Junction field effect transistor

Here   we will learn about the JFET in detail.  we will see

      the construction 

      the working of this JFET.
        Now, in the previous video we have seen thatin case of a field effect transistor, the path through which the charge carrier flowsis known as the channel. And if this channel is made up of n-type materialthen the field effect transistor is known as the n-channel FET. And likewise, if the channel is made up ofp-type semiconductor material, then the field effect transistor is known as the p-channelFET. So, in the case of n-channel JFET, the channelis made up of n-type material. And two small p-type regions are fabricatednear this channel. And if you see the structure, this n-typematerial is the major part of the entire structure. So, through the ohmic contact, the top ofthe n-channel is connected to the drain terminal and the bottom of the n-channel is connectedto the source terminal. And two p-type regions are connected togetherto the gate terminal. So, in this n-channel JFET, due to this p-typeregions two p-n junctions are formed. And due to that, the depletion region is alsoformed near this junction. Now, whenever we apply the voltage betweenthe drain and the source terminal, then the current starts flowing through the devicebetween the drain and the source terminal. And by applying the voltage between this gateand the source terminal, this current can be controlled.
      So, the working of the JFET can be explainedusing the tap-water analogy. So, as we know, in case of a tap-water thewater flows from the source towards the drain. And the flow of water can be controlled usingthis knob. So, similarly, in case of this JFET, the voltagebetween this gate and the source terminal controls the current which is flowing betweenthe drain and the source terminal. So, now let's exactly see how this JFET worksby taking the example of the n-channel JFET. And let's also see what should be the exactpolarity of the voltages between this drain and the source as well as between the gateand the source terminal. So, first of all, let's assume that this gateand the source terminals are connected together. So, as you can see over here, the source terminalis connected to the ground and the gate and the source terminals are connected together. 
        Now, here in case of this n-channel JFET,the voltage between this drain and the source terminal should be positive. So, here let's say this voltage Vdd is appliedbetween this drain and the source terminal. So, this voltage Vds should be positive. That means the drain terminal should be morepositive than this source terminal. So, now once we apply this voltage then theelectrons start flowing from the source terminal to the drain terminal. And if we see the conventional current, thenthe current starts flowing from the drain terminal towards the source terminal. so, here let's say Id is the current whichis flowing into the drain terminal. And the Is is current which is flowing outof the source terminal. And as you can see, this current Id is equalto Is. So, instead of defining these two currentsas a separate current, we will only define this drain current Id. So, now considering this Vgs is equal to zeroand Vds is positive, let's see how this n-channel JFET works. So, whenever this Vds is positive, then thesetwo PN junctions will become reversed biased. And due to this reverse bias connection, thewidth of the depletion region will increase.
         Now, if you notice over here, the depletionregion is wider at the top of this p-type region. And it is narrower at the bottom of this p-typeregion. So, first of all, let's understand the reasonbehind it. Now, during the operation, this n-channelact like a resistor. And let's assume, the uniform resistance throughoutthis n-channel. So, this n-channel can be modeled as a seriesof distributed resistors between the drain and the source terminal. And whenever this drain current Id flows,then there will be a voltage drop across each resistor. So, let's say the voltage at the top end isequal to 2V. And as we move towards the source terminalthen there will be a voltage drop across each resistor. And due to that, the upper region of the p-typematerial will be more reverse biased compared to the lower region. And we had seen in the earlier videos, aswe increase the applied reverse bias voltage, the width of the depletion region will increase. So, due to that, the depletion region is widerat the top portion. And it is narrow at the bottom portion. So, in short, due to the applied voltage Vdd,these two PN junctions will become reversed biased. And due to that only a small amount of reversesaturation current will flow through this PN junction. And for the practical cases, we can considerthat the current Ig through this gate terminal is equal to zero. So, due to this reverse biased PN junctions,the input impedance of this JFET is very high. So, now as we increase this voltage Vds fromzero to few volts, then the current which is flowing through the channel will increase. And if we plot this current Id versus Vds,then initially it will almost look like a straight line. So, this curve of Id vs Vds is known as theoutput characteristic of this JFET. Or sometimes it is also known as the draincurves for the JFET. So, as you can see, for the low voltages,this curve is an almost straight line. Meaning that for the low voltages, the resistanceof the channel remains constant. But if we keep on increasing this voltageVds then the width of the depletion region will become wider. And due to that, the channel will become narrowerand the narrower. So, due to this reduced channel width, nowthe channel resistance will increase. And that is also evident from the graph. So, if you see this region of the graph, theslope of the line changes and it becomes more and more horizontal. So, basically, it indicates that as we increasethe voltage Vds, then the channel resistance will increase. And now if we further increase the voltageVds, then at one particular voltage, the depletion regions will touch each other. So, this condition is known as the pinch-offcondition. And the voltage at which this occurs is knownas the pinch-off voltage. So, let's denote this pinch-off voltage asVp. So, whenever this Vds is greater than or equalto Vp, then this pinch-off condition will occur. So, the name pinch-off suggests, once thiscondition occurs, then the current Id should drop to the zero. Because now, there is no path for the chargecarriers to flow from this side towards this side. But in reality, if you see, that is not thecase. And in fact, once the pinch-off conditionis reached, the current Id reaches the saturation level. So, let's understand why is it so. So, first of all, let's assume that once thepinch-off condition is reached, then this Id is equal to 0. 
     So, if that is the case, then the absenceof the drain current would remove the possibility of the different potential levels across thisn-channel. And due to that, the reverse bias across thePN junction would be removed. And that would result in the loss of depletionregion which causes the pinch-off at the first place. So, basically, this current Id will not becomezero. And in fact, at the pinch-off condition, thiscurrent Id is the maximum current. So, this saturation current is denoted asIdss. And Idss is the maximum current of the JFETwhenever the Vgs is equal to zero and Vds is more than pinch-off voltage. So, whenever this Vds is more than this pinch-offvoltage then the current which is flowing through the device is almost constant. And in this region of operation, the JFETworks as a constant current source. So, basically, under this region of operation,we will get a constant current through this device. Now, so far in our discussion, we have assumedthat the voltage Vgs is equal to zero. But as discussed earlier, this gate to sourcevoltage can control this drain current.
        So, now let's see, how the voltage level ofVgs can control the drain current. Or in a way, how it can affect the drain curvesor the output characteristic of this JFET. So, now what we will do, we will make thisVgs more and more negative with respect to zero volts. And we will find the drain currents for thedifferent values of the Vgs. So, first, let's assume that the Vgs is equalto -1V. So, due to this negative voltage now the depletionregion will get created across this PN junction. And as we keep on increasing this voltageVds, between the drain and the source terminal, then the width of the depletion region willincrease. But now, the pinch-off condition or the saturationof the drain current will be reached at the lower voltage of Vds. Because due to this negative voltage of Vgs,the PN junction is already reverse biased.  
         So, in case of this Vgs is equal to -1V, ifwe see the drain curve, then it will look like this. So, as you can see, at Vgs is equal to -1V,the saturation value of the drain current has been reduced. And in fact, it will continuously reduce,once we reduce the value of Vgs below this zero volt. So, as you can see, if we reduce the valueof Vgs, from -1V to -2V, then further this saturation value of the drain current willreduce. And whenever, this Vgs is equal to -Vp thenthe saturation current will essentially become zero. So, this region of operation is known as thecut-off region of operation. Or we can say that whenever this Vgs is equalto -Vp, at that time the device is turned off. 
        So, in this way, this JFET can be operatedin the three different regions. The ohmic region, the saturation region, andthe cut-off region. So, in this ohmic region, the JFET will workas a resistor. And for the fixed value of Vgs, it providesalmost constant resistance. But as we reduce the value of this Vgs, thenthe resistance of the channel will increase. So, basically in this region, the JFET canbe operated as a variable resistor. And by changing the voltage between the gateand the source terminal, we can control the resistance of this JFET. Then the second region of operation is thesaturation region. So, in this region, whenever this drain tosource voltage or Vds is more than this pinch-off voltage Vp, at that time the drain currentwill almost remain constant. And the third region of operation is the cut-offregion. So, whenever this Vgs is greater than or equalto Vp at that time, this drain current Id will be approximately equal to 0. And we can say that the device is turned off. 
     So, apart from these three regions, thereis one more region. And it is known as the breakdown region. And like a diode, this region of operationshould be avoided. So, in this saturation region of operationif we increase this voltage Vds beyond a certain limit, then there is a vertical rise in thisdrain current. Or we can say that the breakdown has occurred. And now the current is limited solely by theexternal circuit. So, generally in the datasheet, the maximumvalue of the Vds has been defined. So, during the operation, the value of thisVds should be less than this rated value.
        So, this all about the output characteristicsor the drain curves of the JFET. Now, so far in our discussion, we have onlydiscussed about the n-channel JFET. But the p-channel JFET also works in a similarway. So, in case of this p-channel JFET, the channelis made up of p-type semiconductor. And two small n-type regions are fabricatednear this channel. And in case of p-channel JFET, now the polarityof the biasing voltage also reversed. So, now in case of this p-channel JFET, thedrain to source voltage should be negative and the gate to source voltage should be positive. Apart from that in case of this p-channelJFET, now the charge carriers will be holes. 
     And when we apply the voltage between thedrain and the source terminal, then they will start moving from the source towards the drainterminal. So, now if we see the Id versus Vds curveor the output characteristics of the p-channel JFET, then it will look quite similar to then-channel JFET. But in this case, this voltage Vds is negative. That means if you see the voltages on thishorizontal axis, they will be negative. So, now in case of this p-channel JFET, aswe increase the voltage Vgs, then the saturation value of the drain current will reduce. And whenever this Vgs is equal to Vp, or thepinch-off voltage, then the drain current Id will be approximately equal to zero amperes. 
      And similarly to the n-channel JFET, thereis also a breakdown region in case of this p-channel JFET. That means if we go beyond the certain valueof this voltage Vds, then the drain current will increase drastically. So, this is all about the output characteristicsof the p-channel JFET. Alright so now let's see the electronic symbolof this n-channel as well as the p-channel JFET. So, these are the symbols of the n-channeland p-channel JFET.
            And if you see these symbol, it has threeterminals. The gate, drain, and the source terminal. And in fact, if you see these two symbols,they almost look identical. But the only difference between the two symbolsis the direction of the arrow. So, in case of an n-channel JFET, the arrowis going inwards. While in case of a p-channel JFET, the arrowis going outwards. And basically, these arrow indicates the directionof the flow of current whenever the PN junction is forward biased. So, in the case of p-channel JFET, wheneverthe PN junction is forward biased, then the current will flow in the outward direction. While in case of n-channel JFET, wheneverthe PN junction is forward biased, then the current will flow in the inward direction. And apart from these symbols, sometimes thissymbol is also used for the n-channel and the p-channel JFT. So, I hope in this video, you understood theconstruction as well as the working of this JFET. So, in the next video, we will find the relationshipbetween the voltage Vgs and the drain current Id. And we will find the transfer characteristicsof this JFET. So, if you have any question or suggestion,do let me know here in the comment section below. 
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