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Chapter 30 Field Effect Transistors Topics Covered in Chapter 30 30-1: JFETs and Their Characteristics 30-2: Biasing Techniques for JFETs 30-3: JFET Amplifiers 30-4: MOSFETs and Their Characteristics 30-5: MOSFET Biasing Techniques 30-6: Handling MOSFETs © 2007 The McGraw-Hill Companies, Inc. All rights reserved. 30-1: JFETs and Their Characteristics Fig. 30-1 (a) in the next slide, shows the construction of an n-channel JFET. There are four leads: the drain, source, and two gates. The area between the source and drain terminals is called the channel. Because n-type semiconductor material is used for the channel, the device is called an n-channel JFET. Embedded on each side of the n-channel are two smaller p-type regions called gates. McGraw-Hill © 2007 The McGraw-Hill Companies, Inc. All rights reserved. 30-1: JFETs and Their Characteristics JFET P-Channel N-Channel Fig. 30-1 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-1: JFETs and Their Characteristics Fig. 30-2 (a) is the schematic symbol for the n-channel JFET, and Fig. 30-2 (b) shows the symbol for the p-channel JFET. The only difference is the direction of the arrow on the gate lead. Fig. 30-2 (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Fig. 30-2 (b) 30-1: JFETs and Their Characteristics Fig. 30-3 illustrates the current flow in an n-channel JFET with ptype gates disconnected. The amount of current depends upon two factors: The value of the drainsource voltage, VDS The drain-source resistance, designated rDS Fig. 30-3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-1: JFETs and Their Characteristics The gate regions in a JFET are embedded on each side of the channel to help control the amount of current flow in the channel. Fig. 30-4 (a) shows an n-channel JFET with both gates shorted to the source. Fig. 30-4 (b) shows how an n-channel JFET is normally biased. Fig. 30-4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-1: JFETs and Their Characteristics Fig. 30-5 (a) shows an n-channel JFET connected to the proper biasing voltages. The drain is positive and the gate is negative, creating the depletion layers. Fig. 30-5 (c) shows a complete set of drain curves for the JFET in Fig. 30-5 (a). Fig. 30-5 (a) (c) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-2: Biasing Techniques for JFETs Many techniques can be used to bias JFETs. In all cases, the gate-source junction is reverse- biased. The most common biasing techniques are Gate Self Voltage-divider Current-source 30-2: Biasing Techniques for JFETs Fig. 30-7 (a) shows an example of gate bias. Fig. 30-7 (b) shows how an ac signal is coupled to the gate of a JFET. If RG were omitted, as shown in (c), no ac signal would appear at the gate because VGG is at ground for ac signals. Fig. 30-7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-2: Biasing Techniques for JFETs One of the most common ways to bias a JFET is with self-bias. (See Fig. 30-8 a) Only a single power supply is used, the drain supply voltage, VDD. Fig. 30-8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-2: Biasing Techniques for JFETs Fig. 30-9 shows a JFET with voltage-divider bias. Since the gate-source junction has extremely high resistance, the R1 – R2 voltage divider is practically unloaded. Voltage-divider bias is more stable than either gate or self-bias. Fig. 30-9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-2: Biasing Techniques for JFETs Fig. 30-10 shows one of the best ways to bias JFETs, called currentsource bias. The npn transistor with emitter bias acts like a current source for the JFET. The drain current , ID, equals the collector current, IC, which is independent of the value of VGS. Fig. 30-10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-3: JFET Amplifiers JFETs are commonly used to amplify small ac signals. One reason for using a JFET instead of a bipolar transistor is that very high input impedance, Zin, can be obtained. A big disadvantage, however, is that the voltage gain, AV, obtainable with a JFET is much smaller. JFET amplifier configurations are as follows: Common-source (CS) Common-gate (CG) Common-drain (CD) 30-3: JFET Amplifiers Fig. 30-12 (a) shows a common-source amplifier. For a common-source amplifier, the input voltage is applied to the gate and the output is taken at the drain. Fig. 30-12 (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-3: JFET Amplifiers The ac equivalent circuit is shown in Fig. 30-12 (b) On the input side, RG = Zin, which is 1 MΩ. This occurs because with practically zero gate current, the gate-source resistance, designated RGS, approaches infinity. Fig. 30-12 (b) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-3: JFET Amplifiers Fig. 30-13 (a) shows a common-drain amplifier, usually referred to as a source follower. A source follower has a high input impedance, low output impedance, and a voltage gain of less than one, or unity. Fig. 30-13 (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-3: JFET Amplifiers A common-gate amplifier has a moderate voltage gain. Its big drawback is that Zin is quite low. Fig. 30-14 (a) shows a CG amplifier. Fig. 30-14 (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-4: MOSFETs and Their Characteristics The metal-oxide semiconductor field effect transistor has a gate, source, and drain just like the JFET. The drain current in a MOSFET is controlled by the gate-source voltage VGS. There are two basic types of MOSFETS: the enhancement-type and the depletion-type. The enhancement-type MOSFET is usually referred to as an E-MOSFET, and the depletion-type, a DMOSFET. The MOSFET is also referred to as an IGFET because the gate is insulated from the channel. 30-4: MOSFETs and Their Characteristics Fig. 30-15 (a) shows the construction of an n-channel depletion-type MOSFET, and Fig. 30-15 (b) shows the schematic symbol. Fig. 30-15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-4: MOSFETs and Their Characteristics Fig. 30-19 shows the construction and schematic symbol for a p- channel, depletion-type MOSFET. Fig. 30-19 (a) shows that the channel is made of p-type semiconductor material and the substrate is made of n-type semiconductor material. Fig. 30-19 (b) shows the schematic symbol. Fig. 30-19 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-4: MOSFETs and Their Characteristics Fig. 30-20 (a) shows the construction of an n-channel, enhancement-type MOSFET. The p-type substrate makes contact with the SiO2 insulator. Because of this, there is no channel for conduction between the drain and source terminals. Fig. 30-20 (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-5: MOSFET Biasing Techniques Zero-bias can be used only with depletion-type MOSFETs. Even though zero bias is the most commonly used technique for biasing depletion-type MOSFETs, other techniques can also be used. Biasing techniques include Self Voltage-divider Current-source Drain-feedback bias is often used to bias E-MOSFETs 30-5: MOSFET Biasing Techniques Fig. 30-22 (a) shows a popular biasing technique that can be used only with depletion-type MOSFETs. This form of bias is called zero bias because the potential difference between the gate-source region is zero. Fig. 30-22 (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 30-6: Handling MOSFETs One disadvantage of MOSFET devices is their extreme sensitivity to electrostatic discharge (ESD) due to their insulated gate-source regions. The SiO2 insulating layer is extremely thin and can be easily punctured by an electrostatic discharge. The following is a list of MOSFET handling precautions Never insert or remove MOSFETs from a circuit with the power on. 30-6: Handling MOSFETs MOSFET handling precautions (Continued) Never apply input signals when the dc power supply is off. Wear a grounding strap on your wrist when handling MOSFET devices. When storing MOSFETs, keep the device leads in contact with conductive foam, or connect a shorting ring around the leads.