Field Effect Transistors learnabout electronics org

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www learnabout electronics org Semiconductors Module 4 Field Effect Transistors. JFET Construction, The construction of JFETs can be theoretically quite simple but in reality difficult requiring very. pure materials and clean room techniques JFETs are made in different forms some being made as. discrete single components and others using planar technology as integrated circuits. Fig 4 1 1 shows the theoretically simplest form,of construction for a Junction FET JFET using. diffusion techniques It uses a small slab of N type. semiconductor into which are infused two P type,areas to form the Gate Current in the form of. electrons flows through the device from source to,drain along the N type silicon channel As only. one type of charge carrier electrons carry current. in N channel JFETs these transistors are also,called Unipolar devices.
Fig 4 1 2 shows the cross section of a N channel,planar Junction FET JFET The load current. flows through the device from source to drain,along a channel made of N type silicon In the. planar device the second part of the gate is,formed by the P type substrate. JFET Circuit Symbols, P channel JFETs are also available and the principle. of operation is the same as the N channel type, described here but polarities of the voltages are of.
course reversed and the charge carriers are holes,Notice in the JFET circuit symbols shown in Fig. 4 1 3 that the arrowhead on the gate connection,always points towards the negative connection. indicating the polarity either P or N channel of the. SEMICONDUCTORS MODULE 4 PDF 2 E COATES 2020, www learnabout electronics org Semiconductors Module 4 Field Effect Transistors. Module 4 2,How a JFET Works, What you ll learn in Module 4 The JFET is a Voltage Controlled Transistor. The JFET is a voltage controlled transistor that has two distinct. Operation Below Pinch Off areas of operation depending on the whether the voltage. Operation Above Pinch Off applied to the Source and Drain terminals is greater or less that. the transistor s Pinch Off Voltage,JFET Output Characteristic The Pinch Off Voltage.
The Pinch Off value of the JFET refers to the voltage applied. JFET Transfer Characteristic, between Drain and Source with the Gate voltage at zero volts. JFET Video at which maximum current flows Operating with the. Drain Source voltage below this value is classed is the Ohmic. Region as the JFET will act rather like a resistor Operating with the Drain Source voltage above. Pinch Off is known as the Saturation Region as the JFET is acting like a saturated transistor that. is any increase in voltage does not produce a relative increase in current. Operation Below Pinch Off,In the planar construction N. channel JFET shown in Fig,4 2 1 the N channel is,sandwiched between two P. type regions the gate and the,substrate that are connected. together and are at 0V This,forms the gate The N type.
channel is connected to the,source and drain terminals via. more heavily doped N type, regions The drain is connected to a positive supply and the source to zero volts N type silicon. has a lower resistivity than N type This gives it a lower resistance increasing conduction and. reducing the effect of placing standard N type silicon next to the aluminium connector which. because aluminium is a tri valent material having three valence electrons whilst silicon has four. would tend to create an unwanted junction similar in effect to a PN junction at this point. The P type gate is at 0V and is therefore negatively biased compared to the channel which has a. potential gradient on it as one end is connected to 0 volts the source and the other end to a. positive voltage the drain Any point on the channel apart from the extreme end near the source. terminal must therefore be more positive than the gate Therefore the two PN junctions formed. between the N type conducting channel and the P type areas of the gate substrate are both reverse. biased and so have a depletion layer that extends into the channel as shown in Fig 4 2 1. The shape of the depletion layer is not symmetrical as can be seen from Fig 4 2 1 It is generally. thicker towards the drain end of the channel because the voltage on the drain is more positive than. that on the source due to the voltage gradient that exists along the channel This causes a larger. potential across the junctions nearer the drain and so a thickening of the depletion layer The effect. becomes more marked when the voltage between drain and source is greater than about 1 volt or so. SEMICONDUCTORS MODULE 4 PDF 3 E COATES 2020, www learnabout electronics org Semiconductors Module 4 Field Effect Transistors. Operation Above Pinch Off,When a voltage is applied between. drain and source VDS current,flows and the silicon channel acts.
rather like a conventional resistor,The Ohmic Region Now if VDS. is increased with VGS held at,zero volts towards what is called. the pinch off value VP the drain,current ID also at first increases. The transistor is working in the,ohmic region as shown in Fig. However as drain source voltage VDS increases the depletion layers at the gate junctions are also. becoming thicker and so narrowing the N type channel available for conduction There comes a. point Pinch Off where the conducting channel has become narrow enough to cancel out the effect. of current increasing with the applied voltage VDS as shown in fig 4 2 2 Above this Pinch Off. point there is little further increase in drain current and the transistor is said to operating in the. Saturation Region, However if the JFET is biased with VDS at Pinch Off voltage VP a small change in VGS can be.
used to control the current through the source drain channel from zero current to its maximum. saturated value,JFET Characteristics,This type of operation is shown in the. fairly flat top to the output,characteristics shown in Fig 4 2 3. Notice that each curve is drawn for a,particular value of negative voltage. between gate and source and that,when sufficient reverse bias is applied. to the gate e g more than 2 5V the,lowest value on the graph the drain.
current ceases completely,In the JFET output characteristics. shown in Fig 4 2 3 the Drain current,ID increases in a linear manner like a. resistor at values of Gate Source, voltage VGS below pinch off The Ohmic Region but above VP The Saturation Region shows. very little change and the curves are very nearly horizontal at voltages greater than the pinch off. voltage VP Almost all of the expected increase in current due to the increase in voltage between. Source and Drain VDS is offset by the narrowing of the conducting channel due to the growing. depletion layers,SEMICONDUCTORS MODULE 4 PDF 4 E COATES 2020. www learnabout electronics org Semiconductors Module 4 Field Effect Transistors. The tranconductance characteristic for a,JFET which shows the change in Drain.
current ID for a given change in Gate,Source voltage VGS is shown in Fig. 4 2 4 Because the JFET input the,Gate is voltage operated the gain of. the transistor cannot be called current,gain as with bipolar transistors The. drain current is controlled by the Gate,Source voltage so the graph shows. milli amperes per volt mA V and as,current divided by Voltage I V is.
CONDUCTANCE the inverse of resistance V I the slope of this graph the gain of the device is. called the FORWARD or MUTUAL TRANSCONDUCTANCE which has the symbol gm. Therefore the higher the value of gm the greater the amplification. Notice that VGS is always shown as being negative in reality it may be zero or slightly above zero. but the gate is always more negative than the N type channel between source and drain Note also. that the slope of the curve in the transfer characteristic is less steep than that of the Mutual. Conductance characteristic for a typical bipolar transistor compare Fig 4 2 4 and Fig 3 5 4 on the. Bipolar Transistors Current Gain page This means that a JFET will have a lower gain than that of. a bipolar transistor, This disadvantage is offset by the advantage of having an extremely high input resistance A typical. input resistance for a JFET would be in the region of 1 x 1010 ohms 10 000 Megohms compared. with 2K to 3K Ohms for a bipolar device, This makes the JFET ideal for applications where the circuit or device driving the JFET amplifier. cannot supply any appreciable current an example being the Electret microphone which uses a. FET within the microphone to amplify the tiny voltage variations appearing across the vibrating. diaphragm element, Another feature of the JFET that makes it more suited to very high frequency use than bipolar. transistors is the absence of junctions in the JFET conducting channel In a bipolar transistor two. PN junctions forming tiny capacitances exist between base and emitter and base and collector due. to the PN junctions These capacitances will limit high frequency performance as they provide. negative feedback paths at high frequencies Because the JFET is in effect just a slab of silicon. between Source and Drain the stray capacitances that exist in bipolar devices are absent so high. frequency performance is improved making JFETs usable even at hundreds of MHz. Download a datasheet for a typical N Channel JFET from On Semiconductor. Video is available at https learnabout electronics org Semiconductors fet 02 php. SEMICONDUCTORS MODULE 4 PDF 5 E COATES 2020, www learnabout electronics org Semiconductors Module 4 Field Effect Transistors. Module 4 3,Enhancement Mode MOSFETs, What you ll learn in Module 4 The Insulated Gate FET IGFET.
The Metal Oxide Silicon FET MOSFET or Metal, Section 4 3 The Enhancement Mode MOSFET Oxide Silicon Transistor M O S T has an even. higher input resistance typically 1012 to 1015 ohms. The IGFET Insulated Gate FET than that of the JFET In the MOSFET device the. MOSFET IGFET Construction gate is completely insulated from the rest of the. transistor by a very thin layer of metal oxide Silicon. MOSFET IGFET Operation dioxide SiO2 Hence the general name applied to any. device of this type is the IGFET or Insulated Gate. MOSFET IGFET Circuit Symbols FET,Handling Precautions for MOSFETS. Planar Technology, There are several ways in which an insulated gate transistor may be constructed All the methods. used however make use of planar technology in which the various parts of the device are laid down. as planes or layers on the upper surface of a SUBSTRATE in a similar way to that shown on the. Planar Transistors page in the BJT section, The layers are laid down one by one by diffusing various semiconductor materials with suitable. doping levels as well as layers of insulation into the surface of the device under carefully. controlled conditions at high temperatures Parts of a layer may be removed by etching using. photographic masks to make the required pattern of the electrodes etc before the next layer is. added The insulating layers are made by laying down very thin layers of silicon dioxide and. conductors are created by evaporating a metal such as aluminium on to the surface The transistors. produced in this way have a much higher quality than is possible using other methods and many. transistors can be produced at one time on a single slice of silicon before the silicon slice is cut up. into individual transistors or integrated circuits. MOSFET IGFET Construction,The basic construction of a MOSFET.
is shown in Fig 4 3 1 A body or,substrate of P type silicon is used then. two heavily doped N type regions are,diffused into the upper surface to form. a pair of closely spaced strips,A very thin about 10 4 mm layer of. silicon dioxide is then evaporated,onto the top surface forming an. insulating layer Parts of this layer,are then etched away above the N.
type regions using a photographic,mask to leave these regions. uncovered On top of the insulating,layer between the two N type. regions a layer of aluminium is,deposited This acts as the GATE. electrode Metal contacts are also deposited on the N type regions which act as the SOURCE and. DRAIN connectors,SEMICONDUCTORS MODULE 4 PDF 6 E COATES 2020. www learnabout electronics org Semiconductors Module 4 Field Effect Transistors. MOSFET IGFET Operation, The gate has a voltage applied to it that makes it positive with respect to the source This causes.
holes in the P type layer close to the silicon dioxide layer beneath the gate to be repelled down into. the P type substrate and at the same time this positive potential on the gate attracts free electrons. from the surrounding substrate material These free electrons form a thin layer of charge carriers. beneath the gate electrode they can t reach the gate because of the insulating silicon dioxide layer. bridging the gap between the heavily doped source and drain areas This layer is sometimes called. an inversion layer because applying the gate voltage has caused the P type material immediately. under the gate to firstly become intrinsic with hardly any charge carriers and then an N type. layer within the P type substrate, Any further increase in the gate voltage attracts more charge carriers into the inversion layer so. reducing its resistance and increasing current flow between source and drain Reducing the gate. source voltage reduces current flow When the power is switched off the area beneath the gate. www learnabout electronics org Semiconductors Module 4 Field Effect Transistors SEMICONDUCTORS MODULE 4 PDF 4 E COATES 2020 Operation Above Pinch Off When a voltage is applied between drain and source V DS current

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