6.4.3 Effect of real surfaces

Departure from the ideal case is due to • Work function difference between the doped polysilicon gate and substrate• The inevitably charges at the Si-SiO2 interface and within the oxide

• Фms= Фm - Фs is always negative for this case.• Most negative foe heavily doped p-type Si.

Work function difference

mq sq

To accommodate the work function difference • The metal is positively charged and the semiconductor surface is negatively charged at equilibrium.• A tilt in the oxide conduction band (implying an electric field)• The bend down near the semiconductor surface.• In fact, if the Фms is sufficiently negative, an inversion region can exist with no external voltage applied.• To obtain the flat band conduction, we must apply a negative voltage to the metal (VFB= Фms.

Equilibrium Diagram

Charges in oxide • Alkali metal ions (particularly Na+) can be incorporated inadvertently in the oxide.• Sodium ions introduce positive charges (Qm) in the oxide, which in turn induce negative charges in the semiconductor. (distance sensitive)Charges at O/S interface (Qit)• Result from sudden termination of the semiconductor crystal lattice at the oxide surface.Fixed charges in transition layer (Qf)• Near the interface is a transition layer (SiOx) containing fixed charges (Qf)

• Uncompensated Si bonds• Some ionic Si left

• Qit and Qf are about 1010 charges/cm2 for carefully treated interfaces with {100} surfaces.

Interface charge

• For simplicity we will include the various oxide and interface charges in an effective positive at the interface Q i (C/cm2)• The effect of this charge is to induce an equivalent negative charge in the semiconductor.

i

imsFB C

QV

Since the difference in work function and the positive interface charge both tend to bend the bands down at the semiconductor surface, a negative voltage must be applied to the metal relative to the semiconductor to achieve the flat band conduction.

Flat band condition

6.4.4 Threshold voltage

i

imsFB C

QV Fi

d

i

imsT C

QCQV 2

0 smms 0iQ 0dQ 0F for p-type semiconductor

VFB

Influence of material parameters on threshold voltage

• All four terms give negative contributions in the p-channel case. Negative threshold voltage (VT) for p-channel devices.• N-channel devices may have either positive or negative threshold voltages, depending on the relative values of terms.

Influence of material parameters on threshold voltage

• All terms except Qi/Ci depend on the doping in the substrate.• Фms and F have relatively small variations as EF is moved up or down by the doping.• Large changes can occur in Qd, which varies with the square root of the doping impurity concentration.

Fi

d

i

imsT C

QCQV 2

• p-channel: VT is always negative.• n-channel

• for lightly doped p-type substrates, negative flat band voltage terms can dominate resulting in a negative VT.• for more heavily doped substrates, the increasing contribution of Na to the Qd term dominates, and VT becomes positive.

Threshold voltage

• In a p-channel devices (VT <0 always)• A negative VT means that the negative voltage we apply must be larger than VT in order to achieve strong inversion.

• We expect to apply a negative voltage from metal to semiconductor in order to induce the positive charges in the channel.

• In an n-channel devices• A positive value for VT means the applied voltage must be larger than this threshold value to obtain strong inversion and a conducting n channel.

• We expect to apply a positive voltage to the metal to induce the n channel. (enhancement mode, normally off)

• A negative VT means that a channel exists at V=0 due to the Фms and Qi effects.• We must apply a negative voltage VT to turn the device off. (depletion mode, normally on)

Fi

d

i

imsT C

QCQV 2

• Since lightly doped substrates are desirable to maintain a high breakdown voltage for the drain junction, VT will be negative for n-channel devices made by standard processing.

6.4.5 MOS capacitance-voltage analysis

Various parameters of a MOS device can be determined from the C-V characteristics.

1.Type of substrate doping2.Insulator capacitance Ci =i/d → insulator thickness d3.The minimum depletion capacitance Cmin → Cdmin

4.Minimum depletion capacitance Cdmin =s/Wm → substrate doping5.Substrate doping → flat-band capacitance CFB

6.flat-band capacitance CFB → VFB 7.Ci, VFB, substrate doping → VT

8.Fast interface state density Dit

9.Mobile ion charges Qm

1. Type of substrate doping

• If the high-frequency capacitance is large for negative gate biases and small for positive biases, it is a p-type substrate, and vice versa.• For the low frequency C-V curve for p-type material, as the gate bias is made more positive (or less negative), the capacitance goes down slowly in depletion and then rises rapidly in inversion.

• As a result, the low frequency C-V is not quite symmetric in shape.

high-frequency

low-frequency

2. Insulator capacitance Ci =i/d → insulator thickness d

The insulator capacitance Ci =i/d in accumulation or strong inversion (at low frequencies) gives us the insulator thickness d.

iC

min

min

di

di

CCCC

3. The minimum depletion capacitance Cmin → Cdmin

The minimum MOS capacitance, Cmin, is the series combination of Ci and minimum depletion capacitance Cdmin =s/Wm, corresponding to the maximum depletion width.We can in principle use the measurement of Cmin to deduce Cdmin.

iC

min

minmin

di

di

CCCCC

4. Minimum depletion capacitance Cdmin =s/Wm → substrate doping

The minimum MOS capacitance, Cmin, is the series combination of Ci and minimum depletion capacitance Cdmin =s/Wm, corresponding to the maximum depletion width.We can in principle use the measurement of Cmin to deduce Cdmin.We can use deduced Cdmin to find Wm and then to determine the substrate doping Na.

2/1

2

2/1)/ln(2.)(2

a

ias

a

ssm Nq

nNkTqNinvW

5. Substrate doping → flat-band capacitance CFB

Debye length is determined from the substrate doping.

02 pqkTL s

D

The semiconductor capacitance at flat band CFB is determined from the Debye length capacitance.

D

sDebye LC

The overall MOS flat band capacitance, CFB, is the series combination of CDebye and Ci.

Debyei

DebyeiFB CC

CCC

6. Flat-band capacitance CFB → VFB

We can thus determine VFB corresponding to the CFB.

iC

min

minmin

di

di

CCCCC

FBC

FBV

7. Ci, VFB, substrate doping → VT

Once Ci, VFB, and substrate doping are obtained, all terms in the VT expression are known.

Fi

d

i

imsT C

QCQV 2

VFB

Interestingly, the threshold voltage VT does not correspond to exactly the minimum of the C-V characteristics, Cmin, but a slightly higher capacitance marked as point 4.In fact, it corresponds to the series combination of Ci and 2Cdmin.

min

minmin

111

211

1 C

CCCC

C

didi

T

The reason for this is that when we change the gate bias around strong inversion, the change of charge in the semiconductor is the sum of the change in depletion charge and the mobile inversion charge, where the two are equal in magnitude at the onset of strong inversion.

8. Fast interface state density, Dit

These defects can change their charge state relatively fast in response to changes of the gate bias.

• A fast interface state moving above the Fermi level would tend to give up its trapped electron to the semiconductor (or equivalently capture a hole).• The same fast interface state below the Fermi level captures an electron (or gives up a hole).(Note: To talk in terms of electron or holes depends on which is the majority carrier in the semiconductor.)

The fast interface states give rise to a capacitance which is in parallel with the depletion capacitance in the channel(and hence is additive), and this combination is in series with the insulator capacitance Ci. The fast interface states contribute to the low frequency capacitance CLF, but not the high frequency capacitance CHF.

121

eVcmCCCC

CCCC

qD

HFi

HFi

LFi

LFiit

8. Mobile ion charge, Qm

Bias-temperature stress test• Heat up the MOS device to ~200-300oC (to make the ions more mobile) and apply a positive gate bias to generate a field of ~1MV/cm within the oxide.• After cooling the capacitor to room temperature, the C-V characteristics are measured.• The capacitor is heated up again, a negative bias is applied so that the ions drift to the gate electrode.• And another C-V measurement is made.

The positive bias repels positive mobile ions such as Na+ to the oxide-silicon interface so that they contribute fully to a flat band voltage we can call VFB

+. The negative bias attracts positive mobile ions, so they are too far away from the interface to affect the semiconductor band-bending, but induce an equal and opposite charge on the gate electrode. VFB

+ is determined. from the difference of the two flat band voltages, we can determine the mobile ion content using

)( FBFBim VVCQ

6.4.6 Time dependent capacitance measurements

Deep depletion•If the gate bias is varied rapidly from accumulation to inversion, the depletion width can momentarily become greater than the theoretical maximum for gate bias beyond VT.•This causes the MOS capacitance to drop below the theoretical minimum, Cmin, for a transient period.

Zerbst technique• This capacitance transient, C-t, forms the basis of a powerful technique to measure the lifetime.

Deep depletion and Zerbst technique

6.4.7 Current-Voltage characteristics of MOS gate oxides

There can be some leakage current for real insulators.

Fowler-Nordheim and direct tunneling

Fowler-Nordheim tunnelingCurrent for electrons going from the Si conduction band to the conduction band of SiO2, and then having the electrons “hop” along in the oxide to the gate electrode.

Direct tunnelingAs the gate oxides are made thinner, that the electrons in the conduction band of Si can tunnel through the gate oxide and emerge in the gate, without having to go via the conduction band of the gate oxide.

Tunneling currents are becoming a major problem in modern devices because the useful feature of high input impedance for MOS devices is degraded.

Pursuit of high gate capacitance

It is necessary to increase the gate capacitance Ci (=i/d) in order to increase the drain current.

High-k dielectricsUse insulators with a dielectric constant higher than SiO2, instead of reducing the gate oxide thickness d.Reducing d too much will increase the gate oxide field and cause the tunneling.

)exp(2

oxoxFN E

BΕI

BEI

ox

FN Fowler-Nordheim tunneling current

Electric field in the gate oxide

Constant, function of mn* and barrier height

Fowler-Nordheim tunneling current as a function of electric field across the oxide

Time-dependent dielectric breakdown(TDDB)

Prolonged charge transport through gate oxides can ultimately cause catastrophic electrical breakdown of the oxides, known as time-dependent dielectric breakdown (TDDB).

1.Electron tunneling into the conduction band of the gate oxide from the negative electrode cathode), then gaining energy from the electric field, thus becoming “hot” electrons in the gate oxide.2.If they gain sufficient energy, they can cause impact ionization within the oxide and create electron-hole pairs.3.These impact-generated holes, with very low mobilities, are trapped at defect sites within the oxide, near the cathode.4.The resulting band diagram is altered by this sheet of trapped positive charge, which causes the internal electric field between this point and the gate to increase.5.As a result, the barrier for electron tunneling from the gate into the oxide is reduced.6.More electrons can tunnel into the oxide, and cause more impact ionization.7.We get a positive feedback effect that can lead to a runaway TDDB process.

Distortion of band edges by trapped holes and electrons from impact ionization leading TDDB

6.1 Transistor Operation6.2 The Junction FET6.3 The Metal-Semiconductor FET6.4 The Metal-Insulator-Semiconductor FET6.5 The MOS Field-Effect Transistor

We analyze the conductance of the channel and find the ID-VD characteristics as a function of gate voltage VG.