Field Effect Transistor (FET) FET 1-1
Outline MOSFET transistors ntroduction to MOSFET MOSFET Types epletion-type MOSFET Characteristics Biasing Circuits and Examples Comparison between JFET and epletion-type MOSFET Enhancement-type MOSFET Operation Characteristics FET 1-
MOSFET MOSFET stands for Metal-Oxide Semiconductor Field-Effect Transistor Metal-Oxide means there are metal (or polysilicon) and silicon oxide (SiO ) involved in its construction Extremely large input resistance Four terminal devices types of MOSFET: epletion-type MOSFET (MOSFET) Enhancement-Type MOSFET (EMOSFET) FET 1-3
MOSFET Key elements: nversion layer (or conducting channel) under gate depending on gate voltage inversion layer to electrically connect source and drain the layer is formed when applying voltage at gate terminal Heavily doped regions underneath gate providing carriers supply and collector SS MOSFET 1-4
MOSFET Two complementary devices n-channel device (n-mosfet) on p-substrate uses electron inversion layer p-channel device (p-mosfet) on n-substrate uses hole inversion layer Qualitative Operation rain Current ( ) proportional to inversion charge and the velocity that the charge travels from source to drain Gate-Source Voltage (V GS ) controls amount of inversion charge that carries the current rain-source Voltage (V S ) controls the electric field that drifts the inversion charge from the source to drain MOSFET 1-5
epletion-type MOSFET Construction n-channel depletion-type MOSFET will be discussed first The construction is the same with JFET except the addition of SiO under the gate terminal contact and an n-channel between two n-material or polysilicon FET 1-6
epletion-type MOSFET Operation Let us apply some positive voltage connected to the drain-source terminal while remaining the gate voltage to 0 Electrons will flow from source to drain and this will result in current flows from drain to source The result will be the same as in JFET and saturation current will be obtained when the pinch-off voltage (V P ) is reached FET 1-7
epletion-type MOSFET Operation Now, let us apply some negative voltage for gate terminal The negative voltage will push the electrons in the n- channel away from the gate, thus the channel will become smaller n addition, the holes in p- material substrate will be attracted to the electrons in the channel and the recombination process will take place, helping the channel to become smaller FET 1-8
n-channel epletion-type MOSFET Symbols or MOSFET 1-9
epletion-type MOSFET Characteristic the Shockley equation can be applied for the MOSFET in the depletion mode SS V 1 GS VP The Shockley s equation can also be applied for the enhancement mode, but, V GS will have positive voltage values (in case of n-channel) This will be difference between epletion-type MOSFET and JFET characteristic MOSFET 1-10
Comparison between JFET and epletion-type MOSFET JFET epletion -type MOSFET MOSFET 1-11
Example (1) Sketch the transfer curve defined by SS = 10 ma and V P = -4 V Obtain the four plot points that is in the depletion region: V GS 0 V SS = 10 ma 0.3 V p = -1. V SS / = 5 ma 0.6 V p = - V SS /4 =.5 ma V p = -4 V 0 ma MOSFET 1-1
Example (1) cont d Obtain the extra plot points that is in the enhancement region (apply V GS = +1 V): SS 1 V V GS P 10 m 1 1 4 15.63mA V GS +1 V 15.63 ma MOSFET 1-13
Example (1) cont d Plotting: Sketching: MOSFET 1-14
MOSFET Biasing Circuits Same with JFET s fixed-bias configuration except for the device is change to depletion-type MOSFET device All the calculation are the same as in JFET, but an extra point when plotting for the transfer curve for positive value of V GS MOSFET 1-15
Example () etermine Q and V GSQ then find V S G S MOSFET 1-16
Example () cont d All the calculation for voltage-divider bias configuration are all the same as in JFET s voltage-divider bias configuration V G 10M 18* 10M 110M 1.5 V S V S 750 V V V 1.5 750 GS G S We need another equation for V GS and MOSFET 1-17
Example () cont d Using the Shockley s equation and substituting V GS in terms of using the equation in the previous slide for calculating value: 375 SS V 1 V 5.5 13.510 Solving the equation, we get: GS P 610 3 3 1.5 750 1 3 0 b b 4ac 5.5 a 11.55mA and 3.1 ma ( 5.5) 4(375)(13.510 (375) 3 ) MOSFET 1-18
Example () cont d = 3.1 ma is acceptable value = 11.55 ma has exceed the limit of SS, but remember that can exceed SS for depletiontype MOSFET (in the enhancement mode) To make sure which value is more acceptable, check the value by inserting into the V GS equation: V 1.5 750 GS For For 11.55mA, V 3.1mA, V GS GS 1.5 750(11.55m) 7.16V 1.5 750(3.1m) 0.84V From the result above, for = 11.55 ma, the V GS obtained has exceed the limit of V P = - 3 V. Thus, the value for = 3.1 ma is taken VS V VS 181.8k 750 10V MOSFET 1-19
Example () Graphical approach Using the graphical approach to get the Shockley s curve: V GS 0 V SS = 6 ma 0.3V P = -0.9 V SS / = 3 ma 0.5V P = -1.5 V SS /4 = 1.5 ma V P = -3 V 0 ma + 1 V 10.67 ma For the extra plot point when V GS is a positive value, take V GS = +1V due to V P = -3V and when V GS is positive it rise more rapidly Using Shockley s equation, for V GS = +1V, = 10.67mA MOSFET 1-0
Example () - Graphical approach From the circuit, equation of V GS is: V 1.5 750 GS Take two points for plotting: f V GS = 0 V, = ma (0,) f = 0 ma, V GS = 1.5 V, (1.5,0) The Q-point is at 3.1 ma which is very close to the value of obtained by using mathematical approach MOSFET 1-1
p-channel epletion-type MOSFET or Enhancement mode epletion mode Construction Transfer Curve Characteristics MOSFET 1-
Enhancement-Type MOSFET Construction n-channel enhancement-type MOSFET will be discussed first The device is the same as depletiontype MOSFET, but notice that there is no channel between the drain and source terminal MOSFET 1-3
Enhancement-Type MOSFET Operation Because there is no channel, so no current will flow no matter what voltage applied (V S ) to the drain and source terminal ( = 0 for V GS < V T ) So, a certain voltage (threshold voltage, V T ) must be applied to the gate terminal so that a channel will develop and the current will flow between drain and source terminal MOSFET 1-4
Enhancement-Type MOSFET Operation By setting V G higher than V T, a channel will develop As for that, when V S (formerly known as V ) is increased, the pinch-off situation will happen and a saturation current SS will be obtained (same as in JFET and depletion-type MOSFET) Pinch-off voltage V S(sat) (formerly known as V P ) will became higher when V G is increase due to the widening of the channel developed The pinch-off or saturation voltage obtain is defined by the MOSFET 1-5 equation VS ( sat) VGS VT
Enhancement-Type MOSFET Characteristic MOSFET 1-6
Enhancement-Type MOSFET Symbol n-channel E-Type MOSFET p-channel E-Type MOSFET MOSFET 1-7
Enhancement-Type MOSFET Characteristic k V GS VT for VGS VT k V V GS ( on) ( on) T MOSFET 1-8
Enhancement-Type MOSFET Characteristic k V V 8 GS ( on) ( on) T 10m 0.78 ma/v V 0.78m GS MOSFET 1-9
Enhancement-Type MOSFET Transfer Curve Plotting all the V GS(on) from the characteristic curve, the transfer curve can be obtained: V 0.78m GS MOSFET 1-30
Enhancement-Type MOSFET mportant Relationships k G 0 k S V GS V T (on) V V GS(on) T for V GS V T MOSFET 1-31
Enhancement-Type MOSFET C Biasing Fixed-bias, self-bias and many more bias configuration can be applied to enhancement-type MOSFET Two most popular MOSFET biasing configurations Feedback-bias configuration Voltage-divider bias configuration MOSFET 1-3
Feedback-Bias Configuration As the situation G = 0 still applied, the resistor R G will be ignored resulting in the drain and gate terminal to have the same voltage (V G = V ) MOSFET 1-33
Example (1) etermine V GSQ and Q for the E-Type MOSFET shown in the figure MOSFET 1-34
Example (1) Solution For the enhancement-type MOSFET s equation, the value of k have to be obtained first: k V V 8 3 GS(on) (on) the equation for the device: V V 1 G T 0.4m V 6m 3 for V 3 GS Since G equals zero, then GS 0.4mA/V nserting the V GS equation into the device equation: V V V 1 0 1 GS G S MOSFET 1-35
Example (1) Solution Substituting the equation for the MOSFET 960 9.64 19.44m 0 0.4m V 3 0.4m 1 3 0.4m 9 GS 0.4m 81 36 4 19.44m 8.64 960 Solving the equation, we get: b b 4ac 9.64 a ( 9.64) 4(960)(19.44m) (960) 7.5mA and.79 ma Enhancement-type MOSFET doesn t have limitation for saturation current ( SS ), the true value of is the smaller one (since V GS should be greater than V T ).79 ma and V 1 6.4 V GS Q MOSFET 1-36
Example (1) Solution For graphical approach, several plot points have to be obtained first: 0.4m V V GS 3 for V 3 GS 3 V 0 ma GS 4 V 0.4 ma 5 V 0.96 ma 6 V.16 ma 7 V 3.84 ma 8 V 6 ma For the bias line, only two plot points are required: V GS V GS 1 1 V 0 ma 0 V 6 ma MOSFET 1-37
Example (1) Solution Plots all the device transfer curve and device representation points: V T MOSFET 1-38