Lecture 15 Field Effect Transistor (FET) Wednesday 29/11/2017 MOSFET 1-1
Outline MOSFET transistors Introduction to MOSFET MOSFET Types epletion-type MOSFET Characteristics Comparison between JFET and epletion-type MOSFET MOSFET 1-2
MOSFET MOSFET stands for Metal-Oxide Semiconductor Field-Effect Transistor Metal-Oxide means there are metal (or polysilicon) and silicon oxide (SiO 2 ) involved in its construction Extremely large input resistance Four terminal devices 2 types of MOSFET: epletion-type MOSFET (MOSFET) Enhancement-Type MOSFET (EMOSFET) MOSFET 1-3
MOSFET Key elements: Inversion 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 (I ) 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 2 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 In 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 I I SS V 1 GS VP 2 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 I SS = 10 ma and V P = -4 V Obtain the four plot points that is in the depletion region: V GS I 0 V I SS = 10 ma 0.3 V p = -1.2 V I SS /2 = 5 ma 0.6 V p = -2 V I SS /4 = 2.5 ma V p = -4 V 0 ma MOSFET 1-12
Example (1) cont d Obtain the extra plot points that is in the enhancement region (apply V GS = +1 V): I I SS 1 V V GS P 2 10 m 1 1 4 2 15.63mA V GS I +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 (2) etermine I Q and V GSQ then find V S G S MOSFET 1-16
Example (2) 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 IS I V S 750I V V V 1.5 750I GS G S We need another equation for V GS and I MOSFET 1-17
Example (2) cont d Using the Shockley s equation and substituting V GS in terms of I using the equation in the previous slide for calculating I value: I I 375I SS 2 V 1 V 5.5I 13.510 Solving the equation, we get: GS P 2 610 3 3 1.5 750I 1 3 0 2 I 2 b b 4ac 5.5 2a 11.55mA and 3.12 ma ( 5.5) 2 4(375)(13.510 2(375) 3 ) MOSFET 1-18
Example (2) cont d I = 3.12 ma is acceptable value I = 11.55 ma has exceed the limit of I SS, but remember that I can exceed I 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 750I GS For For I I 11.55mA, V 3.12mA, V GS GS 1.5 750(11.55m) 7.16V 1.5 750(3.12m) 0.84V From the result above, for I = 11.55 ma, the V GS obtained has exceed the limit of V P = - 3 V. Thus, the value for I = 3.12 ma is taken VS V VS 181.8kI 750I 10V MOSFET 1-19
Example (2) Graphical approach Using the graphical approach to get the Shockley s curve: V GS I 0 V I SS = 6 ma 0.3V P = -0.9 V I SS /2 = 3 ma 0.5V P = -1.5 V I 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, I = 10.67mA MOSFET 1-20
Example (2) - Graphical approach From the circuit, equation of V GS is: V 1.5 750 GS I Take two points for plotting: If V GS = 0 V, I = 2 ma (0,2) If I = 0 ma, V GS = 1.5 V, (1.5,0) The Q-point is at I 3.1 ma which is very close to the value of I obtained by using mathematical approach MOSFET 1-21
p-channel epletion-type MOSFET or Enhancement mode epletion mode Construction Transfer Curve Characteristics MOSFET 1-22
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 1-23
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 (I = 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 1-24
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 I 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 1-25 equation VS ( sat) VGS VT
Enhancement-Type MOSFET Characteristic MOSFET 1-26
Lecture Summary Covered material MOSFET transistors Introduction to MOSFET MOSFET Types epletion-type MOSFET Characteristics Comparison between JFET and epletion-type MOSFET Enhancement-type MOSFET Operation Characteristics Material to be covered next lecture Continue Enhancement-type MOSFET Characteristics Biasing Circuits and Examples MOSFET 1-27