المملكة العربية السعودية وزارة التعليم العالي - جامعة أم القرى كلية الهندسة و العمارة الإسلامية قسم الهندسة الكهربائية ELECTRONIC DEVICES K INGDOM OF S AUDI A RABIA Ministry of Higher Education Umm Al-Qura University College of Engineering and Islamic Architecture Electrical Engineering Department Lecture 10 By: Dr Tarek Abdolkader

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader2 At the end of this lecture, the student should be able to: 1.Describe the construction of Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) 2.Differentiate between Enhancement and Depletion MOSFETs and their symbols 3.Explain the operation of D-MOSFET and E-MOSFET 4.Demonstrate D-MOSFET and E-MOSFET characteristics 5.Define the different parameters of MOSFET 6.Describe the different methods of MOSFET biasing

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader3 Depletion-type MOSFET (D-MOSFET) Enhancement-type MOSFET (E-MOSFET) There are basically two types of MOSFETs: Depletion-type MOSFET (D-MOSFET) and Enhancement-type MOSFET(E-MOSFET)

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader4 D-MOSFET

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader5 E-MOSFET

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7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader7 A D-MOSFET has parameters of V GS(off) = -6V and I DSS = 1 mA. Plot the transconductance curve for the device.

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader8 D-MOSFET is very similar to JFET: 1.In D-MOSFET no gate current at all because of the insulator between gate and channel 2.If V GS < 0, both devices have the same characteristics. 3.For V GS > 0, JFET cannot be used. However, D-MOSFET can be used with drain currents larger than I DSS. n -channel D-MOSFET with V GS 0, is said to be in Enhancement mode

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader9 More negative V GS depletes more electrons from the n -channel and decrease its conductivity. So, lower V GS leads to less drain current. V GS(off) for n -channel D-MOSFET is negative More positive V GS attracts more electrons to the n-channel and increase its conductivity. So, higher V GS leads to more drain current. n-channel p-channel More positive V GS depletes more holes from the p -channel and decrease its conductivity. So, higher V GS leads to less drain current. V GS(off) for p -channel D-MOSFET is positive More negative V GS attracts more holes to the p -channel and increase its conductivity. So, lower V GS leads to more drain current.

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader10 For a certain D-MOSFET, I DSS = 10 mA and V GS(off) = ‒ 8 V. (a) Is this an n -channel or a p -channel? (b) Calculate I D at V GS = ‒ 3 V. (c) Calculate I D at V GS = +3 V.

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader11 There is no built-in channel in E-MOSFET. A positive gate bias has to be applied on the n-channel. Minimum value of V GS needed to form the n-channel and passes drain current is called “ threshold voltage V th “

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader12

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader13 V th for n-channel E-MOSFET is positive n-channel p-channel V th for p-channel E-MOSFET is negative

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader14 For a 2N7002 E-MOSFET, I D(on) = 500 mA (for a point well above the threshold voltage) at V GS = 10 V and V GS(th) = 1 V. Determine the drain current for V GS = 5 V.

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader15 D-MOSFET can be operated with either positive or negative gate bias ( V GS ). A simple method is to set V GS = 0. R G is used for protecting ac signal input from being shorted Remember that at V GS = 0, the drain current I D = I DSS Remember that for JFET, there was a resistance R S at the source terminal to make V GS negative. D-MOSFET

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader16 Determine the drain-to-source voltage in the circuit shown. I DSS = 12 mA and V GS(off) = ‒ 8 V.

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader17 For E-MOSFET, V GS must be greater than threshold voltage V GS(th). So, zero bias cannot be used. Two methods of biasing are shown: (a) voltage-divider bias, (b) Drain-feedback bias. Note that here also R S is not used because it will raise the potential of source terminal and decrease V GS. In BJT, we were using R E to increase the input impedance Z in but here we do not need it because Z in is already high E-MOSFET

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader18 Determine V GS and V DS for the E- MOSFET circuit shown. Assume that I D(on) = 200 mA at V GS = 4 V and V GS(th) = 2 V.

7/6/1433 Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader 19 Determine the values of I D and V DS for the circuit shown. The data sheet for this particular MOSFET gives I D(on) = 10 mA when V GS = V DS

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader20 Graphical Picture Change in V GS around the V GSQ leads to change in I D around I DQ We use the symbol V gs, I d, … for the change in V GS, I D, … around the Q point (ac components) The transconductance:

7/6/1433Electronic devices (802311) Lecture 10 Dr Tarek Abdolkader21 Equivalent Circuit In ac analysis we replace the transistor by its equivalent circuit: r gs represents the resistance between gate and source which is infinitely large r ds represents the dependence of drain current on drain to source voltage which is very small. Although the dc resistance of channel R DS = I DQ / V DSQ is small, the ac resistance r ds is very high (a) Complete equivalent circuit (b) Simplified equivalent circuit

13/4/1432Electronic Circuits (802312) Lecture 5 Dr Tarek Abdolkader22 Electrostatic Discharge Excess static charge may be accumulated on the gate of a MOSFET. All MOS devices are subject to damage due to Electro-Static Discharge (ESD) Humans don't feel ESD transfers less than 3,500 volts, but MOS devices are sensitive to charges at less than half of this level. So, when you work on MOS devices, you may damage the components in the computer via ESD, and not even feel it. MOSFETs are usually shipped in conductive foam. MOSFETs are usually shipped with a wire ring around the leads, which is removed just prior to installing the MOSFET in a circuit. All instruments and metal benches used in assembly or testing of MOSFETs should be connected to earth ground. Handler’s rest should be connected to a grounding strap

13/4/1432Electronic Circuits (802312) Lecture 5 Dr Tarek Abdolkader23 MOSFET scaling