Chapter 6: Field-Effect Transistors slamic University of Gaza Dr. Talal Skaik
FETs vs. BJTs Similarities: Amplifiers Switching devices mpedance matching circuits Differences: FETs are voltage controlled devices. BJTs are current controlled devices. FETs have a higher input impedance. BJTs have higher gains. FETs are less sensitive to temperature variations and are more easily integrated on Cs. 2
FET Types JFET: Junction FET MOSFET: Metal Oxide Semiconductor FET D-MOSFET: Depletion MOSFET E-MOSFET: Enhancement MOSFET 3
JFET Construction There are two types of JFETs n-channel p-channel The n-channel is more widely used. There are three terminals: Drain (D) and Source (S) are connected to the n-channel Gate (G) is connected to the p- type material Water analogy for the JFET control mechanism. 4
JFET Operating Characteristics: GS = 0, DS some positive value When GS = 0 and DS is increased from 0 to a more positive voltage: The depletion region between p- gate and n-channel increases. ncreasing the depletion region, decreases the size of the n- channel which increases the resistance of the n-channel. Even though the n-channel resistance is increasing, the current ( D ) from source to drain through the n-channel is increasing. This is because DS is increasing. 5
JFET Operating Characteristics: GS = 0, DS some positive value D versus DS for GS = 0. 6
JFET Operating Characteristics: Pinch Off f GS = 0 and DS is further increased to a more positive voltage, then the depletion zone gets so large that it pinches off the n-channel. As DS is increased beyond P, the level of D remains the same ( D = DSS ). DSS is the maximum drain current for a JFET and is defined by the conditions GS =0 and DS > P. 7
JFET Operating Characteristics, GS <0 As GS becomes more negative, the depletion region increases. The more negative GS, the resulting level for D is reduced. Eventually, when GS = P (-ve) [ P = GS(off) ], D is 0 ma. (the device is turned off. The level of GS that results in D =0 ma is defined by GS = P, with P being a negative voltage for n-channel devices and a positive voltage for p-channel JFETs. Application of a negative voltage to the gate of a JFET. 8
JFET Operating Characteristics n-channel JFET characteristics with DSS = 8 ma and P = -4. 9
JFET Operating Characteristics: oltage-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 GS controls the drain-source resistance (r d ). As GS becomes more negative, the resistance (r d ) increases. r d 1 r o GS P 2 where r o is the resistance with GS =0 and r d is the resistance at a particular level of GS. 10
p-channel JFETS The p-channel JFET behaves the same as the n-channel JFET, except the voltage polarities and current directions are reversed. 11
p-channel JFET Characteristics As GS increases more positively The depletion zone increases D decreases ( D < DSS ) Eventually D = 0 A Also note that at high levels of DS the JFET reaches a breakdown situation: D increases uncontrollably if DS > DSmax. 12
JFET Symbols JFET symbols: (a) n-channel; (b) p-channel. 13
(a) GS = 0, D = DSS ; (b) cutoff ( D = 0 A) GS less than (more negative than) the pinch-off level; (c) D is between 0 A and DSS for GS 0 and greater than the pinch-off level. 14
JFET Transfer Characteristics n a BJT, indicates the relationship between B (input) and C (output). n a JFET, the relationship of GS (input) and D (output) is a little more complicated (Shockley s equation): D DSS 1 GS P 2 William Bradford Shockley (1910 1989) 15
JFET Transfer Curve This graph shows the value of D for a given value of GS. 16
Plotting the JFET Transfer Curve Using DSS and p ( GS(off) ) values found in a specification sheet, the transfer curve can be plotted according to these three steps: Step 1 Solving for GS = 0 Step 2 D DSS D = DSS DSS Solving for GS = p ( GS(off) ) D = 0A D 1 1 Step 3 2 GS Solving for GS = 0 to p D DSS 1 i.e. For GS = -1 2 GS P GS P 1 D 8mA 1 4.5mA 4 2 P 2 Conversely, for a given D, GS can be obtained: GS P 1 D DSS 17
Example 6.1 Sketch the transfer curve defined by DSS =12 ma and P =-6. D DSS 1 GS P 2 GS P 1 D DSS 18