Field effect transistor FET

FIELD EFFECT TRANSISTOR
6.1 INTRODUCTION

1. The Field effect transistor is abbreviated as FET , it is an another semiconductor device like a BJT  which can be used as an amplifier or  switch.
2. The Field effect transistor is a voltage operated device. Whereas Bipolar junction transistor is a current controlled device. Unlike BJT a FET requires virtually no input current.
3. This gives it an extremely high input resistance , which is its most important advantage over a bipolar transistor.
4. FET is also a three terminal device, labeled  as source, drain and gate.
5. The source can be viewed as BJT’s emitter, the drain as collector, and the gate as the counter part of the base.
6. The material that connects the source to drain is referred to as the channel.

7. FET operation depends only on the flow of majority carriers ,therefore they are called uni polar  devices. BJT operation depends on both minority and majority carriers.
8. As FET has conduction through only majority carriers it is less noisy than BJT.
9. FETs are much easier to fabricate and are particularly suitable for ICs because they occupy less space than BJTs.
10. FET amplifiers have low gain bandwidth product due to the junction capacitive effects and produce more signal distortion except for small signal operation.
11. The performance of FET is relatively unaffected by ambient temperature changes. As it has  a negative temperature coefficient at high current levels, it prevents the FET from thermal breakdown. The BJT has a positive temperature coefficient at high current levels which leads to thermal breakdown.

6.2 CLASSIFICATION OF FET:
There are two major categories of field effect transistors:
1. Junction Field Effect Transistors
2. MOSFETs
These are further sub divided in to P- channel and N-channel devices.
MOSFETs are further classified  in to two types Depletion MOSFETs  and Enhancement . MOSFETs
When the channel is of N-type the JFET is referred to as an N-channel JFET ,when the channel is of P-type the JFET is referred to as P-channel JFET.
The schematic symbols for the P-channel and N-channel JFETs are shown in the figure.
                                 
6.3 CONSTRUCTION AND OPERATION OF N- CHANNEL FET
If the gate is an N-type material, the channel must be a P-type material.
CONSTRUCTION OF N-CHANNEL JFET

 

A piece of N- type material, referred to as channel has two smaller pieces of P-type material attached to its sides, forming PN junctions. The  channel ends are designated as the drain and source . And the two pieces of P-type material are connected together and their terminal is called the gate. Since this channel is in the N-type bar, the FET is known as N-channel JFET.
OPERATION OF N-CHANNEL JFET:- 
The overall operation of the JFET is based on varying the width of the channel to control the drain current.
A piece of N type material referred to as the channel, has two smaller pieces of P type material attached to its sites, farming PN –Junctions. The channel’s ends are designated the drain and the source. And the two pieces of P type material are connected together and their terminal is called the gate. With the gate terminal not connected and the potential applied positive at the drain negative at the source a drain current Id flows. When the gate is biased negative with respective to the source the PN junctions are reverse biased and depletion regions are formed. The channel is more lightly doped than the P type gate blocks, so the depletion regions penetrate deeply into the channel. Since depletion region is a region depleted of charge carriers it behaves as an Insulator. The result is that the channel is narrowed. Its resistance is increased and Id is reduced. When the negative gate bias voltage is further increased, the depletion regions meet at the center and Id is cut off completely.
There are two ways to control the channel width

1. By varying the value of Vgs
2. And by Varying the value of Vds  holding Vgs constant

1 By varying the value of Vgs :- 
We can vary the width of the channel and in turn vary the amount of drain current. This can be done by varying the value of Vgs. This point is illustrated in the fig  below. Here we are dealing with N channel FET. So channel is of N type and gate is of P type that constitutes a PN junction. This PN junction is always reverse biased in JFET operation .The reverse bias is applied by a battery voltage Vgs connected between the gate and the source terminal i.e positive terminal of the battery is connected to the source and negative terminal to gate.

1. When a PN junction is reverse biased the electrons and holes diffuse across junction by leaving immobile ions on the N and P sides , the region containing these immobile ions  is known as depletion regions.
2. If both P and N regions are heavily doped then the depletion region extends symmetrically on both sides.
3. But in N channel FET P region is heavily doped than N type thus depletion region extends more in N region than P region.
4. So when no Vds is applied the depletion region is symmetrical and the conductivity becomes Zero. Since there are no mobile carriers in the junction.
5. As the reverse bias voltage is increases the thickness of the depletion region also increases.  i.e. the effective channel width decreases .
6. By varying the value of Vgs we can vary the width of the channel.

2 Varying the value of Vds  holding Vgs constant :-

1. When no voltage is applied to the gate i.e. Vgs=0 , Vds is applied between source and drain the electrons will flow from source to drain through the channel constituting drain current Id .
2. With Vgs= 0 for Id= 0 the channel between the gate junctions is entirely open .In response to a small applied voltage Vds , the entire bar acts as a simple semi conductor resistor and the current Id increases linearly with Vds .
3. The channel resistances are represented as rd and rs as shown in the fig.

 

1. This increasing drain current Id produces a voltage drop across rd which reverse biases the gate to source junction,(rd> rs) .Thus the depletion region is formed which is not symmetrical .
2. The depletion region i.e. developed penetrates deeper in to the channel near drain and less towards source because Vrd >> Vrs. So reverse bias is higher near drain than at source.
3. As a result growing depletion region reduces the effective width of the channel. Eventually a voltage Vds is reached at which the channel is pinched off. This is the voltage where the current Id begins to level off and approach a constant value.
4.  So, by varying the value of Vds we can vary the width of the channel holding Vgs constant.

 
When both Vgs and Vds is applied:-

It is of course in principle not possible for the channel to close Completely and there by reduce the current Id to Zero for,  if such indeed, could be the case the gate voltage Vgs is applied in the direction to provide additional reverse bias

1. When voltage is applied between the drain and source with a battery Vdd, the electrons flow from source to drain through the narrow channel existing between the depletion regions. This constitutes the drain current Id, its conventional direction is from drain to source.
2. The value of drain current is maximum  when no external voltage is applied between gate and source and is designated by Idss.

 

 

 

1. When Vgs is increased beyond Zero the depletion regions are widened. This reduces the effective width of the channel and therefore controls the flow of drain current through the channel.
2. When Vgs is further increased a stage is reached at which to depletion regions touch each other that means the entire channel is closed with depletion region. This reduces the drain current to Zero.

 

6.4 CHARACTERISTICS OF N-CHANNEL JFET :-
The family of curves that shows the relation between current and voltage are known as characteristic curves.
There are two important characteristics of a JFET.

1. Drain or VI Characteristics
2. Transfer characteristics

1. Drain Characteristics:-

                                   Drain characteristics shows the relation between the drain to source voltage Vds and drain current Id. In order to explain typical drain characteristics let us  consider the curve with Vgs= 0.V.

1. When Vds is applied and it is increasing the drain current ID also increases linearly up to knee point.
2. This shows that FET behaves like an ordinary resistor.This region is called as ohmic region.
3. ID increases with increase in drain to source voltage. Here the drain current is increased slowly as compared to ohmic region.

1.  
2.  
3.  

 

 

 

 

4) It is because of the fact that there is an increase in VDS .This in turn increases the reverse bias voltage across the gate source junction .As a result of this depletion region grows in size thereby reducing the effective width of the channel.

5) All the drain to source voltage corresponding to point the channel width is reduced to a minimum value  and is known as pinch off.

6) The drain to source voltage at which channel pinch off occurs is called pinch off voltage(Vp).

PINCH OFF Region:-

1. This is the region shown by the curve  as saturation region.
2. It is also called as saturation region or constant current region. Because of the channel is occupied with depletion region , the depletion region is more towards the drain and less towards the source, so the channel is limited, with this only limited number of carriers are only allowed to cross this channel from source drain causing a current that is constant in this region. To use FET as an amplifier it is operated in this saturation region.
3. In this drain current remains constant at its maximum value IDSS.

1. The drain current in the pinch off region depends upon the gate to source voltage and is given by the relation

 

                                 Id =Idss [1-Vgs/Vp]2
This is known as shokley’s relation.
BREAKDOWN REGION:-

1. The region is shown by the curve .In this region, the drain current increases rapidly as the drain to source voltage is increased.
2. It is because of the gate to source junction due to avalanche effect.
3. The avalanche break down occurs at progressively lower value of VDS because the reverse bias gate voltage adds to the drain voltage thereby increasing effective voltage across the gate junction

        This causes

1. The maximum saturation drain current is smaller
2. The ohmic region portion decreased.
1. It is important to note that the maximum voltage VDS which can be applied to FET is the lowest voltage which causes available break down.

1.  TRANSFER CHARACTERISTICS:-

                      These curves shows the relationship between drain current ID  and gate to source voltage VGS   for different values of VDS.

1. First adjust the drain to source voltage to some suitable value , then increase the gate to source voltage in small suitable value.
2. Plot the graph between gate to source voltage along the horizontal axis and current ID on the vertical axis. We shall obtain a curve like this.

 

                                                                                   

1. As we know that if Vgs  is more negative curves drain current to reduce . where Vgs is made sufficiently negative, Id is reduced to zero. This is caused by the widening of the depletion region to a point where it is completely closes the channel. The value of Vgs at the cutoff point is designed as Vgsoff

 

1. The upper end of the curve as shown by the drain current value is equal to Idss that is when Vgs = 0 the drain current is maximum.

1. While the lower end is indicated by a voltage equal to Vgsoff
2. If Vgs continuously increasing , the channel width is reduced , then Id =0
3. It may be noted that curve is part of the parabola; it may be expressed as

Id=Idss[1-Vgs/Vgsoff]2
DIFFERENCE BETWEEN Vp AND Vgsoff –
Vp is the value of Vgs that causes the JFET to become constant current component, It is measured at Vgs =0V and has a constant drain current of Id =Idss .Where Vgsoff is the value of Vgs that reduces  Id to approximately zero.
Why the gate to source junction of a JFET be always reverse biased ?
The gate to source junction of a JFET is never allowed to become forward biased because the gate material is not designed to handle any significant amount of current. If the junction is allowed to become forward biased, current is generated through the gate material. This current may destroy the component.
There is one more important characteristic of JFET reverse biasing i.e. J FET ‘s have extremely high characteristic gate input impedance. This impedance is typically in the high mega ohm range. With the advantage of extremely high input impedance it draws no current from the source. The high input impedance of the JFET has led to its extensive use in integrated circuits. The low current requirements of the component makes it perfect for use in ICs. Where thousands of transistors must be etched on to a single piece of silicon. The low current draw helps the IC to remain relatively cool, thus allowing more components to be placed in a smaller physical area.
6.5 JFET PARAMETERS
The electrical behavior of JFET may be described in terms of certain parameters. Such parameters are obtained from the characteristic curves.
A C Drain resistance(rd):
It is also called dynamic drain resistance and is the a.c.resistance between the drain and source terminal,when the JFET is operating in the pinch off or saturation region.It is given by the ratio of small change in drain to source voltage  ∆Vds to  the  corresponding change in drain current   ∆Id for a constant gate to source voltage Vgs.
Mathematically it is expressed as rd=∆Vds/ ∆Id where Vgs is held constant.
TRANCE CONDUCTANCE (gm):
It is also called forward transconductance  . It is given by the ratio of small change in drain current (∆Id) to the corresponding change in gate to source voltage (∆Vds)
Mathematically  the transconductance can be written as
gm=∆Id/∆Vds
AMPLIFICATION FACTOR (µ)
It is given by the ratio of small change in drain to source voltage (∆Vds) to the corresponding change in gate to source voltage (∆Vgs)for a constant drain current (Id).
Thus    µ=∆Vds/∆Vgs   when Id held constant
The amplification factor µ may be expressed as a product of transconductance (gm)and ac drain resistance (rd)
µ=∆Vds/∆Vgs=gm rd

 

6.6 THE FET SMALL SIGNAL MODEL:- 
The linear small signal equivalent circuit for the FET can be obtained in a manner similar to that used to derive the corresponding model for a transistor.
We can express the drain current iD as a function f of the gate voltage and drain voltage Vds.
Id =f(Vgs,Vds)------------------(1)
The transconductance gm and drain resistance rd:-
If both gate voltage and drain voltage are varied, the change in the drain current is approximated by using taylors series considering only the first two terms in the expansion
∆id=|vds=constant .∆vgs|vgs=constant∆vds
we can write ∆id=id
∆vgs=vgs
∆vds=vds
Id=gm v   Vds→(1)
Where gm=|Vds    |Vds
gm=|Vds
Is the mutual  conductance or transconductance .It is also called as gfs or yfs common source forward conductance .
The second parameter rd  is the drain resistance or output resistance is defined as
rd=|Vgs |Vgs=|Vgs   
rd=|Vgs
The reciprocal of the rd is the drain conductance gd .It is also designated by Yos and Gos and called the common source output conductance . So the small signal equivalent circuit for FET  can be drawn in two different ways.
1.small signal current –source model
2.small signal voltage-source model.
A small signal current –source model for FET in common  source configuration can be drawn satisfying  Eq→(1) as shown in the figure(a)
This low frequency model for FET has a Norton’s output circuit with a dependent current generator whose magnitude is proportional to the gate-to –source voltage. The proportionality factor is the transconductance ‘gm’. The output resistance is ‘rd’. The input resistance between the gate and source is infinite, since it is assumed that the reverse biased gate draws no current. For the same reason the resistance between gate and drain is assumed to be infinite.
The small signal voltage-source model is shown in the figure(b).
This can be derived by finding the Thevenin’s equivalent for the output part of fig(a) .
These small signal models for FET can be used for analyzing the three basic FET amplifier configurations:
1.common source (CS)  2.common drain (CD) or source follower
3. common gate(CG).
(a)Small Signal  Current source model for FET            (b)Small Signal voltage source model for FET
Here the input circuit is kept open because of having high input impedance and the output circuit satisfies the equation for ID
6.7 MOSFET:-
We now turn our attention to the insulated gate FET or metal oxide semi conductor FET which is having the greater commercial importance than the junction FET.
Most MOSFETS however are triodes, with the substrate  internally connected to the source. The circuit symbols used by several manufacturers are indicated in the Fig below.

(a) Depletion type MOSFET                                 (b) Enhancement type MOSFET
                                    Both of them are P- channel
Here are two basic types of MOSFETS
(1) Depletion type           (2) Enhancement type MOSFET.
D-MOSFETS can be operated in both the depletion mode and the enhancement mode. E MOSFETS are restricted to operate in enhancement mode. The primary difference between them is their physical construction.
The construction difference between the two is shown in the fig given below.

 

           As we can see the D MOSFET have physical channel between the source and drain terminals(Shaded area)

          The E MOSFET on the other hand has no such channel physically. It depends on the gate voltage to form a channel between the source and the drain terminals.
Both MOSFETS have an insulating layer between the gate and the rest of the component. This insulating layer is made up of SIO2 a glass like insulating material. The gate material is made up of metal conductor .Thus going from gate to substrate, we can have metal oxide semi conductor which is where the term MOSFET comes from.
Since the gate is insulated from the rest of the component, the MOSFET is sometimes referred to as an insulated gate FET or IGFET.
The foundation of the MOSFET is called the substrate. This material is represented in the schematic symbol by the center line that is connected to the source.
In the symbol for the MOSFET, the arrow is placed on the substrate. As with JFET an arrow pointing in represents an N-channel device, while an arrow pointing out represents p-channel device.

CONSTRUCTION OF AN N-CHANNEL MOSFET:-
The N- channel MOSFET consists of a lightly doped p type substance into which two heavily doped n+ regions are diffused as shown in the Fig. These n+ sections , which will act as source and drain.         A thin layer of insulation silicon dioxide (SIO2) is grown over the surface of the structure, and holes are cut into oxide layer, allowing contact with the source and drain. Then the gate metal area is overlaid on the oxide, covering the entire channel region.Metal contacts are made to drain and source and the contact to the metal over the channel area is the gate terminal.The metal area of the gate, in conjunction with the insulating dielectric oxide layer and the semiconductor channel, forms a parallel plate capacitor. The insulating layer of sio2
Is the reason why this device is called the insulated gate field effect transistor. This layer results in an extremely high input resistance (10 10 to 10power 15ohms) for MOSFET.
6.7.1 DEPLETION MOSFET
The basic structure of D –MOSFET is shown in the fig. An N-channel is diffused between source and drain with the device an appreciable drain current IDSS flows foe zero gate to source voltage, Vgs=0.

Depletion mode operation:-

1. The above fig shows the D-MOSFET operating conditions with gate and source terminals shorted together(VGS=0V)

1. At this stage ID= IDSS where VGS=0V, with this voltage VDS, an appreciable drain current IDSS flows.

 

1. If the gate to source voltage is made negative  i.e. VGs is negative .Positive charges are induced in the channel through the SIO2 of the gate capacitor.

1. Since the current in a FET is due to majority carriers(electrons for an N-type material) , the induced positive charges make the channel less conductive and the drain current drops as Vgs is made more negative.

 

1. The re distribution of charge in the channel causes an effective depletion of majority carriers , which accounts for the designation depletion MOSFET.

1. That means biasing voltage Vgs depletes the channel of free carriers This effectively reduces the width of the channel , increasing its resistance.

 

1. Note that negative Vgs has the same effect on the MOSFET as it has on the JFET.

                                                                                 

1. As shown in the fig above, the depletion layer generated by Vgs (represented by the white space between the insulating material and the channel) cuts into the channel, reducing its width. As a result ,Id<Idss.The actual value of ID depends on the value of Idss,Vgs(off) and Vgs.

Enhancement mode operation of the D-MOSFET:-

1. This operating mode is a result of applying a positive gate to source voltage Vgs to the device.
2. When Vgs is positive the channel is effectively widened.  This reduces the resistance of the channel allowing ID to exceed the value of IDSS
3. When Vgs is given positive the majority carriers in the p-type are holes. The holes in the p type substrate are repelled by the +ve gate voltage.
4. At the same time, the conduction band electrons (minority carriers) in the p type material are attracted towards the channel by the +gate voltage.
5. With the build up of electrons near the channel , the area to the right of the physical channel effectively becomes an N type material.
6. The extended n type channel now allows more current, Id> Idss

 

                                                                                   

Characteristics of Depletion MOSFET:-
The fig. shows the drain characteristics for the N channel depletion type MOSFET

1. The curves are plotted for both Vgs positive and Vgs negative voltages

.

1. When Vgs=0 and negative the MOSFET operates in depletion mode when Vgs is positive ,the MOSFET operates in the enhancement mode.
2. The difference between JFET and D MOSFET is that JFET does not operate for positive values of Vgs.

1. When Vds=0, there is no conduction takes place between source to drain, if Vgs<0 and Vds>0 then Id increases linearly.

 

1. But as Vgs,0 induces positive charges holes in the channel, and controls the channel width. Thus the conduction between source to drain is maintained as constant, i.e. Id is constant.

1. If Vgs>0 the gate induces more electrons in channel side, it is added with the free electrons generated by source. again the potential applied to gate determines the channel width and maintains constant current flow through it as shown in Fig

 

 

                                                                           

 

TRANSFER CHARACTERISTICS:-
The combination of 3 operating states i.e. Vgs=0V, VGs<0V, Vgs>0V is represented by the D MOSFET transconductance curve shown in Fig.

1. Here in this curve it may be noted that the region AB of the characteristics similar to that of JFET.

1. This curve extends for the positive values of Vgs

 

1. Note that Id=Idss for Vgs=0V when Vgs is negative,Id< Idss when Vgs= Vgs(off) ,Id is reduced to approximately omA.Where Vgs is positive Id>Idss.So obviously Idss is not the maximum possible value of Id for a MOSFET.

1. The curves are similar to JFET so thet the D MOSFET have the same transconductance equation.

 

6.7.2 E-MOSFETS
The E MOSFET is capable of operating only in the enhancement mode.The gate potential must be positive w.r.t to source.

1. when the value of Vgs=0V, there is no channel connecting the source and drain materials.

1. As aresult , there can be no significant amount of drain current.

 

1. When Vgs=0, the Vdd supply tries to force free electrons from source to drain but the presence of p-region does not permit the electrons to pass through it. Thus there is no drain current at Vgs=0,

1. If Vgs is  positive, it induces a negative charge in the p type substrate just adjacent to the SIO2 layer.

 

1. As the holes are repelled by the positive gate voltage, the minority carrier electrons attracted toward this voltage. This forms an effective N type bridge between source and drain providing a path for drain current.

1. This +ve gate voltage forma a channel between the source and drain.

 

1. This produces a thin layer of N type channel in the P type substarate.This layer of free electrons is called N type inversion layer.

 

                                                                    

 

1.  The minimum Vgs which produces this inversion layer is called threshold voltage and is designated by Vgs(th).This is the point at which the device turns on is called the threshold voltage Vgs(th)
2.  When the voltage Vgs is <Vgs (th) no current flows from drain to source.

1. How ever when the voltage Vgs > Vgs (th) the inversion layer connects the drain to source and we get significant values of current.

 

CHARACTERISTICS OF E MOSFET:-  

1. DRAIN CHARACTERISTICS

The volt ampere drain characteristics of an N-channel enhancement mode MOSFET are given in the fig.

 

1. TRANSFER CHARACTERISTICS:-
1. The current Idss at Vgs≤ 0 is very small beinf of the order of a few nano amps.
2. As Vgs is made +ve , the current Id increases slowly at forst, and then much more rapidly with an increase in Vgs.
3. The standard transconductance formula will not work for the E MOSFET.
4. To determine the value of ID at a given value of VGs we must use the following relation

Id =K[Vgs-Vgs(Th)]2
Where K is constant for the MOSFET . found as
K=
From the data specification sheets, the 2N7000 has the following ratings.
Id(on)= 75mA(minimum).
And     Vgs(th)=0.8(minimum)

6.8 APPLICATION OF MOSFET
One of the primary contributions to electronics made by  MOSFETs  can be found in the area of digital (computer electronics). The signals in digital circuits are made up of rapidly switching dc levels. This signal is called as a rectangular wave ,made up of two dc levels (or logic levels). These logic levels are 0V and +5V.
A group of circuits with similar circuitry and operating characteristics is referred to as a logic family. All the circuits in a given logic family respond to the same logic levels, have similar speed and power-handling capabilities  , and can be directly connected together. One such logic family is complementary MOS (or CMOS)  logic. This logic family is made up entirely of  MOSFETs.
6.9 BIASING FET:-
For the proper functioning of a linear FET amplifier, it is necessary to maintain the operating point Q stable in the central portion of the pinch off region The Q point should be independent of device parameter variations and ambient temperature variations
This can be achieved by suitably selecting the gate to source voltage VGS and drain current ID which is referred to as biasing
JFET biasing circuits are very similar to BJT biasing circuitsThe main difference between JFET circuits and BJT circuits is the operation of the active components themselves
There are mainly two types of Biasing circuits

1. Self bias
2. Voltage divider bias.

6.9.1 SELF BIAS
Self bias is a JFET biasing circuit that uses a source resistor to help reverse bias the JFET gate. A  self bias circuit is shown in the fig. Self bias is the most common type of JFET bias. This JFET must be operated such that gate source junction is always reverse biased. This condition requires a negative VGS for an N channel JFET and a positive VGS for P channel JFET. This can be achieved using the self bias arrangement as shown in Fig. The gate resistor RG doesn’t affect the bias because it has essentially no voltage drop across it, and : the gate remains at 0V .RG is necessary only to isolate an ac signal from ground in amplifier applications. The voltage drop across resistor RS makes gate source junction reverse biased.

For the dc analysis coupling capacitors are open circuits.
For the N channel FET in Fig (a)
IS produces a voltage drop across RS and makes the source positive  w.r.t ground. In any JFET circuit all the source current passes through the device to the drain circuit .This is due to the fact that there is no significant gate current.
We can define source current as  IS = ID
(VG =0 because there is no gate current flowing in RG So VG across RG is zero)
VG =0 then VS= ISRS =ID RS
VGS = VG-VS =0-ID RS=- ID RS
DC analysis of self Bias:-
In the following DC analysis, the N channel J FET shown in the fig. is used for illustration.
For DC analysis we can replace coupling capacitors by open circuits and we can also replace the resistor RG by a short circuit equivalent.:. IG = 0.The relation between ID and VGS is given by
Id=Idss[1-]2                                  
VGS for N channel JFET is =-id Rs
Substuting  this value in the above equation
Id=Idss[1-]2
Id=Idss[1+]2
For the N-chanel FET in the above figure
Is produces a voltage drop across Rs and makes the source positive w.r.t ground in any JFET circuit all the source current passes through the device to drain circuit this is due to the fact that there is no significant gate current. Therefore we can define source current as Is=Id and Vg=0 then
Vs= Is Rs =IdRs
Vgs=Vg-Vs=0-IdRs=-IdRs
Drawing the self bias line:-
Typical transfer characteristics for a self biased JFET are shown in the fig.
The maximum drain current is 6mA and the gate source cut off voltage is -3V. This means the gate voltage has to be between 0 and -3V.

Now using the equation VGS = -IDRS and assuming RS of any suitable value we can draw the self bias line.
Let us assume RS = 500Ω
With this Rs , we can plot two points corresponding to ID = 0 and Id = IDSS
for ID = 0
VGS = -ID RS
VGS = 0X (500.Ω) = 0V
So the first point is (0 ,0)
( Id, VGS)
For ID= IDSS=6mA
VGS = (-6mA) (500 Ω) = -3V
So the 2nd  Point will be (6mA,-3V)
By plotting these two points, we can draw the straight line through the points. This line will intersect the transconductance curve and it is known as self bias line.The intersection point gives the operating point of the self bias JFET for the circuit.
At Q point , the ID is slightly  > than 2mA and VGS is slightly > -1V. The Q point for the self bias JFET depends on the value of Rs.If Rs is large, Q point far down on the transconductance curve ,ID is small, when Rs is small Q point is far up on the curve , ID is large.
6.9.2 VOLTAGE DIVIDER BIAS:-

The fig. shows N channel JFET with voltage divider bias. The voltage at the source of JFET must be more positive than the voltage at the gate in order to keep the gate to source junction reverse biased. The source voltage is
VS = IDRS
The gate voltage is set by resistors R1 and R2 as expressed by the following equation using the voltage divider formula.
Vg=Vdd
For dc analysis              

Applying KVL to the input circuit
VG-VGS-VS =0
:: VGS = VG-Vs=VG-ISRS
VGS = VG-IDRS       :: IS = ID
Applying KVL to the input circuit we get
VDS+IDRD+VS-VDD =0
::VDS = VDD-IDRD-IDRS
VDS = VDD-ID ( RD +RS )
The Q point of a JFET amplifier , using the voltage divider bias is 
IDQ  = IDSS [1-VGS/VP]2
VDSQ = VDD-ID ( RD+RS )
COMPARISON OF MOSFET WITH JFET

1. In enhancement and depletion types of MOSFET, the transverse electric field induced across an insulating layer deposited on  the semiconductor material controls the conductivity of the channel.
2. In the JFET the transverse electric field across the reverse biased PN junction controls the conductivity of the channel.
3. The gate leakage current in a MOSFET is of the order of 10-12A. Hence the input resistance of a MOSFET is very high in the order of 1010  to  1015 Ω.  The gate leakage current of a JFET  is of the order of 10-9A., and its input resistance is of the order of 108Ω.
4. The output characteristics of the JFET are flatter than those of the MOSFET, and hence the drain resistance of a JFET (0.1 to 1MΩ)  is  much higher than that of a MOSFET (1 to 50kΩ).
5. JFETs are operated only in the depletion mode. The depletion type MOSFET may be operated in both depletion and enhancement mode.
6. Comparing to JFET, MOSFETs  are easier to fabricate.
7. Special digital CMOS circuits are available which involve near zero power dissipation and very low voltage and current requirements. This makes them suitable for portable systems.