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Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols MOSFET Characteristics Dr DC Hendry September 2007 Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols Outline I 1 Qualitative Description Eﬀect of VG on the Channel 2 IDS as a function of VDS and VGS Resistive Mode Saturation Mode or Triode Mode Summary of MOSFET Equations 3 MOSFET Symbols Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols Deﬁnition of W and L The variation of IDS versus VGS and VDS is a basic property of a MOS device. The values of L and W, the length and width, of the transistor channel directly aﬀect IDS . Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols Deﬁnition of W and L The variation of IDS versus VGS and VDS is a basic property of a MOS device. The values of L and W, the length and width, of the transistor channel directly aﬀect IDS . W S G D L Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols Deﬁnition of W and L The variation of IDS versus VGS and VDS is a basic property of a MOS device. The values of L and W, the length and width, of the transistor channel directly aﬀect IDS . W S G D L Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols Eﬀect of VG on the Channel Suppose that both the source and the drain of an n-channel MOSFET are at ground, that is, 0V. Now consider the eﬀect of varying the gate voltage VG . Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols Eﬀect of VG on the Channel Suppose that both the source and the drain of an n-channel MOSFET are at ground, that is, 0V. Now consider the eﬀect of varying the gate voltage VG . Suppose ﬁrst that the gate voltage is negative, VG < 0. VG < 0 + + + + + + + + Source Drain Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols Eﬀect of VG on the Channel Suppose that both the source and the drain of an n-channel MOSFET are at ground, that is, 0V. Now consider the eﬀect of varying the gate voltage VG . Suppose ﬁrst that the gate voltage is negative, VG < 0. VG < 0 + + + + + + + + Source Drain The negative voltage on the gate attracts the positively charged holes causing then to collect beneath the thinox - this is referred to as accumulation. Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols VG > 0 but small The positive gate voltage now repels the majority holes from the area beneath the thinox. Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols VG > 0 but small The positive gate voltage now repels the majority holes from the area beneath the thinox. VG > 0 - - - - - - - - Source + + + + Drain Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols VG > 0 but small The positive gate voltage now repels the majority holes from the area beneath the thinox. VG > 0 - - - - - - - - Source + + + + Drain As holes are repelled from beneath the gate the negative acceptors of the p-type material are exposed. Essentially this is a depletion zone. Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols VG > VT 0 Inversion If VG is made suﬃciently positive then enough electrons are drawn up to the thinox to give n-type semiconductor - this is inversion. Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols VG > VT 0 Inversion If VG is made suﬃciently positive then enough electrons are drawn up to the thinox to give n-type semiconductor - this is inversion. VG > VT 0 - - - - - - - - Source - - - - - - Drain + + + + Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS Eﬀect of VG on the Channel MOSFET Symbols VG > VT 0 Inversion If VG is made suﬃciently positive then enough electrons are drawn up to the thinox to give n-type semiconductor - this is inversion. VG > VT 0 - - - - - - - - Source - - - - - - Drain + + + + The voltage VT 0 is referred to as the threshold voltage. There is now a conducting channel from source to drain of the MOSFET. Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Modes of Operation The MOSFET, whether n-channel or p-channel, has three modes of operation, these are: Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Modes of Operation The MOSFET, whether n-channel or p-channel, has three modes of operation, these are: 1 Cutoﬀ For VGS < VT 0 no conducting channel is formed and there is no current from source to drain. IDS = 0. Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Modes of Operation The MOSFET, whether n-channel or p-channel, has three modes of operation, these are: 1 Cutoﬀ For VGS < VT 0 no conducting channel is formed and there is no current from source to drain. IDS = 0. 2 Resistive or Linear For VGS >= VT 0 and VDS < VGS − VT 0 the current through the device is dependent, initially linearly, on VDS , and on VGS . Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Modes of Operation The MOSFET, whether n-channel or p-channel, has three modes of operation, these are: 1 Cutoﬀ For VGS < VT 0 no conducting channel is formed and there is no current from source to drain. IDS = 0. 2 Resistive or Linear For VGS >= VT 0 and VDS < VGS − VT 0 the current through the device is dependent, initially linearly, on VDS , and on VGS . 3 Saturation or Triode For VGS >= VT 0 and VDS >= VGS − VT 0 the current through the device is dependent only on VGS . Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 1 Assume that VGS >= VT 0 and that VDS < VGS − VT 0 . Since VGS >= VT 0 a conducting inversion layer is formed from the source to the drain. Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 1 Assume that VGS >= VT 0 and that VDS < VGS − VT 0 . Since VGS >= VT 0 a conducting inversion layer is formed from the source to the drain. Deﬁne the Channel Support Voltage as: VCS = (VGS − VT 0 ) − V (x) (1) where V (x) is the channel voltage at a distance x along the channel. L x Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 2 Now assume that V (x) is linear in x (this is not actually true but for long channels is not too far oﬀ), Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 2 Now assume that V (x) is linear in x (this is not actually true but for long channels is not too far oﬀ), so that V (x) can be written as x V (x) = VDS (2) L Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 2 Now assume that V (x) is linear in x (this is not actually true but for long channels is not too far oﬀ), so that V (x) can be written as x V (x) = VDS (2) L The average value of the channel support voltage is then: VDS VCS = (VGS − VT 0 ) − (3) 2 Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 2 Now assume that V (x) is linear in x (this is not actually true but for long channels is not too far oﬀ), so that V (x) can be written as x V (x) = VDS (2) L The average value of the channel support voltage is then: VDS VCS = (VGS − VT 0 ) − (3) 2 Due to the assumption that VDS < VGS − VT 0 the channel support voltage is never zero. Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 3 The charge stored within the inversion layer is: Q = VCS Cg (4) where Cg is the gate capacitance, which is: Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 3 The charge stored within the inversion layer is: Q = VCS Cg (4) where Cg is the gate capacitance, which is: A 0 SiO2 Cg = (5) tox where A is the area of the gate (= WL), 0 is the permittivity of free space, SiO2 is the relative permittivity of silicon dioxide and tox is the thickness of the thinox. Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 4 The charge stored within the channel is driven by the electric ﬁeld from the source to the drain. Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 4 The charge stored within the channel is driven by the electric ﬁeld from the source to the drain. The magnitude of that electric ﬁeld is: VDS E= (6) L Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 4 The charge stored within the channel is driven by the electric ﬁeld from the source to the drain. The magnitude of that electric ﬁeld is: VDS E= (6) L The resulting velocity of the electrons is then: ve = µe E (7) Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 4 The charge stored within the channel is driven by the electric ﬁeld from the source to the drain. The magnitude of that electric ﬁeld is: VDS E= (6) L The resulting velocity of the electrons is then: ve = µe E (7) and so the time to cross from source to drain is: L t= (8) ve Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 5 The current IDS is therefore: Q IDS = t Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 5 The current IDS is therefore: Q IDS = t Q VDS = L µe L Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 5 The current IDS is therefore: Q IDS = t Q VDS = L µe L VDS A 0 SiO2 µe VDS = VGS − VT 0 − 2 tox L2 Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 5 The current IDS is therefore: Q IDS = t Q VDS = L µe L VDS A µe VDS 0 SiO2 (9) = VGS − VT 0 − 2 L2 tox 2 VDS W = Kp L (VGS − VT 0 )VDS − 2 Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Resistive Mode - 5 The current IDS is therefore: Q IDS = t Q VDS = L µe L VDS A µe VDS 0 SiO2 (9) = VGS − VT 0 − 2 tox L2 2 VDS W = Kp L (VGS − VT 0 )VDS − 2 where 0 SiO2 µe Kp = (10) tox Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Saturation Mode When VDS is increased above VGS − VT 0 the end of the channel near the drain is no longer inverted. Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Saturation Mode When VDS is increased above VGS − VT 0 the end of the channel near the drain is no longer inverted. No further increase in current occurs. Setting VDS = VGS − VT 0 then gives: W (VGS − VT 0 )2 IDS(sat) = Kp (11) L 2 Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Saturation Mode When VDS is increased above VGS − VT 0 the end of the channel near the drain is no longer inverted. No further increase in current occurs. Setting VDS = VGS − VT 0 then gives: W (VGS − VT 0 )2 IDS(sat) = Kp (11) L 2 The current IDS is, in this approximation, independent of VDS . Ampliﬁer circuits normally use FETs in saturation mode (also called triode mode for historical reasons) Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Summary of MOSFET Equations 0 VGS < VT 0 IDS = Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Summary of MOSFET Equations 0 VGS < VT 0 2 VDS IDS = Kp W L (VGS − VT 0 )VDS − 2 VGS > VT 0 , VDS < VGS − V Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Summary of MOSFET Equations 0 VGS < VT 0 2 VDS IDS = Kp W L (VGS − VT 0 )VDS − 2 VGS > VT 0 , VDS < VGS − V (VGS −VT 0 )2 Kp W L 2 VGS > VT 0 , VDS >= VGS − Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations Summary of MOSFET Equations 0 VGS < VT 0 2 VDS IDS = Kp W L (VGS − VT 0 )VDS − 2 VGS > VT 0 , VDS < VGS − V (VGS −VT 0 )2 Kp W L 2 VGS > VT 0 , VDS >= VGS − A similar set of equations apply to p-channel devices, but with the polarity of voltages reversed, and µe replaced by µh . Dr DC Hendry MOSFET Characteristics Qualitative Description Resistive Mode IDS as a function of VDS and VGS Saturation Mode or Triode Mode MOSFET Symbols Summary of MOSFET Equations MOSFET Curves For an n-channel device with VT 0 = 0.8V the above curves are as follows: Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols nMOS Enhancement Devices There are a variety of symbols in use for these devices, the two most commonly found are: d g b s Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols nMOS Enhancement Devices There are a variety of symbols in use for these devices, the two most commonly found are: d g - gate d - drain s - source g b b - bulk (substrate) s Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols nMOS Enhancement Devices There are a variety of symbols in use for these devices, the two most commonly found are: d g - gate d - drain s - source g b b - bulk (substrate) s d g s Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols nMOS Enhancement Devices There are a variety of symbols in use for these devices, the two most commonly found are: d g - gate d - drain s - source g b b - bulk (substrate) s d This symbol is most com- monly used for digital cir- g cuits. s Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols pMOS Enhancement Devices Again two symbols are in common usage: d g b s Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols pMOS Enhancement Devices Again two symbols are in common usage: d g - gate d - drain s - source g b b - bulk (substrate) s Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols pMOS Enhancement Devices Again two symbols are in common usage: d g - gate d - drain s - source g b b - bulk (substrate) s d g s Dr DC Hendry MOSFET Characteristics Qualitative Description IDS as a function of VDS and VGS MOSFET Symbols pMOS Enhancement Devices Again two symbols are in common usage: d g - gate d - drain s - source g b b - bulk (substrate) s d This symbol is most com- monly used for digital cir- g cuits. s Dr DC Hendry MOSFET Characteristics

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MOSFET Characteristics, Power MOSFET, Power MOSFETs, Gate Drive, threshold voltage, Drain Source, source voltage, gate voltage, Drive Characteristics, input capacitance

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posted: | 1/31/2011 |

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