UNIT IMOS TRANSISTOR THEORY

AND PROCESS TECHNOLOGY

TOPICS

• NMOS and PMOS transistors • Threshold voltage • Body effect • Design equations• Second order effects • MOS models• Small signal AC characteristics • Basic CMOS technology

MOS TRANSISTOR• MOS Metal Oxide Semiconductor• MOS transistor is a majority carrier device,

current is the conducting channel b/w source & drain.

• 2 types of MOS transistor:n-MOS transistorp-MOS transistor

• Various symbol representation for n-MOS & p-MOS

n-MOS

n-MOS

• Majority carriers electrons• When +ive voltage is applied on gate, no. of

electrons will be increased.• So, conductivity of channel is increased.• If Vg < Vt ,Then the channel is cutoff.

• Threshold voltage is the voltage at which MOS device starts to conduct.

p-MOS

• Majority carriers holes

• 2 types of modes in n-MOS & p-MOS:– Enhancement mode– Depletion mode.

• n-MOS Enhancement mode:– Device will be cut off when Vgs= 0

• n-MOS Depletion mode:– Device will conduct even if Vgs= 0

• p-MOS Enhancement mode:– Above Vtp, device will start to conduct.

• p-MOS Depletion mode:– Device will be in conducting state even if Vgs= 0

Conduction characteristics

n-MOS ENHANCEMENT TRANSISTOR

• It has Moderately doped p - type silicon substrate

• In that p-substrate, Heavily doped n + source and drain.

• Channel – a thin insulating layer made up of Silicon dioxide (SiO 2)

• Gate –polycrystaline silicon (polysilicon)

Working principle• When Vgs= 0, Vds is applied.

• There is no current flow b/w source & drain.• When positive voltage is applied to gate,

electric field is produced across p-substrate.• It attracts electrons towards the channel.• It is continued when gate voltage is increased

further, the region below gate will be converted from p-type to n-type.

• So the channel becomes n-type(n-channel).

• Three types of modes of MOS transistor:– Accumulation mode– Depletion mode– Inversion mode

Accumulation mode

• In this mode Vgs < Vt

• Initially p-substrate is having holes only.

Depletion mode

• In this mode Vgs =Vt

• Depletion region is created in this mode .

• Vgs is increased and reach Vt

• So holes are repelled from the channel.

• Because of this, depletion region is created.

Inversion mode

• In this mode Vgs > Vt

• Voltage increased further, so electrons are attracted towards the region below gate.

• So, the layers of electrons will be formed below the gate.

• Bcoz of this layer, this mode is known as inversion mode.

Operation regions of MOS transistor

• Cut-off mode• Non-saturated mode(linear or resistive or

unsaturated mode)• Saturated mode

Cut-off mode

• When Vgs > Vt and Vds = 0.

• Depletion layer is created.• So the region is completely cut-off.

Non-saturated mode

• When Vds < Vgs - Vt

• Deep channel is created in this mode.• Inversion region is weak in this region.

Saturated mode

• Here, Vds > Vgs - Vt and Vgd < Vt

• The channel becomes pinched off.

• Inversion is strong.• Channel current is

controlled by gate voltage & it is independent of drain voltage

• Ids depends following:– Distance b/w source & drain– Channel width– Threshold voltage– Thickness of oxide layer– Dielectric constant of gate insulator– Carrier mobility(μ)

Conclusion

• Three conduction regions are available in nMOS enhancement transistor.

Cut-off region (no current flow)Non-saturated (Id depends Vg and Vd)

Saturated mode (Id independent of Vds )

p-MOS ENHANCEMENT TRANSISTOR