UNIT I MOS TRANSISTOR THEORY AND PROCESS TECHNOLOGY

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Presentation transcript:

UNIT I MOS 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 transistor p-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