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Field Effect Transistors. Introduction Two main types of FET: - JFET –Junction field effects transistor -MOSFET – Metal oxide semiconductor field effect.

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Presentation on theme: "Field Effect Transistors. Introduction Two main types of FET: - JFET –Junction field effects transistor -MOSFET – Metal oxide semiconductor field effect."— Presentation transcript:

1 Field Effect Transistors

2 Introduction Two main types of FET: - JFET –Junction field effects transistor -MOSFET – Metal oxide semiconductor field effect transistor -D-MOSFET ~ Depletion MOSFET -E-MOSFET ~ Enhancement MOSFET Similarities: -Amplifiers -Switching devices -Impedance matching circuits Differences: - FETs are voltage controlled devices whereas BJTs are current controlled devices. -FETs also have a higher input impedance, but BJTs have higher gains. -FETs are less sensitive to temperature variations and more easily integrated on ICs. - FETs are also generally more static sensitive than BJTs.

3 Construction and characteristics of JFET N-channel device will appear as the prominent device with paragraph and section devoted to the impact of using a p-channel. Major part of structure is n-type material. Top of the n-type channel is connected through an ohmic contact to a terminal referred to as the drain (D) The lower end-connected through an ohmic contact to a terminal referred as source (S) P-type materials are connected together and to the gate (G) terminal. JFET has two p-n junctions under no- bias conditions.

4 JFET operation can be compared to a water faucet: The source of water pressure – accumulated electrons at the negative pole of the applied voltage from Drain to Source The drain of water – electron deficiency (or holes) at the positive pole of the applied voltage from Drain to Source. The control of flow of water – Gate voltage that controls the width of the n-channel, which in turn controls the flow of electrons in the n-channel from source to drain. Construction and characteristics of JFET

5 N-Channel JFET Circuit Layout Construction and characteristics of JFET

6 JFET Operating Characteristics There are three basic operating conditions for a JFET: A.V GS = 0, V DS increasing to some positive value B. V GS < 0, V DS at some positive value C. Voltage-Controlled Resistor

7 V GS = 0, V DS increasing to some positive value Three things happen when V GS = 0 and V DS is increased from 0 to a more positive voltage: the depletion region between p-gate and n-channel increases as electrons from n-channel combine with holes from p-gate. increasing the depletion region, decreases the size of the n-channel which increases the resistance of the n-channel. But even though the n-channel resistance is increasing, the current (I D ) from Source to Drain through the n-channel is increasing. This is because V DS is increasing.

8 The flow of charge is relatively uninhibited and limited solely by the resistance of the n-channel between drain and source. The depletion region is wider near the top of both p-type materials. ID will establish the voltage level through the channel. The result: upper region of the p-type material will be reversed biased by about 1.5V with the lower region only reversed biased by 0.5V (greater applied reverse bias, the wider depletion region). V GS = 0, V DS increasing to some positive value

9 I G =0A p-n junction is reverse-biased for the length of the channel results in a gate current of zero amperes. As the V DS is increased from 0 to a few volts, the current will increase as determined by Ohms Law. V DS increase and approaches a level referred to as Vp, the depletion region will widen, causing reduction in the channel width. (p large, n small). Reduced part of conduction causes the resistance to increase. If V DS is increased to a level where it appears that the 2 depletion regions would touch (pinch-off) V GS = 0, V DS increasing to some positive value

10 Vp = pinch off voltage. I D maintain the saturation level defined as I DSS Once the V DS > V P, the JFET has the characteristics of a current source. As shown in figure, the current is fixed at I D = I DSS, the voltage V DS (for level >V p ) is determined by the applied load. I DSS is derived from the fact that it is the drain-to-source current with short circuit connection from gate to source. I DSS is the max drain current for a JFET and is defined by the conditions V GS =0V and V DS > | V p |. V GS = 0, V DS increasing to some positive value

11 At the pinch-off point: any further increase in V GS does not produce any increase in I D. V GS at pinch-off is denoted as V p. I D is at saturation or maximum. It is referred to as I DSS. The ohmic value of the channel is at maximum. V GS = 0, V DS increasing to some positive value

12 Typical JFET operation

13 JFET modeling when I D =I DSS, V GS =0, V DS >V P

14 V GS < 0, V DS at some positive value V GS is the controlling voltage of the JFET. For n-channel devices, the controlling voltage V GS is made more and more negative from its V GS = 0V level. The effect of the applied negative V GS is to establish depletion regions similar to those obtained with V GS =0V but a lower level of V DS to reach the saturation level at a lower level of V DS.

15 When V GS = -V p will be sufficiently negative to establish saturation level that is essentially 0mA, the device has been turn off. The level of the V GS that results in I D = 0 mA is defined by V GS = V p, with V p being a negative voltage for n-channel devices and a positive voltage or p-channel JFETs. In this region, JFET can actually be employed as a variable resistor whose resistance is controlled by the applied gate to source voltage. A V GS becomes more and more negative; the slope of each curve becomes more and more horizontal. VGS < 0, VDS at some positive value

16 The region to the right of the pinch-off locus of the figure is the region typically employed in linear amplifiers (amplifiers with min distortion of the applied signal) and is commonly referred to as the constant-current, saturation, or linear amplification region. VGS < 0, VDS at some positive value

17 Characteristic curves for N- channel JFET

18 Voltage-Controlled Resistor The region to the left of the pinch-off point is called the ohmic region. The JFET can be used as a variable resistor, where V GS controls the drain-source resistance (rd). As V GS becomes more negative, the resistance (rd) increases.

19 And as summary in practical…

20 p-Channel JFETS p-Channel JFET acts the same as the n-channel JFET, except the polarities and currents are reversed.

21 P-Channel JFET Characteristics As V GS increases more positively: the depletion zone increases I D decreases (I D < I DSS ) eventually I D = 0A Also note that at high levels of V DS the JFET reaches a breakdown situation. I D increases uncontrollably if V DS > V DSmax.

22 JFET Symbols

23 Transfer Characteristics The transfer characteristic of input-to-output is not as straight forward in a JFET as it was in a BJT. In a BJT, indicated the relationship between IB (input) and I C (output). In a JFET, the relationship of V GS (input) and I D (output) is a little more complicated:

24 Transfer Curve From this graph it is easy to determine the value of ID for a given value of VGS. Transfer Characteristics

25 Plotting the Transfer Curve Shockleys Equation Methods. Using IDSS and Vp (VGS(off)) values found in a specification sheet, the Transfer Curve can be plotted using these 3 steps: Step 1: Solving for VGS = 0V: Step 2: Solving for VGS = Vp (VGS(off)): Step 3: Solving for VGS = 0V to Vp:

26 Shorthand method V GS IDID 0I DSS 0.3V P I DSS /2 0.5I DSS /4 VPVP 0mA Plotting the Transfer Curve

27 Specification Sheet (JFETs)

28 This information is also available on the specification sheet. Case Construction and Terminal Identification

29 MOSFETs MOSFETs have characteristics similar to JFETs and additional characteristics that make then very useful. There are 2 types: 1.Depletion-Type MOSFET 2.Enhancement-Type MOSFET

30 Depletion-Type MOSFET Construction The Drain (D) and Source (S) connect to the to n-doped regions. These N- doped regions are connected via an n-channel. This n-channel is connected to the Gate (G) via a thin insulating layer of SiO 2. The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS.

31 V GS is set to 0V by the direct connection from one terminal to the other. V DS is applied across the drain-to-source terminals. The result is an attraction for the positive potential at the drain by the free electron of the n-channel and a current similar to that established through the channel of the JFET. In the figure, V GS has been set at a negative voltage (-1V) Depletion-Type MOSFET Construction

32 Negative potential at gate will tend to pressure electron towards the p- type substrate and attract holes from the p-type substrate. Depending on negative bias established by V GS, a level recombination between electron and hoes will occur.--- it will reduce the number of free electron in the n-channel available for conduction. The more negative bias, the higher the rate of recombination I D decrease, negative bias for V GS increase Depletion-Type MOSFET Construction

33 Basic Operation A Depletion MOSFET can operate in two modes: Depletion or Enhancement mode.

34 Depletion-type MOSFET in Depletion Mode Depletion mode The characteristics are similar to the JFET. When V GS = 0V, I D = I DSS When V GS < 0V, I D < I DSS The formula used to plot the Transfer Curve still applies:

35 Depletion-type MOSFET in Enhancement Mode Enhancement mode V GS > 0V, I D increases above I DSS The formula used to plot the Transfer Curve still applies: (note that V GS is now a positive polarity)

36 p-Channel Depletion-Type MOSFET The p-channel Depletion-type MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed.

37 Symbols

38 Enhancement-Type MOSFET Construction The Drain (D) and Source (S) connect to the to n-doped regions. These n-doped regions are connected via an n-channel. The Gate (G) connects to the p-doped substrate via a thin insulating layer of SiO 2. There is no channel. The n-doped material lies on a p-doped substrate that may have an additional terminal connection called SS.

39 V GS =0, V DS some value, the absence of an n-channel will result in a current of effectively 0A V DS some positive voltage, V GS =0V, and terminal SS is directly connected to the source, there are in fact 2 reversed-biased p-n junction between the n-doped regions and p substrate to oppose any significant flow between drain and source. V DS and V GS have been set at some positive voltage greater than 0V, establishing the D and G at a positive potential with respect to the source The positive potential at the gate will pressure the holes in the p substrate along the edge of the SiO 2 layer to leave the area and enter deeper regions of the p-substrate The result is a depletion region near the SiO 2 insulating layer void of holes Enhancement-Type MOSFET Construction

40 The electrons will in the p substrate will be attracted to the +G and accumulate in the region near the surface of the SiO 2 layer The SiO 2 layer and its insulating qualities will prevent the negative carriers from being absorbed at the gate terminal V GS increase, the concentration of electrons near the SiO 2 surface increase until eventually the induced n-type region can support a measurable flow between D and S The level of V GS that results in the significant increase in drain current is called the threshold voltage, V T. V GS increase beyond the V T level the density of the carriers in the induced channel will increase and I D also increase Enhancement-Type MOSFET Construction

41 If V GS constant and increase the level of V DS, I D will eventually reach a saturation level as occurred for the JFET Applying Kirchoffs voltage law to the terminal voltage of the MOSFET V DG = V DS - V GS If V GS fixed at some value, 8V, V DS increased from 2 – 5V, the V DG will drop from - 6V to -3V and the gate will become less and less positive with respect to the drain Enhancement-Type MOSFET Construction

42 V GS is always positive As V GS increases, I D increases But if V GS is kept constant and V DS is increased, then I D saturates (I DSS ) The saturation level, V DSsat is reached. The Enhancement-type MOSFET only operates in the enhancement mode. Enhancement-Type MOSFET Construction

43 To determine I D given V GS : where V T = threshold voltage or voltage at which the MOSFET turns on. k = constant found in the specification sheet. k can also be determined by using values at a specific point and the formula: V DSsat can also be calculated: Enhancement-Type MOSFET Construction

44 p-Channel Enhancement-Type MOSFETs The p-channel Enhancement-type MOSFET is similar to the n- channel except that the voltage polarities and current directions are reversed.

45 Symbols

46 Specification Sheet

47 MOSFET Handling MOSFETs are very static sensitive. Because of the very thin SiO2 layer between the external terminals and the layers of the device, any small electrical discharge can stablish an unwanted conduction. Protection: Always transport in a static sensitive bag Always wear a static strap when handling MOSFETS Apply voltage limiting devices between the Gate and Source, such as back-to-back Zeners to limit any transient voltage.

48 VMOS VMOS – Vertical MOSFET increases the surface area of the device. Advantage: This allows the device to handle higher currents by providing it more surface area to dissipate the heat. VMOSs also have faster switching times.

49 CMOS CMOS – Complementary MOSFET p-channel and n-channel MOSFET on the same substrate. Advantage: Useful in logic circuit designs Higher input impedance Faster switching speeds Lower operating power levels

50 Summary Table


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