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MOS fundamentals
Metal-oxide-semiconductor FET is the most important device in modern microelectronics.
In this chapter, we will study:– Ideal MOS structure electrostatics– MOS band diagram under applied bias– Gate voltage relationship– I-V characteristics, transfer characteristics– Enhancement and depletion MOS– Channel length modulation, body effect– Biasing of MOSFETs, capacitances in MOS .
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MOS Transistor
• An MOS structure is created by superimposing several layers of conducting, insulating, and transistor forming materials to create sandwich like structure shown in fig 1.
• A MOS transistor can be modeled as a 3-terminal device that acts like a voltage controlled resistance. As suggested by Figure 2 an input voltage applied to one terminal controls the resistance between the remaining two terminals.
• In digital logic applications, a MOS transistor is operated so its resistance is always either very high (and the transistor is “off”) or very low (and the transistor is “on”).
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Fig. 1
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MOS Transistor
Figure 1 The MOS transistor as a voltage-controlledresistance.
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MOS Transistor
The basic electrical properties of the semiconductor (Si), the equilibrium concentration of mobile carriers in Si ,always obeys the
Mass Action Law given by,
Assuming that the substrate is uniformly doped with an accepter concentration NA, the Equilibrium electron and hole concentration in the P type substrate.
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Energy Band Diagram For P-Type Silicon
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Energy Band Diagram For P-Type Silicon
Fermi Potential
For P-type Fermi Potential
For n-type Fermi Potential
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Energy Band Diagram For P-Type Silicon
Electron affinity: It is the potential difference between conduction band level and vacuum (free space) level.It is denoted by qX.
Work Function: The energy required for an electron to move from Fermi level into free space.
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Energy Band Diagram of the components of MOS
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Energy Band Diagram of the Combined MOS System
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MOS Transistor
•Compared to the bipolar junction transistor (BJT), the MOS transistor occupies a relatively smaller silicon area, and its fabrication used to involve fewer processing steps.•There are two types of MOS transistors, n-channel and p-channel; the names refer to the type of semiconductor material used for the resistance-controlled terminals. The circuit symbol for an n-channel MOS (NMOS) transistor is shown in Figure 2.
Figure 2 Circuit symbol for an n-channel MOS (NMOS) transistor.
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• The terminals are called gate, source, and drain. The voltage from gate to source (Vgs) in an NMOS transistor is normally zero or positive. If Vgs = 0, then the resistance from drain to source (Rds) is very high, in the order of a megohm (106 ohms) or more. As we increase Vgs (i.e.,increase the voltage on the gate), Rds decreases to a very low value, 10 ohms or less in some devices.
• In an n-MOS transistor the majority carriers are electrons.• A positive voltage applied on the gate with respect to the
substrate enhances the number of electrons in the channel and hence increases the conductivity of the channel.
MOS Transistor
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MOS system under External Bias
Accumulation Region:
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MOS system under External Bias
Depletion Region:
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Energy band diagrams and charge density diagrams describing MOS capacitor in p-type Si
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MOS system under External Bias
The thickness xd of Depletion Region: Assume mobile hole charge in thin horizontal layer parallel to the surface is
The change in surface potential
Integrating along with vertical dimension
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MOS system under External Bias
Thus depth of depletion region
And the depletion region charge density,
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MOS system under External Bias
Inversion Region:
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MOS system under External Bias
The maximum depletion depth xdm
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Structure and Operation of MOSFET
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Circuit symbol of n-type MOSFET
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Circuit symbol of p-type MOSFET
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Formation of depletion region in n-type enhancement MOSFET
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Band diagram for MOS underneath gate
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Formation of inversion region in n-type enhancement MOSFET
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Threshold Voltage
Work function difference is
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Threshold Voltage
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Threshold Voltage
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MOSFET Operation
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MOSFET Operation
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MOSFET Operation
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MOSFET Current-Voltage Characteristics
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MOSFET Current-Voltage Characteristics
Gradual Channel Approximation
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MOSFET Current-Voltage Characteristics
Boundary Conditions for channel voltage Vc(y) are:
Now assume that the entire channel region between the source and The drain is inverted,
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MOSFET Current-Voltage Characteristics
Let QI(y) be the total mobile electron charge in the surface inversionlayer then,
Now consider the incremental resistance dR of the differential channel segment shown in fig (a) then,
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MOSFET Current-Voltage Characteristics
Fig (a)
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MOSFET Current-Voltage Characteristics
Applying Ohm’s law for this segment yields the voltage dropalong the incremental segment dy, in the y direction.
This equation can be integrated along the channel,
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MOSFET Current-Voltage Characteristics
Now put the value of QI(y), we get
Assume that the channel voltage Vc is the only variable, the Drain current is found as follows.
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MOSFET Current-Voltage Characteristics
This current equation can be rewritten as
Where the parameters k and k’ are defined as
Eq. -1
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MOSFET Current-Voltage Characteristics
The drain current equation-1 has been derived under following assumptions,
Which guarantee that entire channel region between source and drain is inverted. This condition is corresponds to the linearoperating region for the MOSFET.
Beyond the linear region boundary MOS transistor is assumed To be in saturation.
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MOSFET Current-Voltage Characteristics
Boundary for Saturation are
In Saturation region VDS = VDSAT , the saturation current can be foundby substituting in eq. -1
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MOSFET ID-VGS Transfer Characteristics
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Channel Length Modulation
Consider the inversion layer charge QI (y), that represents the total electron charge on the surface, the inversion layer charge at thesource end of the channel,
the inversion layer charge at the drain end of the channel,
Now at the edge of the saturation,
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Channel Length Modulation
In saturation region inversion layer charge at drain end becomesZero,
This condition is called pinched-off at the drain end at y=L.
If VDS > VDSAT , the large portion of the channel becomespinched-off .This is called channel length modulation. Under this condition length
of the channel is called effective channel length.
Where ∆L is the length of the channel segment with QI = 0shown in fig (b)
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Channel Length Modulation
Fig (b)
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Channel Length Modulation
Hence the pinch-off point moves from drain end of the channeltowards source with increase in VDS the remaining portion is in depletion mode. The channel voltage at L’<y<L ,
The drain current under this condition
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Channel Length Modulation
Put L’ = L - ∆L, then saturation current can be rewrite
But
To simplify the analysis even further, we will use following Empirical relation between ∆L and VDS
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Channel Length Modulation
Here λ is an empirical model parameter and is called channel length modulation coefficient .Assume λ. VDS << 1, the saturation current can be written
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Channel Length Modulation
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Body Effect
The threshold voltage is not constant with respect to the VSB .This is known as substrate bias effect or body effect.
Where γ is called substrate bias or body effect coefficient.
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Terminal Voltages and Currents for N,P Channel MOSFET
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Current Voltages Equation for N- Channel MOSFET
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Current Voltages Equation for P- Channel MOSFET
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MOSFET Capacitances
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MOSFET Capacitances
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MOSFET CapacitancesOxide Related Capacitances:
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MOSFET Capacitances
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MOSFET Capacitances
Cut Off Mode: Cgs = Cgd = 0 and
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MOSFET Capacitances
Linear Mode: Cgb = 0 and
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MOSFET Capacitances
Saturation Mode: Cgd = Cgb = 0 and
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MOSFET Capacitances
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MOSFET Capacitances
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MOSFET Capacitances
Junction Capacitances (Csb and Cdb):
Fig-1
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MOSFET Capacitances
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MOSFET Capacitances
The depletion region thickness (xd):
Where φ0 is built in potential
The depletion region charge Qj
Eq. – 1Here A is the junction area.
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MOSFET CapacitancesThe junction capacitances associated with the depletion region is defined as
Differentiating eq.-1 with respect to V we get expression forJunction capacitance
In general form
Eq-2
Eq-3
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MOSFET Capacitances
The parameter m in eq-3 is called grading coefficient and Cj0 is the zero bias junction capacitance per unit area.
The equivalent large signal capacitance can be defined as
Eq-4
Bu substituting eq-3 into eq-4, we obtain
Eq-5
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MOSFET CapacitancesFor a special case of abrupt p-n junction eq-5 becomes
Eq-6
This equation can be written in a simpler form
Where Keq is the dimension less coefficient is called voltage equivalence factor (0 < Keq < 1)
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MOSFET Capacitances
Assuming that NA (sw) is the sidewall doping density, thezero bias junction capacitance per unit area Cj0 (sw).
Where φ0sw is built in potential. Since all the sidewalls have a same depth xj , the zero bias junction capacitance per unit length
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MOSFET Capacitances
The voltage equivalence factor Keq (sw)
The equivalent large signal junction capacitance Ceq (sw) for a sidewall of length ( perimeter) P can be calculated as
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MOSFET Capacitances
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MOSFET Capacitances
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MOS Transistor
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nMOS Enhancement transistor
P-Si
electrons
N-channelMOSFET(NMOS)uses p-type substrate
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MOSFET operation
ID
VD
Pinch-off
VG1
VG2
VG3
VG3 > VG2 > VG1
When a positive voltage VG is applied to the gate relative to the substrate, mobile negative charges (electrons) gets attracted to Si-oxide interface. These induced electrons form the channel.
For a given value of VG, the current ID increases with VD, and finally saturates.
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Ideal MOS capacitor
Oxide has zero charge, and no current can pass through it.No charge centers are present in the oxide or at the oxide-semiconductor interface.Semiconductor is uniformly doped
M = S
= + (EC – EF)FB
Let us consider a simple MOS capacitor and call it “ideal”
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Equilibrium energy band diagram for an ideal MOS structure
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Effect of an applied bias
Let us ground the semiconductor and start applying different voltages, VG, to the gate
VG can be positive, negative or zero with respect to the semiconductor
EF, metal – EF, semiconductor = – q VG
(Since electron energy = q V, when V < 0, electron energy increases)
Since oxide has no charge, d Eoxide / dx = / = 0; i.e. the E-field inside the oxide is constant.
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Consider p-type Si, apply VG < 0
EC
Ei
EVEFs
GqV
m' Accumulation
of holes
xqx
ioxide
oxide 1const.0 EEE
The oxide energy band has constant slope as shown. No current flows in Si EF in Si is constant.
Negative voltage attracts holes to the Si-oxide interface.This is called accumulation condition.Ei – EF shouldincreases near thesurface of Si.
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Accumulation condition, VG < 0, p-type Si
––––
+
+
Sheet of holes
smallcharge density
E
EM O p-type Si
VG < 0
Sheet ofelectrons
x
x
Accumulation of holes nearsilicon surface, and electronsnear the metal surface.
Similar to a parallel platecapacitor structure.
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Consider p-type Si, apply VG > 0 (Depletion condition)
EFM
EC
EiEFsEV
DepletionE
OM S
positive
0negative
+
+
+- - - -
- - - -
E
Finite depletion layerwidth
E
x
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Consider p-Si, apply VG >> 0 (Inversion condition)
EC
Ei
EV
EFM
+
+
+
+
- - - - - - -
- - - - - - --
-
Immobile acceptors
Mobile electrons
x
EFM
EFS
E
E
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Inversion condition
If we continue to increase the positive gate voltage, the bands at the semiconductor bends more strongly. At sufficiently high voltage, Ei can be below EF indicating large concentration of electrons in the conduction band.
We say the material near the surface is “inverted”. The “inverted” layer is not gotten by doping, but by applying E-field. Where did we get the electrons from?
When Ei(surface) – Ei(bulk) = 2 [EF – Ei(bulk)], the condition isstart of “inversion”, and the voltage VG applied to gate is called VT
(threshold voltage). For VG > VT, the Si surface is inverted.
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Energy band diagrams and charge density
diagrams describing MOS
capacitor in n-type Si
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Energy band diagrams and charge density diagrams describing MOS capacitor in p-type Si
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Example 1
Construct line plots that visually identify the voltage ranges corresponding to accumulation, depletion and inversion in ideal n-type Si (i.e. p-channel) and p-type Si (i.e. n-channel) MOS devices.
Answer: