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MOSFET

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Published in: Electronics
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Introduction To MOSFET

Pranav P / Bangalore

6 years of teaching experience

Qualification: 12th (SANT PATHIK SCHOOL - 2015)

Teaches: Basic Computer, Computer for official job, MS Office, School Level Computer, All Subjects, Chemistry, Computer Science, Mathematics, Physics, C / C++, Python Programming, Android Training

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  1. MOSFETs MOSFETs have characteristics similar to JFETs and additional characteristics that make then very useful There are 2 types of MOSFET's: , Depletion mode MOSFET (D-MOSFET) Operates in Depletion mode the same way as a JFET when VGS 0 Operates in Enhancement mode like E-MOSFET when VGS > 0 , Enhancement Mode MOSFET (E-MOSFET) Operates only in Enhancement mode ' IDSS = 0 until VGS > VT (threshold voltage) 1
  2. BJT vs mpsr Output Current Cost ESD Risk Switching Speed and Frequency Response Thermal Runaway Gain MOSFET is controlled by the input gate voltage. More Expensive Easily damaged by ESD Electrostatic Discharge. Faster than Bipolar Better frequency response When MOSFETS heat up, the current flowing through them decreases. They are BJT is controlled by the input base current. Lower Cost ESD is rarely a problem Slower than MOSFETs. Inferior frequency response. When bipolar transistors heat up, the gain increases and so the current through them increases too. This in turn causes further heating and yet more gain and less likely to be destroyed by current. This can cause overheating. catastrophic failure called thermal runaway. Very high current gain which Lower current gain and it is is nearly constant for varying not constant. drain currents.
  3. JFET VS MOSFET JFETs can only be operated in the depletion mode whereas MOSFETs can be operated in either depletion or in enhancement mode In a JFET, if the gate is forward biased, excess- carrier injunction occurs and the gate current is substantial. Thus channel conductance is enhanced to some degree due to excess carriers but the device is never operated with gate forward biased because gate current is undesirable. MOSFETs have input impedance much higher than that of JFETs. This is due to negligibly small leakage current. JFETs have characteristic curves more flat than those of MOSFETs indicating a higher drain resistance. When JFET is operated with a reverse bias on the junction, the gate current IG is larger than it would be in a comparable MOSFET. For the above reasons, and also because MOSFETs are somewhat easier to manufacture, they are more widely used than are the JFET
  4. THE MOSFET - DEPLETION MOSFET Depletion Mode With a negative gate voltage, the negative charges on the gate repel conduction electrons from the channel, leaving positive ions in their place. Thereby, the n channel is depleted of some of its electrons, thus decreasing the channel conductivity. The greater the negative voltage on the gate, the greater the depletion of n-channel electrons. At sufficiently negative gate-to-source voltage V the channel is totally depleted and GS(off)' drain current is zero. Enhancement Mode With a positive gate voltage, more conduction electrons are attracted into the channel, thus increasing (enhancing) the channel conductivity. Gate Drain Source n channel Gate Drain Source p channel D-MOSFET schematic symbols.
  5. Depletion Mode MOSFET Construction
  6. CONSTRUCTION OF N CHANNEL DEPLETION TYPEMOSFET Two highly doped N regions are diffused into a lightly doped p type substrate that may have an additional terminal connection called SS The two highly doped n regions represent source and drain connected via an n-channel. The N-channel is formed by diffusion between the source and drain. The type of impurity for the channel is the same as for the source and drain. Metal is deposited through the holes to provide drain and source terminals, and on the surface area between drain and source, a metal plate is deposited. This layer constitutes the gate. The n-channel is connected to the Gate (G) via a thin insulating layer of Si02 The thin layer of Si02 dielectric is grown over the entire surface and holes are cut through the Si02 (silicon-dioxide) layer to make contact with the N-type blocks (Source and Drain).
  7. OPERATION OF N CHANNEL DEPLETION MOSFET A D-MOSFET may be biased to operate in two modes: the Depletion mode or the Enhancement mode When \/GS and drain is made positive with respect to source current(in the form of free electrons) can flow between source and drain, even with zero gate potential and the MOSFET is said to be operating in Enhancement mode. In this mode of operation gate attracts the charge carriers from the P-substrate to the N- channel and thus reduces the channel resistance which increases the drain- current. negative with respect to the substrate, the gate repels some of the When - negative charge carriers out of the N-channel ,and attracts holes from the p type substrate . This initiates recombination of holes and electrons. This creates a depletion region in the channel and, therefore, reduces the number of free electrons in the n channel, increases the channel resistance and reduces the drain current. The more negative the gate, the less the drain current. In this mode of operation the device is referred to as a depletion-mode MOSFET. Here too much negative gate voltage can pinch-off the channel. The more positive the gate is made, more number of electrons from p substrate due to reverse leakage current and collisions between accelerating particles the more drain current flows.
  8. DRAIN CHARACTERISTICS 2ment DSS DSS DSS GS = ov GS GS GS GS DS GS
  9. DRAINCHARACTERISTICS For V G s exceed zero the device operates in enhancement These drain curves again display an ohmic region, a constant-current source region and a cut-off region. For a specified drain-source voltage VDS, VGS (OFF) is the gate-source voltage at which drain current reduces to a certain specified negligibly small value, For V G s between V ( -ve value)and zero, the GS (OFF) device operates in depletion-mode
  10. ID (mA) Enhancement 6 10.9 Depletion mode 8 —4 -3-2 10 0.3b mode GS 2 DSS 4 0 The transfer characteristics are similar to the JFET In Depletion Mode operation: When VGS = OV, ID = IDSS When VGS < OV, ID < IDSS When VGS > OV, ID > IDSS Enhancement Mode operation In this mode, the transistor operates with VGS > OV, and ID increases above IDSS Shockley's equation, the formula used to plot the Transfer Curve still applies but VGS is positive: 2 10
  11. P CHANNEL DEPLETION MOSFET Vgs > Vthreshold Source p p-channel Gate depletion Drain P n-type Body
  12. Vc;s (a) ID (mA) DSS 12 3 4 (b) ID (mA) GS — 5 6 GS DS (c) The p-channel Depletion mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed 12
  13. THE MOSFET - ENHANCEMENT MOSFET (E-MOSFET) The schematic symbols for the n-channel and p-channel E-MOSFET are shown in Figure below. The conventional enhancement MOSFETs have a long thin lateral channel as shown in structural view in Figure below. Gate Drain Source n channel Gate Drain Source p channel Source n Gate p Substrate Drain Si02 n Channel
  14. Il-doped region Metallic contacts substrate Il-doped region no-channel Substrate
  15. ENHANCEMENT MOSFET CONSTRUCTION Figure shows the construction of an N-channel E-MOSFET. The main difference between the construction of DE-MOSFET and that of E- MOSFET, the E-MOSFET substrate extends all the way to the silicon dioxide (Si02) and no channels are doped between the source and the drain. Two highly doped n regions are diffused into a lightly doped p substrate that may have an additional terminal connection called SS The source and drain are taken out through metallic contacts to n doped regions. These n-doped regions are not connected via an n-channel without an external voltage The Gate (G) connects to the p-doped substrate via a thin insulating layer of Si02 Channels are electrically induced in these MOSFETs, when a positive gate-source voltage VGS is applied to it.
  16. N CHANNEL ENHANCEMENT MOSFET OPERATION This MOSFET operates only in the enhancement mode and has no depletion mode. It operates with large positive gate voltage only. When drain is applied with positive voltage with respect to source and no potential is applied to the gate , a very small drain current that is, reverse leakage current flows. The E MOSFET does not conduct when the gate- O. This is the reason that it is called normally-off source voltage VGS MOSFET When the gate is made positive with respect to the source and the substrate, negative (i.e. minority) charge carriers within the substrate are attracted to the positive gate and accumulate close to the-surface of the substrate. As the gate voltage is increased, more and more electrons accumulate under the gate. Since these electrons can not flow across the insulated layer of silicon dioxide to the gate, so they accumulate at the surface of the substrate just below the gate. When this occurs, a channel is induced by forming what is termed an inversion /ayer(N-type). Now a drain current start flowing.
  17. N CHANNEL ENHANCEMENT MOSFET OPERATION At a particular value of VGS there is measurable current between drain and source. This value of VGS is called threshold voltage VT. For any voltage below VT, there is no channel The strength of the drain current depends upon the channel resistance which, in turn, depends upon the number of charge carriers attracted to the positive gate. Channel does not exist with VGS=O and the conductivity of the channel is enhanced by the positive bias on the gate so this device is also called the enhancement MOSFETor E- MOSFET.
  18. Basic Operation The Enhancement mode MOSFET only operates in the enhancement mode. — 10 VGS is always positive IDSS = O when VGS < VT = -4-8 20 When VGS is greater than VT, the device turns- on and the drain current ID is controlled by the gate voltage. As VGS increases above VT, the density of electrons in the induced channel increases and ID increases. If VGS is kept constant and V DS is increased, then ID saturates (IDSS) The almost vertical components of the curves correspond to the ohmic region, and the horizontal components correspond to the constant current region. Thus E-MOSFET can be operated in either of these regions i.e. it can be used as a variable-voltage resistor (W R) or as a constant current source. 18
  19. TRANSFER CHARACTERSTIC FOR N CHANNEL ENHANCEMENT MOSFET ID (mA) 10 9 8 7 6 5 4 3 2 o I 2 3 4 5 6 7 8 The current IDSS at VGS
  20. E-MOSFET TRANSFER CHARACTERISTIC CHARACTERISTICS AND PARAMETERS - The E-MOSFET for all practical purposes does not conduct until v GS reaches the threshold voltage (V ). I when it is when conducting can be determined by the GS(th) D is a data sheet given formulas below. The constant K must first be determined. I D(on) value. D GS(th) (a) n channel D(on) GS GS(th) GS GS(th) channel D GS(th) An n-channel device requires a positive gate-to-source voltage, and a p-channel device requires a negative gate-to-source voltage. E-MOSFET general transfer characteristic curves. 33
  21. p-Channel Enhancement Mode MOSFETs The p-channel Enhancement mode MOSFET is similar to the n-channel except that the voltage polarities and current directions are reversed. 16=-3 v —5 -4 -1 —6 ID (mA) GS ID (mA) VGs=-6V VGs=-5V v (a) (b) (c) 21
  22. n-channel 1
  23. JFET Summary Table D-MOSFET E-MOSFET
  24. Specification Sheet 2N3797 CASE 22-03. STYLE 2 TO. 18 (TO-206AA) MAXIMUM RATINGS 2N3?9? Voltaec thin Tee TA 23 'C atxn•e WC ELECTRICAL CHARACTERISTICS (T on CHA RACTV.RIST'CS thin Scene • -70V. Curtent ( —-10 V. Vos —0) -10 V. Vos •O. tso•cw Voltage ao 2.0BA.Vos thin-Gate Current 10 V. Is ON CHARACTERISTICS Current s."ALL-SIGNAL CHARACTERISTICS TrNtsfef V. vas 1.0 A&nitt.uxe 10 V. 10 V. Tr•mier C4'Eitame FUNCTIONAL CHARACTV.Rt.s-rtes Not* Figur 10 V. vcs • O. f • 1.0 kHz. R' 3 10 20 20 1.14 t :nit mwrc •c •c 2 MO,SFETs LOW POWER AUDIO N.C"ANNEL - OEPLET'ON Symbol 2NS'V' 1 Yol 9.0 23 29 14 21 os his value as 26

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