MOSFET Drain Current Characteristics

1
MOSFET Drain Current Characteristics

• Week 12

2
Id from channel charge

• Assuming inversion charge is proportional to COX
  times the gate voltage above threshold results in
  the integral
• This is a four terminal model that includes
  substrate bias Vb (V bulk) and all voltages are
  referenced to the bulk contact voltage.

3
Id from channel charge 2

• Doing the integral results in
• If VS VB VSB 0 in CMOS usually the case
  then

4
Id from channel charge 3

• Ignoring the depletion region effect the equation
  becomes the usual FET equation
• Setting the derivative of Id with respect to Vd
  equal to zero gives the Idsat condition
• For the saturation condition the drain current
  is

5
PSPICE LEVEL 1

• The PSPICE level 1 is the same set of equations
  but is not in terms of physical parameters and
  has a LAMBDA term for drain barrier lowering that
  results in a linear increase in ID with VDS
• Again for the saturated case VDSgtVGS-VTO

6
VTO substrate bias VBulk dependence

• At the source the depletion region width with
  substrate bias VB is
• Neglecting variation of channel voltage along the
  channel the effective charge per unit area Qd is
  then
• The Qdeff then changes the threshold by ?VTO
  where the quantity before the brackets defines a
  new constant gamma that appears in PSPICE

7
MOSFET Mobility Models

• MOSFET mobility models have been used to obtain
  more accurate device simulation results (Silvaco
  has some 10 models) and more accurate compact
  circuit models.
• The primary effect is a decrease in mobility with
  increased tangential electric field. The tangent
  field can be taken to be proportional to VG
  VTO. The text suggest the equation
• Here theta ? the mobility degradation parameter
  is a curve fitting parameter.

8
Short Channel Effects

• When Lg becomes very small short channel effects
  occur.
• With decreasing Lg the electric field along the
  channel increases. This has two effects.
• Electron velocity saturation at about 1.0x107
  cm/sec is one effect at high fields and ID then
  becomes a linear function of VG-VTO
• Drain induced barrier lowering at the ND source
  contact increases the channel charge resulting in
  the already mentioned linear increase in drain
  current with VDS modeled in PSPICE with LAMBDA
  the channel length modulation parameter

9
Short Channel and Narrow Channel VTO Thershold
effects.

• With short channels some of the depletion charge
  at the source and drain is shared with the
  heavily doped contacts decrease the effective Qd
  that the gate sees. This decreases VTO for short
  channel devices compared with long channel
  devices on the same circuit.
• Narrow channels have the opposite effect. As the
  channel narrows the field oxide starts to
  increase effective TOX, decreasing COX and
  increasing VTO.

10
Subthreshold Effects

• Below threshold the current really does not go to
  zero but decreases exponentially with gate bias
  lower than threshold.
• One suggested model available in PSPICE for
  VGSltVTO is
• Here n is picked to fit data and IDO is picked to
  provide current continuity.

11
Text Subthresold Current ID

• The text uses a similar but more elaborate
  subthreshold current model that includes a Cit
  interface trapped charge capacitance
• The constant c is given by
• This leads to a slope S given by

12
BISM SPICE Model

• BSIM is the acronym for Berkeley Short Channel
  Insulated Gate FET Model and SPICE is the
  acronym for Simulation Program with Integrated
  Circuit Emphasis.
• Reference MOSFET Models for SPICE Simulation
  including BSIM3v3 and BSIM4, William Liu, 2001,
  Wiley, ISBN 0-471-39697-4.
• BISM3 was the first BISM to introduce gate length
  L and gate width W dependant parameters.
• BISM4 no yet completely adapted accounts for
  added effects such as gate oxide tunneling and
  corrects for addition discontinuities in the
  capacitances as a function of bias voltage.
• BSIM model unit dimensions are all in meters (not
  cm)

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BISM Threshold Dependence

• The threshold voltage in the BISM3 model has
  layout geometry dependence (?NP1 for n
  channel,and -1 for p channel)

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BISM Threshold Dependence 2

• The model includes NCH channel doping to XT and
  NSUB doping of the well.
• VBM -3 default, Minimum voltage for body effect

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BISM Threshold Dependence 3

• The ?VT are

16
BISM Threshold Dependence 4

• Finally for both small gate length and width
• The built in voltage is and NSD default is
  1020cm-3

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BISM Threshold Dependence 5

• The L and Xdep are given by
• And for CV2VBS,eff lt-0.5 or CV2WVBS,efflt-0.5

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Id BISM3 Model

• The BISM3 Id model is (Leff and Weff have
  corrections for photo process for small
  dimensions)
• Where a ? is a fitting parameter for smooth
  transition to saturation.

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Id BISM3 Model

• The ? is related to a parameter Abulk 1? where
  Abulk is

20
BISM3 Mobillity Model

• There are 3 mobility models and for MOBMOD 3
  the mobility is
• This decreases mobility as a function of the
  tangential electric field (VGSVT)/TOX.