Lecture 14 Field Effect Transistor (FET) Sunday 26/11/2017 FET 1-1
Outline Introduction to FET transistors Types of FET Transistors Junction Field Effect Transistor (JFET) Characteristics Construction and Operation of JFET Characteristics Of JFET Examples BJT 1-2
Field Effect Transistor (FET) 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
JUNCTION FIEL EFFECT TRANSISTOR (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 characteristics 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 (think about the equivalent resistance) 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 (I ) is maintained at saturation level (I SS ) FET 1-8
Characteristics of JFET In an n-channel JFET this Gate voltage is negative while for a p-channel JFET the Gate voltage is positive. FET 1-9
Characteristics of JFET For GS < 0 : The characteristic obtained: FET 1-10
Transfer Characteristic of JFET i/p characteristic curve o/p characteristic curve Cut off at GS > p Note that at high levels of S the JFET reaches a breakdown situation. I 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 Input voltage SS I SS : current drain to source at saturation level ( GS = 0 ) GS : voltage from gate to source P : pinch-off voltage I I 1 GS P 2 FET 1-12
Plotting Shockley s Equation GS and I points can be plotted using this table: GS I 0 I SS 0.3 P I SS /2 0.5 P I SS /4 P 0 FET 1-13
Example (1) Sketch the JFET transfer curve defined by I SS = 12 ma and P = -6 Obtain the four plot points: GS I 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: I G 0 I I S I I 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: Solution GSQ, I Q S,, G, S When I G = 0, G = -2 ue to source terminal is grounded, so S = 0 GS can be obtained: GS G S 2 0 2 I I 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 I
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: Solution I S GS GSQ, I Q, S,, G, S S I 1kI G R S S S S 1k 0 1kI 1kI GS 1kI I ISS 1 8m 1 P 6 2 222.24I 3.67I 8m 0 2 2 FET 1-21
Example (3) cont d Solving the equation, we get: I b 13.93 2 b 4ac 2a ma and 3.67 2.58 ma ( 3.67) 2(222.24) 4(222.24)(8m) I = 2.58 ma is taken due to I = 13.93 ma is out of range because the maximum value of I is I SS which is 8 ma When the value of I 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 I 0 I SS = 8 ma 0.3 P = -1.8 I SS /2 = 4 ma 0.5 P = -3 I SS /4 = 2 ma P = -6 0 ma From the circuit, equation of GS is: Take two points for plotting: If I = 0 A, GS = 0 (0,0) If I = 4 ma, GS = -4 (-4,4m) GS 1kI FET 1-23
Example (3) cont d Shockley s curve: GS equation from the circuit: GS 1kI FET 1-24
Example (3) cont d Combining the Shockley s curve and GS equation of the circuit: The Q-point is at I = 2.6 ma which is very close to the value of I 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 Introduction to FET transistors Types of FET Transistors Junction Field Effect Transistor (JFET) Characteristics Construction and Operation of JFET Characteristics Of JFET Examples Material to be covered next lecture Introduction to MOSFET transistors BJT 1-27