Lecture 17 Field Effect Transistor (FET) FET 1-1
Outline ntroduction to FET transistors Comparison with BJT transistors FET Types Construction and Operation of FET Characteristics Of FET Examples FET 1-2
ntroduction Field-effect means that an electric field is established by the charges present, which controls the conduction path of the output circuit without the need to direct contact between the controlling and controlled quantities FET is a voltage-controlled device while BJT is a current-controlled device FET operation depends on majority carriers (unipolar device) FET are more temperature stable than BJT FET are smaller than BJT (FET occupies less area) FET exhibits high input resistance FET 1-3
Types of Field Effect Transistors Junction Field-Effect Transistor (JFET) Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) FET 1-4
JUNCTON FEL EFFECT TRANSSTOR (JFET) FET 1-5
Construction of JFET FET 1-6
Characteristics of JFET By applying voltage at the JFET s terminal ( GS = 0 and S = +ve values), some characteristic can be obtained For the n-type material, electrons will be attracted to the positive terminal of S For the p-type material at gate, holes will be attracted to the negative terminal and further away from the positive terminal of S As for that, depletion region will become larger between the n-type and p-type materials Resistance will increase due to narrowing channel FET 1-7
Characteristics of JFET When the +ve value of S is increased, the depletion regions will become larger and such that it seems to be touching each other and blocks the electron flows from source to drain The condition is called pinch-off and the voltage at that point is called pinch-off voltage ( P ) But in reality, a very small channel still exist and current can flow through it with a very high density the current ( ) is maintained at saturation level ( SS ) FET 1-8
Characteristics of JFET FET 1-9
Characteristics of JFET For GS < 0 : The characteristic obtained: FET 1-10
Transfer Characteristic of JFET Cut off at GS > p Note that at high levels of S the JFET reaches a breakdown situation. increases uncontrollably if S > Smax FET 1-11
Transfer Characteristic of JFET The linear equation just like in BJT characteristic cannot be applied However, Shockley s equation can be applied for that region resulting in: Output Current where nput voltage SS : current drain to source at certain applied voltage at gate SS : current drain to source at saturation level ( GS = 0 ) GS : voltage from gate to source P : pinch-off voltage 1 GS P 2 FET 1-12
Plotting Shockley s Equation GS and points can be plotted using this table: GS 0 SS 0.3 P SS /2 0.5 P SS /4 P 0 SS 1 GS P 2 FET 1-13
Example (1) Sketch the JFET transfer curve defined by SS = 12 ma and P = -6 Obtain the four plot points: GS 0 12 ma -1.8 6 ma -3 3 ma -6 0 FET 1-14
Example (1) cont d Plotting and sketching FET 1-15
n-channel JFET Symbols For n-channel JFET: G 0 S SS 1 GS P 2 FET 1-16
Fixed-Bias Configuration Recall back from BJT s topic, for fixedbias configuration emitter terminal is grounded. Same for FET s fixed-bias configuration: FET 1-17
Example (2) etermine: GSQ, Q S,, G, S When G = 0, G = -2 ue to source terminal is grounded, so S = 0 GS can be obtained: GS G S 2 0 2 SS 1 GS P 2 10 m 1 2 8 2 5.625mA FET 1-18
Example (2) cont d FET 1-19 For S : 4.75 4.75 2 16 5.625 0 S S S k m R
Self-Bias Configuration Self-bias configuration was introduced to eliminate the need for 2 C supplies and a resistor was added at source terminal FET 1-20
Example (3) etermine: S GS GSQ, Q, S,, G, S S 1k G R S S S S 1k 0 1k 1k GS 1k SS 1 8m 1 P 6 2 222.24 3.67 8m 0 2 2 FET 1-21
Example (3) cont d Solving the equation, we get: b 13.93 2 b 4ac 2a ma and 3.67 2.58 ma ( 3.67) 2(222.24) 4(222.24)(8m) = 2.58 ma is taken due to = 13.93 ma is out of range because the maximum value of is SS which is 8 ma When the value of has been obtained, all other values can be calculated easily 2 FET 1-22
Example (3) cont d Using the graphical approach to get the Shockley s curve: GS 0 SS = 8 ma 0.3 P = -1.8 SS /2 = 4 ma 0.5 P = -3 SS /4 = 2 ma P = -6 0 ma From the circuit, equation of GS is: Take two points for plotting: f = 0 A, GS = 0 (0,0) f = 4 ma, GS = -4 (-4,4m) GS 1k FET 1-23
Example (3) cont d Shockley s curve: GS equation from the circuit: GS 1k FET 1-24
Example (3) cont d Combining the Shockley s curve and GS equation of the circuit: The Q-point is at = 2.6 ma which is very close to the value of obtained by using mathematical approach All other values can be obtained just as the same as in mathematical approach FET 1-25
p-channel JFET The device: The characteristic: FET 1-26
Lecture Summary Covered material ntroduction to FET transistors Comparison with BJT transistors Types of FET Construction and Operation of FET Characteristics Of FET Examples Material to be covered next lecture ntroduction to MOSFET Types epletion-type Enhancement-type FET 1-27