1 . Representation of symmetrical junction field effect transistor; channel open at top and pinched-off at bottomHere, the structure is taken as symmetrical in order not to account for the influence of the substrate; however, the results are, nonetheless, applicable for real, asymmetrical components built on a semi-isolating substrate. The width of the depletion zones is calculated for the pn diode (Chapter 1 , section 1.4.3 ), while accounting for the difference in potential between the gate and the channel, that is VGS – V(x); with V(x) = 0 at the source and V(x) = VDS > 0 at the drain. The gate is reverse biased by VGS < 0 and NA >> ND , so that: , where Φ is the diffusion potential or built-in potential of the gate–channel junction (Chapter 1 , section 1.4.1 ).In the following, drain current will be calculated in half the height (dimension a) of the structure drawn in Figure 3.1 . The electric field Ex (x) that is predominant in the channel determines conduction current ID in two different ways according to whether the channel is pinched-off or not:- 1) In a variable resistance regime, occurring in an open channel, the channel’s resistance is dependent on its fixed width Z and its height h(x) = a – W(x) and we can calculate the conduction current by . If we differentiate equation , we can obtain dV according to dh that is carried subsequently into the expression of ID dx. By integrating both members with respect to x from 0 to L and with respect to h from h(0) to h(L), respectively, in cases where h(L) remains non-zero (non-pinched-off channel), we have:
eBook - PDFCurrent Sources and Voltage References
A Design Reference for Electronics Engineers
- Linden T. Harrison(Author)
- 2005(Publication Date)
- Newnes(Publisher)
To control current passing through any type of FET, one uses a voltage at its gate, whereas with the bipolar transistor a combination of base voltage and current is used. The N-channel JFET is a normally-on device, until the gate-to-source voltage (V GS ) becomes negative enough to reverse-bias the gate channel diode, and so turn it off. The point where this occurs is known as V GS (off) , which we will look at in more detail shortly. The P-channel JFET is also a normally-on device, until the gate voltage becomes positive enough to reverse-bias the gate chan-nel diode, and thereby turn it off. In contrast, the bipolar transistor (both NPN and PNP) is a normally-off device until sufficient voltage (V BE ) and base current (I B ) are provided to turn it on. N- and P-channel JFETs work only in the depletion mode (as shown in Figure 6.3A and D). Other types of FETs can operate in either the depletion mode or the enhance-ment mode, which we will cover in a later chapter. The depletion mode operates by a gate voltage depleting or shutting off the majority current carriers in the channel. It Figure 6.3. Showing the structure of the JFET, as well as its various voltages, currents, and polarities. Notice the two-channel diodes that provide forward and reverse bias. 142 Characteristics of the JFET does this by changing the size of the depletion region within the junction area, thus increasing the resistance and reducing the current flow. It requires virtually no gate current to function. The cross-sectional area (L × W) of the JFET’s channel is fixed by the device geometry. However, the thickness and position of the channel is controlled by a combination of the gate-to-source voltage (V GS ) and the drain-to-source voltage (V DS ). These effectively change the resistance in the channel, allowing full, partial, or no conduction. Because there is virtually no gate current, the JFET has an extremely high input impedance (Z in ), typically more than 100 M Ω (more than 1 × 10 8 ).
eBook - PDF- Lloyd Dingle, Michael H Tooley(Authors)
- 2013(Publication Date)
- Routledge(Publisher)
The gate connection of an insulated gate field effect transistor (IGFET), on the other hand, is insulated from the channel and charge is capacitively coupled to the channel. To keep things simple, we will consider only JFET devices. Figure 6.58 shows the basic construction of an N-channel JFET. FETs comprise a channel of P- or N-type material surrounded by material of the opposite polarity.The ends of the channel (in which conduction takes place) form electrodes known as the source and drain. The effective width of the channel (in which conduction takes place) is controlled by a charge placed on the third (gate) electrode. The effective resistance between the source and drain is thus determined by the voltage present at the gate. JFETs offer a very much higher input resistance when compared with bipolar transistors. For example, the input resistance of a bipolar transistor operating in common-emitter mode is usually around 2.5 k. A JFET transistor operating in equivalent common-source mode would typically exhibit an input resistance of 100 M!This feature makes JFET devices ideal for use in applications where a very high input resistance is desirable. 6.58 Construction of an N-channel JFET As with bipolar transistors, the characteristics of an FET are often presented in the form of a set of graphs relating voltage and current present at the transistor’s terminals. KEY POINT The three connections on a JFET are referred to as the gate, source and drain. Inside a JFET there is a resistive connection between the source and drain and a normally reverse-biased junction between the gate and source. KEY POINT In a JFET, the effective resistance between the source and drain is determined by the voltage that appears between the gate and source. FET characteristics A typical mutual characteristic (I D plotted against V GS ) for a small-signal general-purpose N-channel FET operating in common-source mode is shown in Figure 6.59.
eBook - PDFA Guide to Noise in Microwave Circuits
Devices, Circuits and Measurement
- Peter Heymann, Matthias Rudolph(Authors)
- 2021(Publication Date)
- Wiley-IEEE Press(Publisher)
201 11 Field Effect Transistors JFET It is often found in the literature that the channel of the JFET generates thermal noise [1, 2]. This drain noise source is usually described without comment with i 2 D = 2 3 4 kTg m B (11.1) The factor 2 ∕ 3 and the occurrence of the transconductance g m instead of a “real” conductance is based on a complicated approach with a relatively simple solution from van der Ziel. We want to present it here in simplified form. In doing so, we will abstain from an exact mathematical analysis in favor of understanding. But first we will start with the current–voltage characteristic of the FET. Mode of Operation of the FET The drain current in the channel is driven by the drain-source voltage V D and controlled by the gate-source voltage V G . Along the channel, below the gate, the voltage V ( x ) depends on the position x , since it increases from 0 (directly at the source contact) to V D (at the drain contact). The space charge layer under the gate is shaped accordingly. This drain current includes a fluctuation component i 2 D . We derive the V / I -characteristic for the junction field effect transistor with pn-junction. The same applies analogously to the other designs, e.g. MESFET or HEMT with Schottky barrier. We consider the open channel without gate as a piece of material with the conductivity 𝜎 , the cross-section A = a × w and the length L . w is the width of the cuboid, a its height, and L its length. Due to the n-doping the channel has an electron density n . According to the material these electrons have a mobility 𝜇 . With this approach, Ohm’s law applies, which in its general form is J = 𝜎 E (11.2) J is the current density, E the electric field strength. A Guide to Noise in Microwave Circuits: Devices, Circuits, and Measurement , First Edition. Peter Heymann and Matthias Rudolph. © 2022 The Institute of Electrical and Electronics Engineers, Inc. Published 2022 by John Wiley & Sons, Inc.
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