CHAPTER 6 Field Effect Transistors (FETs)

1. CHAPTER 6Field Effect Transistors (FETs) Acknowledged to: Shahrul Ashikin Azmi (PPKSE)

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

3. Introduction • FET – a three-terminal voltage-controlled device used in amplification and switching application. • Field effect transistors controls current by voltage applied to the gate. The FET’s major advantage over the BJT is high input resistance. • 2 basic type of FET: JFET and MOSFET

4. 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. VGGvoltage levels control current flow in theVDD, RD circuit.

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

6. JFET • VDD provide a drain-to-source voltage. • VGG sets the reverse-bias voltage between gate and source. JFET is always operated with gate-source pn junction reverse-biased. Reverse-biasing of the gate-source junction with a –ve gate voltage produces a depletion region along pn junction.

7. JFET Biasing • Gate-to-source junction of JFET always reverse-biased under normal condition. • Gate-to-source junction never allowed to become forward-biased because the gate material is not designed to handle any significant amount of current  may destroy the component. • The fact gate is always reverse-biased leads to important feature  JFET has high gate input impedance; typically in high megaohm range. • This feature result to JFET extensively being used in integrated circuits. Low current draw helps IC remain cool, thus allowing more components to be placed in a smaller physical area.

8. The JFET is always operated with the gate-source pn junction reverse-biased. • Reverse biasing of the gate-source junction with a negative gate voltage produces a depletion region along the pn junction, which extends into the n channel • thus increases its resistance by restricting the channel width. • The channel width and the channel resistance can be controlled by varying the gate voltage, thereby controlling the amount of drain current, ID.

11. JFET Characteristics and Parameters Let’s first take a look at the effects with a VGS of 0V. This is produced by shorting the gate to source junction.

12. JFET Drain Curve

13. Refer to JFET drain curve from point A to B, ID increases proportionally with increases of VDD (VDS increases as VDD increases). In this area, the channel resistance is essentially constant because the depletion region is not large enough to have significant effect. [V=IR] This is called the ohmic region (point A to B) because VDS and ID are related by Ohm’s Law.

14. At point B, the curve levels off and ID becomes constant. • The point when ID ceases to increase regardless of VDD increases is called the pinch-off voltage, VP (point B). • This current is called maximum drain current (IDSS) and always specified for the condition, VGS=0V. This area is called constant-current area. • Breakdown (point C) occur when ID begins to increase rapidly with any increase in VDS. This of course undesirable, so JFETs operation is always well below this value.

15. Illustration of JFET Drain Curve

16. JFET Characteristics and Parameters From this set of curves you can see with increased voltage applied to the gate, ID decrease and JFET reaches pinch-off at values of VDS less than VP.

17. JFET Characteristics and Parameters

18. JFET Characteristics and Parameters

19. JFET Characteristics and Parameters • We know that as VGSis increased IDwill decrease. The point that ID ceases decrease is called cutoff. The amount of VGSrequired to do this is called the cutoff voltage (VGS(off )). • The more negative VGS, the smaller ID becomes. When VGS has sufficiently large negative value, ID is reduced to zero. It is interesting to note that pinch-off voltage (Vp) and cutoff voltage (VGS(off)) are both the same value only opposite polarity.

20. JFET Transfer Characteristic • For n-channel JFET, VGS(off) is negative and for p-channel, VGS(off) is positive. • Bottom end of the curve is at a point on VGS axis equal to VGS(off) and the top end of the curve is at a point on ID axis equal to IDSS (shorted-gate drain current rating of the device). • The operating limits of JFET are: • ID=0 when VGS=VGS(off) • ID=IDSS when VGS = 0 • Transfer characteristic curve can be developed from drain characteristic curves by plotting values of ID for the values of VGS taken from the family of drain curves at pinch-off. Show example 8-1 (pg391)

21. JFET Characteristics and Parameters The transfer characteristic curve illustrates the control VGShas on IDfrom cutoff (VGS(off) ) to pinchoff (VP). Note the parabolic shape. The formula below can be used to determine drain current. All these values are usually available from data sheet. ID = IDSS(1 - VGS/VGS(off))2

22. JFET Forward Transfer Conductance Forward transfer conductance, gm of JFETs is the changes in ID based on changes in VGS with VDS is constant. Forward transfer conductance referred to as gm = ∆ID /∆VGS. Unit is Siemens (s) The value is larger at the top of the curve (near VGS=0) but become smaller as you increase VGS (near VGS(off)).

23. Transconductance At VGS =0, the parameter is known as minimum transfer conductance, gmoand can be calculated using this equation: gmo = 2IDSS/|VGS(off)| and gm = gmo(1 - VGS/VGS(off)) gmo can be read from the datasheet as gfs or yfs and sometimes written as Forward Transfer Admittance.

24. Example

25. 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 IGSS. This internal input resistance can be calculated at different VGS : RIN=|VGS/IGSS| As IGSS increases with temperature, RIN will decrease.

26. JFET Input Resistance Example 1: Calculate RIN if IGSS=-2nA and VGS=-20V Solution: RIN=|VGS/IGSS|=|-20/-2n|=10G

27. JFET Biasing Circuit • Just as we learned that the bi-polar junction transistor must be biased for proper operation, the JFET also must be biased for operation. • Let’s 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 IDSS. • 4 types of bias method are self-bias, gate-bias, voltage-divider bias and current-source bias.

28. JFET Biasing- Self bias • Self-bias is the most common type of biasing method for JFETs. • Notice there is no voltage applied to the gate, VG=0V. • However, the voltage from gate to source (VGS) will be negative for n channel and positive for p channel to keep the junction reverse biased.

29. JFET Biasing- Self bias • Uses a source resistor to help reverse bias JFET gate. The gate is returned to ground via RG, and RS has been added to source circuit. • This voltage can be determined using the formulas below. • ID= ISfor all JFET circuits. VG=0 and VS=IDRS. • VGS = VG - VS • (n channel) VGS = 0-IDRS • =-IDRS • (p channel) VGS = 0-(-IDRS ) • =IDRS

30. JFET Biasing – self bias • Keep in mind that analysis of p-channel is the same as n-channel except for opposite polarity voltages. • The drain voltage with respect to ground is: VD = VDD – IDRD • Since VS = IDRS, VDS is: VDS = VD – VS = VDD – ID(RD+RS)

32. JFET Biasing-self bias • Setting the Q-point requires us to determine a value of RS that will give us the desired ID and VGS. The formula below shows the relationship. RS = | VGS/ID | • For a desired value of VGS,ID can be determinedfrom the either the transfer characteristic curve or more practically from the formula below. The data sheet provides the IDSSand VGS(off). ID = IDSS(1 - VGS/VGS(off))2

34. JFET Midpoint Biasing-self bias-formula method Midpoint biasing- desirable to bias a JFET near the midpoint of its transfer characteristic curve where ID =IDSS / 2. IDis approximately one-half of IDSSwhen: VGS VGS(off)/3.4

35. JFET Midpoint Biasing-self bias-formula method The value of RSneeded to establish VGS can be determined by the relationship below. RS = |VGS/ID | To set the drain voltage at midpoint (VD=VDD/2), select a value of RD to produce the desired voltage drop. The value of RDneeded can be determined by taking half of VDD and dividing it by ID. RD = (VDD/2)/ID

37. JFET Biasing- self-bias Remember the purpose of biasing is to set a dc operating point (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 RSand RD. Setting it at midpoint on the drain curve is most common. One thing not mentioned in the discussion is RG. It’s value is arbitrary large to prevent loading on the driving stage in a cascaded amplifier arrangement.

38. JFET Midpoint Biasing-self bias-graphical method 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, establish dc load line by calculating VGS. VGS = -IDRS for ID=0 and ID=IDSS With 2 points (ID=0 and ID=IDSS), draw dc load line on the transfer characteristic curve. The point where the two lines intersect gives us the ID and VGS(Q-point) needed for mid-point bias. Note that load line extends from VGS(off)(ID= 0A) to VP(ID = IDSS)

39. JFET Midpoint Biasing-self bias-graphical method

41. JFET Biasing- voltage divider bias-formula • Voltage-divider bias can also be used to bias a JFET. R1and R2are used to keep the gate-source junction in reverse bias. Operation is no different from self-bias. Determining VGS for a JFET voltage-divider circuit with givenVD can be calculated with the formulas below. Source voltage,VS = IDRS Gate voltage, VG =(R2/R1+R2)VDD Gate-to-source voltage.VGS=VG –VS Source voltage, VS = VG - VGS

42. JFET Biasing- voltage-divider bias - formula • VS must be more +ve than VG in order to keep VGS reverse-biased (-ve value). Drain current, ID = (VDD – VD)/RD or Since ID=IS, then ID=VS/RS

44. JFET Biasing-voltage-divider bias-graphical • In using the transfer characteristic curve to determine the approximate Q-point we must establish the 2 points for the load line. • 1st step draw dc load line: • The first point is ID = 0 and VGS =VG. VS=IDRS=(0)RS=0V VGS=VG-VS=VG-0=VG • The 2nd point is for VGS=0, ID=(VG-VGS) / RS = VG / RS ID=VG / RS and VGS=0.

45. The point at which the load line intersect with transfer characteristic curve is Q-point. Dc load line for JFET with voltage-divider bias

48. JFET Biasing – Current Source Bias • Current source bias  provides high Q-point stability by making value of ID independently of JFET. • From figure, JFET drain current equals BJT collector current. IDQ = IC • In this circuit, a BJT acts as the constant-current source because its emitter current is essentially constant if • A FET can also be used as a constant current source.

49. JFET Biasing- Current source bias • Advantage: provide the most stable Q-point value of ID. • Disadvantage: circuit complexity makes it undesirable for most applications.