Principles and Circuits

F PRINCIPLES AND CIRCUITS Part 1 E Field-Effect Transistors by Ray Marston Ray Marston explains FET (Field-Effect Transistor) basics in this opening episode of this new T four-part series. ield-Effect Transistors (FETs) are unipolar devices, and have two big advantages over bipolarF transistors: one is that they have a near-infinite input resistance and thus offer near- Figure 1. Comparison of infinite current and power transistor and gain; the other is that their JFET symbols, switching action is not marred notations, and by charge-storage problems, supply polarities. and they thus outperform most bipolars in terms of digi- tal switching speeds. Figure 2. Basic structure of Several different basic a simple n-channel JFET, types of FETs are available, showing how channel width is and this opening episode controlled via the gate bias. looks at their basic operating rial with a drain terminal at one end becomes so deep that conduction principles. Parts 2 to 4 of the and a source terminal at the other. A ceases. an n-channel FET, -ve for a p-channel series will show practical ways p-type control electrode or gate sur- Thus, the basic JFET of Figure 2 FET), a drain current (ID) flows and of using FETs. rounds (and is joined to the surface passes maximum current when its can be controlled via a gate-to-source of) the middle section of the n-type gate bias is zero, and its current is bias voltage VGS. bar, thus forming a p-n junction. reduced or ‘depleted’ when the gate (2). ID is greatest when VGS = 0, FET BASICS In normal use, the drain terminal bias is increased. It is thus known as and is reduced by applying a reverse is connected to a positive supply and a ‘depletion-type’ n-channel JFET. A bias to the gate (negative bias in an An FET is a three-terminal ampli- the gate is biased at a value that is p-channel version of the device can n-channel device, positive bias in a fying device. Its terminals are known negative (or equal) to the source volt- (in principle) be made by simply p-type). The magnitude of VGS need- as the source, gate, and drain, and age, thus reverse-biasing the JFET’s transposing the p and n materials. ed to reduce ID to zero is called the correspond respectively to the emit- internal p-n junction, and account- ‘pinch-off’ voltage, VP, and typically ter, base, and collector of a normal ing for its very high input imped- JFET DETAILS has a value between 2 and 10 volts. transistor. Two distinct families of ance. The magnitude of ID when VGS = 0 is FETs are in general use. The first of With zero gate bias applied, a Figure 3 shows the basic form of denoted IDSS, and typically has a these is known as ‘junction-gate’ current flow from drain to source via construction of a practical n-channel value in the range 2 to 20mA. types of FETs; this term generally a conductive ‘channel’ in the n-type JFET; a p-channel JFET can be made (3). The JFET’s gate-to-source being abbreviated to either JUGFET bar is formed. When negative gate by transposing the p and n materials. junction has the characteristics of a or (more usually) JFET. bias is applied, a high resistance All JFETs operate in the depletion silicon diode. When reverse-biased, The second family is known as region is formed within the junction, mode, as already described. Figure 4 gate leakage currents (IGSS) are only either ‘insulated-gate’ FETs or Metal and reduces the width of the n-type shows the typical transfer character- a couple of nA (1nA = .001µA) at Oxide Semiconductor FETs, and conduction channel and thus istics of a low-power n-channel JFET, room temperature. Actual gate sig- these terms are generally abbreviat- reduces the magnitude of the drain- and illustrates some important fea- nal currents are only a fraction of an ed to IGFET or MOSFET, respectively. to-source current. As the gate bias is tures of this type of device. The most nA, and the input impedance of the ‘N-channel’ and ‘p-channel’ versions increased, the ‘depletion’ region important characteristics of the JFET gate is typically thousands of of both types of FET are available, spreads deeper into the n-type chan- are as follows: megohms at low frequencies. The just as normal transistors are avail- nel, until eventually, at some ‘pinch- gate junction is shunted by a few pF, able in npn and pnp versions. Figure off’ voltage value, the depletion layer (1). When a JFET is connected to so the input impedance falls as fre- 1 shows the symbols and supply a supply with quency rises. polarities of both types of bipolar the polarity If the JFET’s gate-to-source junc- transistor, and compares them with shown in tion is forward-biased, it conducts both JFET versions. Figure 1 like a normal silicon diode. If it is Figure 2 illustrates the basic con- (drain +ve for excessively reverse-biased, it struction and operating principles of a simple n-channel JFET. It consists of a bar of n-type semiconductor mate- Figure 5. Figure 4. An n-channel Idealized JFET can be Figure 3. transfer used as a Construction characteristics voltage- of n-channel of an controlled JFET. n-channel resistor. JFET. 1 MAY 2000/Nuts & Volts Magazine ©T & L Publications, Inc. All rights reserved. configuration can be terminal device, or may be internally obtained by using connected to the source, making a the basic Figure 11 three-terminal device. circuit. In practice, An important point about the fairly accurate bias- IGFET/MOSFET is that it is also avail- ing techniques (dis- able as an enhancement-mode device, cussed in Part 2 of in which its conduction channel is nor- this series) must be mally closed but can be opened by Figure 6. An n-channel Figure 7. An n-channel JFET can be used as used in these cir- applying forward bias to its gate. JFET can be used as a an electronic chopper. cuits. Figure 13 shows the basic con- voltage-controlled switch. struction and the symbol of the n- THE IGFET/MOSFET channel version of such a device. Here, no n-channel drain-to-source The second (and most impor- conduction path exists through the p- tant) family of FETs are those known type substrate, so with zero gate bias under the general title of IGFET or there is no conduction between drain Figure 8. An n-channel JFET MOSFET. In these FETs, the gate ter- and source; this feature is indicated in can be used as a minal is insulated from the semicon- the symbol of Figure 13(b) by the constant-current ductor body by a very thin layer of sil- gaps between source and drain. generator. icon dioxide, hence the title To turn the device on, significant ‘Insulated Gate Field Effect positive gate bias is needed, and Transistor,’ or IGFET. Also, the devices when this is of sufficient magnitude, it generally use a ‘Metal-Oxide Silicon’ starts to convert the p-type substrate avalanches like a zener diode. In stant as VDS is increased beyond the semiconductor material in their con- material under the gate into an n- either case, the JFET suffers no dam- knee value. Thus, when VDS is below struction, hence the alternative title channel, enabling conduction to take age if gate currents are limited to a the JFET’s knee value, the drain-to- of MOSFET. place. few mA. source terminals act as a resistor, RDS, Figure 12 shows the basic con- Figure 14 shows the typical trans- (4). Note in Figure 4 that, for with a value dictated by VGS, and can struction and the standard symbol of fer characteristics of an n-channel each VGS value, drain current ID rises thus be used as a voltage-variable the n-channel depletion-mode FET. It enhancement-mode IGFET/MOSFET, linearly from zero as the drain-to- resistor, as in Figure 5. resembles the JFET, except that its and Figure 15 shows the VGS/ID source voltage (VDS) is increased Typically, RDS can be varied from gate is fully insulated from the body curves of the same device when from zero up to some value at which a few hundred ohms (at VGS = 0) to of the FET (as indicated by the Figure powered from a 15V supply. Note a ‘knee’ occurs on each curve, and thousands of megohms (at VGS = VP), 12(b) symbol) but, in fact, operates that no ID current flows until the gate that ID then remains virtually con- enabling the JFET to be used as a on a slightly different principle to the voltage reaches a ‘threshold’ (VTH) voltage-controlled switch (Figure 6) JFET. value of a few volts, but that beyond or as an efficient ‘chopper’ (Figure 7) It has a normally-open n-type this value, the drain current rises in a that does not suffer from offset-volt- channel between drain and source, non-linear fashion. age or saturation-voltage problems. but the channel width is controlled by Also note that the transfer graph Also note in Figure 4 that when the electrostatic field of the gate bias. is divided into two characteristic VDS is above the knee value, the ID The channel can be closed by applying regions, as indicated (in Figure 14) by value is controlled by the VGS value suitable negative bias, or can be the dotted line, these being the ‘tri- and is almost independent of VDS, increased by applying positive bias.

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