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Chapter 6 Field Effect Transistors (FETs) By: Muhamad Sani Bin Mustafa

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Objectives Explain the operation and characteristics of junction field effect transistors (JFET). Understand JFET parameters Explain the operation and characteristics of metal oxide semiconductor field effect transistors (MOSFET) Discuss and analyze how MOSFET are biased Troubleshoot FET circuits. Discuss and analyze how JFETs are biased

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Introduction Field effect transistors controls current by voltage applied to the gate. The FETs major advantage over the BJT is high input resistance. Overall the purpose of the FET is the same as the BJT.

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The JFET The junction field effect transistor, like a BJT, controls current flow. The difference is the way this is accomplished. The JFET uses voltage to control the current flow. As you will recall the transistor uses current flow through the base- emitter junction to control current. JFETs can be used as an amplifier just like the BJT. V GG voltage levels control current flow in theV DD, R D circuit.

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The JFET The terminals of a JFET are the source, gate, and drain. A JFET can be either p channel or n channel.

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The JFET The current is controlled by a field that is developed by the reverse biased gate-source junction (gate is connected to both sides). With more V GG (reverse bias) the field (in white) grows larger. This field or resistance limits the amount of current flow through R D. With low or no V GG current flow is at maximum.

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JFET Characteristics and Parameters Lets first take a look at the effects with a V GS of 0V. I D increases proportionally with increases of V DD (V DS increases as V DD is increased). This is called the ohmic region (point A to B). Reverse-bias between Gate-Drain = Increasing resistance

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JFET Characteristics and Parameters The point when I D ceases to increase regardless of V DD increases is called the pinch-off voltage (point B). This current is called maximum drain current (I DSS ). Breakdown (point C) is reached when too much voltage is applied. This of course undesirable, so JFETs operation is always well below this value. DSS = Drain-to-Source current when gate Shorted

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JFET Characteristics and Parameters

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From this set of curves you can see with increased voltage applied to the gate the I D is limited and of course the pinch-off voltage is lowered as well.

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JFET Characteristics and Parameters

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We know that as V GS is increased I D will decrease. The point that I D ceases increase is called cutoff. The amount of V GS required to do this is called the cutoff voltage (V GS(off ) ). The field (in white) grows such that it allows practically no current to flow through. It is interesting to note that pinch-off voltage (V p ) and cutoff voltage (V GS(off) ) are both the same value only opposite polarity.

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JFET Characteristics and Parameters The transfer characteristic curve illustrates the control V GS has on I D from cutoff (V GS(off) ) to pinchoff (V P ). Note the parabolic shape. The formula below can be used to determine drain current. I D = I DSS (1 - V GS /V GS(off) ) 2

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JFET Characteristics and Parameters Forward transfer conductance of JFETs is sometimes considered. It is the changes in I D based on changes in V GS. Input resistance for a JFET is high since the gate -source junction is reverse biased, however the capacitive effects can offset this advantage particularly at high frequencies. Drain-to-source resistance is the ratio of changes of V DS to I D.

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Transconductance Forward transfer conductance referred to as g m = I D /V GS. The value is larger at the top of the curve but becaome smaller as you increase V GS. At V GS =0, the parameter is known as g mo and can be calculated using this equation: g mo = 2I DSS /|V GS(off) | and g m = g mo (1-V GS /V GS(off) )

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Transconductance g mo can be read from the datasheet as g fs or y fs and sometimes written as Forward Transfer Admittance. g mo = 2I DSS /|V GS(off) | and g m = g mo (1-V GS /V GS(off) )

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JFET Input Resistance Since JFET is reverse-biased for operation, its input resistance becomes so large. This is an advantage of using JFET. Looking at the datasheet, you may calculate the resistance value by using the Gate Reverse Current I GSS. This internal input resistance can be calculated at different V GS voltages as: R IN =|V GS /I GSS | As I GSS increases with temperature, RIN will decrease in hotter environment as the JFET is heated.

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JFET Input Resistance Calculate RIN if IGSS=-2nA and VGS=-20V R IN =|V GS /I GSS |=|-20/-2n|=10G

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JFET Biasing Just as we learned that the bi-polar junction transistor must be biased for proper operation, the JFET too must be biased for operation. Lets look at some of the methods for biasing JFETs. In most cases the ideal Q-point will be the middle of the transfer characteristic curve which is about half of the I DSS.

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Normal Biasing

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JFET Biasing Self-bias is the most common type of biasing method for JFETs. Notice there is no voltage applied to the gate. The voltage to ground from here will always be 0V. However, the voltage from gate to source (V GS ) will be negative for n channel and positive for p channel keeping the junction reverse biased. This voltage can be determined by the formulas below. I D = I S for all JFET circuits. (n channel) V GS = 0-I D R S (p channel) V GS = 0-(-I D R S )

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JFET Biasing

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Setting the Q-point requires us to determine a value of R S that will give us the desired I D and V GS.. The formula below shows the relationship. R S = | V GS /I D | To be able to do that we must first determine the V GS and I D from the either the transfer characteristic curve or more practically from the formula below. The data sheet provides the I DSS and V GS(off). V GS is the desired voltage to set the bias. I D = I DSS (1 - V GS /V GS(off) ) 2

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JFET Biasing Since midpoint biasing is most common lets determine how this is done. The values of R S and R D determine the approximate midpoint bias. Half of I DSS would be I D that is midpoint. The V GS to establish this can be determined by the formula below. V GS V GS(off) /3.4

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JFET Biasing The value of R S needed to establish the computed V GS can be determined by the previously discussed relationship below. R S = | V GS /I D | The value of R D needed can be determined by taking half of V DD and dividing it by I D. R D = (V DD /2)/I D

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JFET Biasing Remember the purpose of biasing is to set a point of operation (Q-point). In a self-biasing type JFET circuit the Q-point is determined by the given parameters of the JFET itself and values of R S and R D. Setting it at midpoint on the drain curve is most common. One thing not mentioned in the discussion was R G. Its value is arbitrary but it should be large enough to keep the input resistance high.

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JFET Biasing The transfer characteristic curve along with other parameters can be used to determine the mid-point bias Q-point of a self- biased JFET circuit. First determine the V GS at I DSS from the formula below. V GS = -I D R S Where the two lines intersect gives us the I D and V GS (Q- point) needed for mid-point bias. Note that load line extends from V GS(off) (I D = 0A) to V P (I D = I DSS ) Please try Ex. 8-10

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JFET Biasing

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Voltage-divider bias can also be used to bias a JFET. R 1 and R 2 are used to keep the gate-source junction in reverse bias. Operation is no different from self-bias. Determining I D, V GS for a JFET voltage-divider circuit with V D given can be calculated with the formulas below. I D = V DD - V D /R D V S = I D R S V G = (R 2 /R 1 + R 2 )V DD V GS = V G - V S

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JFET Biasing In using the transfer characteristic curve to determine the approximate Q-point we must establish the two points for the load line. The first point is I D = 0 and V GS (note that V GS = V G when I D = 0). V GS = V G = (R 2 /R 1 + R 2 )V DD The second point is I D when V GS is 0. I D = V G /R S

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JFET Biasing Transfer characteristics can vary for JFETs of the same type. This would adversely affect the Q-point. The voltage- divider bias is less affected by this than self-bias. This is an undesirable problem that in extreme cases would require trying several of the same type until you find one that works within the desired range of operation.

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Experiment: JFET Self Bias Circuit Transfer characteristics can vary for JFETs of the same type. This would adversely affect the Q-point. The voltage- divider bias is less affected by this than self-bias. This is an undesirable problem that in extreme cases would require trying several of the same type until you find one that works within the desired range of operation.

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Experiment: JFET Self Bias Circuit We can use both hand calculations and lab-based measurements to understand the operation of circuits and devices. For the circuit shown here, we can use equations provided in the lab-manual to analyze the circuit.

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Experiment: JFET Self Bias Circuit If we know IDSS, gmo, Rs, RD and VDD, the values of many circuit parameters can easily be calculated.

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Experiment: JFET Self Bias Circuit JFET dc gate to source cutoff voltage =(13.1) Quiescent dc drain (source) current I D = (13.2a) =(13.2b) Quiescent dc gate to source voltage V S =I DRS (13.3) =-V GS that V G =0

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Experiment: JFET Self Bias Circuit Quiescent drain voltage V D =V DD – I DRD (13.4) Quiescent dc drain to source voltage V DS =V DD – I D (R D + R S )(13.5) JFET forward transconductance at Q point gm= (13.6)

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The MOSFET The metal oxide semiconductor field effect transistor (MOSFET) is the second category of FETs. The chief difference is that there no actual pn junction as the p and n materials are insulated from each other. MOSFETs are static sensitive devices and must be handled by appropriate means. There are depletion MOSFETs (D-MOSFET) and enhancement MOSFETs (E-MOSFET). Note the difference in construction. The E-MOSFET has no structural channel.

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The MOSFET The D-MOSFET can be operated in depletion or enhancement modes. To be operated in depletion mode the gate is made more negative effectively narrowing the channel or depleting the channel of electrons.

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The MOSFET To be operated in the enhancement mode the gate is made more positive, attracting more electrons into the channel for better current flow. Remember we are using n channel MOSFETs for discussion purposes. For p channel MOSFETs, polarities would change.

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The MOSFET The E-MOSFET or enhancement MOSFET can operate in only the enhancement mode. With a positive voltage on the gate the p substrate is made more conductive.

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The MOSFET The lateral double diffused MOSFET (LDMOSFET) and the V- groove MOSFET (VMOSFET) are specifically designed for high power applications. Dual gate MOSFETs have two gates which helps control unwanted capacitive effects at high frequencies.

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MOSFET Characteristics and Parameters Since most of the characteristics and parameters of MOSFETs are the same as JFETs we will cover only the key differences.

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MOSFET Characteristics and Parameters For the D-MOSFET we have to also consider its enhancement mode. Calculating I D with given parameters in the enhancement mode and depletion mode is the same. Note this equation is no different for I D than JFETs and that the transfer characteristics are similar except for its effect in the enhancement mode. I D = I DSS (1 - V GS /V GS(off) ) 2 Remember n and p channel polarity differences.

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MOSFET Characteristics and Parameters The E-MOSFET for all practical purposes does not conduct until V GS reaches the threshold voltage (V GS(th) ). I D when it is when conducting can be determined by the formulas below. The constant K must first be determined. I D(on) is a data sheet given value. K = I D(on) /(V GS - V GS(th) ) 2 I D = K(V GS - V GS(th) )2

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MOSFET Biasing The three ways to bias a MOSFET are zero-bias, voltage-divider bias, and drain-feedback bias. For D-MOSFET zero biasing as the name implies has no applied bias voltage to the gate. The input voltage swings it into depletion and enhancement mode.

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MOSFET Biasing For E-MOSFETs zero biasing cannot be used. Voltage-divider bias must be used to set the V GS greater than the threshold voltage (V GS(th) ). I D can be determined as follows. To determine V GS, normal voltage divider methods can be used. The following formula can now be applied. K = I D(on)/ (V GS - V GS(th) ) 2 I D = K(V GS -V GS(th) ) 2 V DS can be determined by application of Ohms law and Kirchhoffs voltage law to the drain circuit.

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MOSFET Biasing With drain-feedback bias there is no voltage drop across R G making V GS = V DS. With V GS given determining ID can be accomplished by the formula below. I D = V DD - V DS /R D

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Troubleshooting As always, having a thorough knowledge of the devices makes for easier troubleshooting circuits utilizing them. We will discuss some the common faults associated with FET circuits. Experience in troubleshooting is the best teacher having basic theoretical knowledge is extremely helpful.

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Troubleshooting If V D = V DD in a self-biased JFET circuit it could be one of several opens. It is a clear indication of no drain current. Use of senses to check for obvious failures the first and easiest step. Replace the FET only if associated components are known to be good. If V D is less than normal in a self-biased JFET circuit an open in the gate circuit is more than likely the problem. The low drain voltage would be indicative of more drain current flowing than normal.

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Troubleshooting In a zero-biased D-MOSFET or drain-feedback biased E-MOSFET an open in the gate circuit is more difficult to detect. It may seem to be biased properly with dc voltages but will fail to work properly when an ac signal is applied.

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Troubleshooting With a voltage-divider biased E-MOSFET circuit faults are more easily detected. With an open R 1 there is no drain current, so the V D = V DD. With an open R 2 full V DD is applied to the gate turning it on fully. V D = 0

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Summary JFETs are unipolar devices. JFETs have three terminals: Source, Gate, and Drain. JFETs have a high input resistance since the gate- source junction is reverse biased. IDSS for all FETs is the maximum amount of current flow in the drain circuit when V GS is 0V. All FETs must be biased for proper operation. Midpoint is most common for use in amplifiers. Unwanted capacitance associated with FETs can be dealt with by using dual gate type FETs.

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Summary E-MOSFETs have no physical channel. A channel is induced with V GS greater than V GS(th). There are special MOSFET designs for high power applications. E-MOSFETs have no I DSS parameter. MOSFETs differ in construction in that the gate is insulated from the channel. D-MOSFETs can operate in both depletion and enhancement modes. E-MOSFETs can only operate in the enhancement mode.

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