MOS Terminology

Basic MOS & CMOS Terminology




What is the Full Form of SPICE and it application?
SPICE full form is Simulation Program with integrated Circuit Emphasis. it is a general purpose circuit simulator tool. SPICE uses three model to analyze the I-V characteristics of the                    MOSFET.

What are three different MOSFET model used by SPICE for Simulations?.
    1. Level-1(MOS1) - is Described by square law of I-V Characteristics.
    2. Level-2(MOS2) - is a detailed analytical MOSFET model. 
    3. Level-3(MOS3) - is a semi empirical model.
MOS2 & MOS3 include the second order effect such as short channel threshold voltage,                          subthreshold conduction, scattering effect, velocity saturation etc.

When second order effect come into picture?. and what are different the second order effects?
When the channel length is reduced to the order of the depletion width of source and drain region in the MOSFET. so many non ideal effect occurs that is know as second order effects. example of                    second order effects are:-
    • Hot carrier effects
    • channel length modulation
    • velocity saturation
    • DIBL
    • Body effect
    • mobility degradation
How you can increase the switching speed (reduce delay) in CMOS Circuit?.
    • By Increasing the W/L ratio
    • By layout modification
    • to make compact placement & routing of interconnect to reduce parasitic

what are different VLSI Design Styles?.
    • FPGA
    • Full Custom Design
    • Semi Custom Design (Standard cell Based Design)
    • Gate Array Design
Explain Electro migration.
Electromigration is phenomenon in which atom of the metal wire interconnect displaces from one place to other place caused by the flow of high current density. 
effect of Electromigration
    • voids are created by Electromigration
    • Hillocks are formed by Electromigration
how to avoid Electromigration
    • metal slotting can reduce Electromigration
    • Increase metal width (reducing resistance)
    • by increasing more number of contacts or via from one metal layer to other.
Explain Electrostatic discharge(ESD).
Electrostatic Discharge (ESD) Occurs when the charge stored in machine or human body are discharged to the chip on contact  or by the static induction.
                                                                                  or
ESD Occurs when two different charged body come into contact for very short duration of time.

Electrostatic discharge is the one of the most prevalent causes for the chip failures in both chip manufacturing and field operations.

ESD Protection
    • by making Clamping circuit which reduces signal overshoot and prevents damage from Electrostatic Discharge (ESD).
Define Lithography.
Lithography is a process of generating pattern on the wafer to fabricate the circuit elements and provide for component interconnections.

Photoresist:-
Photoresist is a light-sensitive , acid-resistant organic polymer which is used to cover the certain area of the device structure. Photoresist are basically two types
    • Positive Photoresist:- Photoresist which are initially insoluble and becomes soluble after the exposure of the UV light is called Positive photoresist.
    • Negative Photoresist:-Photoresist which are initially soluble and becomes insoluble(hardened) after the exposure of UV light is called Negative photoresist.
Mass Action Law:-
Under thermal equilibrium the product of the free electron concentration and free hole concentration is equal to a constant equal to the square of intrinsic carrier concentration.

where n and p denote the mobile carrier concentration of electron and holes.ni denotes the intrinsic carrier concentration of silicon.

Define Work Function
The energy required for an electron to move from fermi level into free space is called work function.

What are different region of operation of MOSFET?.
There are three region of operation MOSFET.

    1. Accumulation Region:- 
    2. Depletion Region:-
    3. Inversion Region:-
Here, We will understand Region of operation for n-channel MOSFET.
  1. Accumulation Region:- For n-channel MOSFET substrate is of p-type and substrate has both hole as a majority charge carriers and electron as a minority charge carriers. Hence, when we will apply the negative voltage at the gate terminal then direction of electric field will be from the substrate to gate electrode so hole in the p-type substrate will attracted towards the semiconductor oxide interface. The majority charge carriers concentration near the surface will be larger than the equilibrium hole concentration in the substrate, that is why this region is called as accumulation region.
  2. Depletion Region:-when small positive gate bias VG is applied to the gate electrode, then electric field will be directed towards the substrate in this case. The majority carriers(i.e. hole) will be repelled back into the substrate and these hole will leave negatively charged fixed ion behind. Thus negatively charged ion region is called as depletion region of MOSFET
  3. Inversion Region:-when gate voltage is applied greater than the threshold voltage, then minority charge carriers(electron) from the substrate is attracted at interface hence-type region is created at interface this n-type region is called as inversion layers, and this condition is called surface inversion 
or

A surface is said to be inverted when density of mobile electron become equal to the density of holes in the bulk(p-type) substrate. 

 

Difference between Enhancement mode MOSFET and Depletion mode MOSFET

Enhancement Mode MOSFET:-A MOSFET which has no conducting channel at zero gate bias is called as Enhancement mode (enhancement type) MOSFET.

Depletion Mode MOSFET:-when conducting channel already exist at zero gate bias is known as Depletion mode (Depletion type) MOSFET.


Explain Threshold Voltage of MOSFET
the value of gate to source (Vgs) voltage at which surface inversion occurs(conducting channel creates) is called threshold voltage VT0.

Threshold voltage depends on following physical components.
  1. The work function difference between gate and the channel.
  2. The gate voltage components to change the surface potential.
  3. The gate voltage components to offset the depletion region change.
  4. The voltage component to offset the fixed oxide charges in the gate oxide and in the silicon-oxide interface.

Explain Cutoff Region, Linear Region & Saturation Region of Operation of NMOS

  • Cutoff Region:-At VDS=0,When applied gate voltage is greater than ZERO and less than threshold voltage (0<VGS<VTO),then no channel is formed at interface so no current will flow this region of operation is called cutoff region of operation.

  • Linear Region:-when we apply gate to source voltage VGS>VTO and small drain voltage VDS>0 is applied then current will flow proportional to VDS from source to drain through channel, and this region of operation is called linear region.

  • Saturation Region:-when VGS>VTO, and VDS is increased and reached at point greater than VDsat, then depletion region forms at adjacent to drain and this region grows towards the source with increasing voltages. This region of operation is called saturation region.in this region current looses control over voltage and current become constant.
What is Pinch Off Point?.

when the drain voltage is increased, the inversion layer charge and the channel depth at drain end start to decrease, eventually for VDS=VDSAT, the inversion charge at drain is reduced to zero, which is called pinch-off point.



Explain Channel Length Modulation

When the drain to source voltage is increased beyond the saturation voltage (VDS>VDSAT) larger portion of channel  become pinched-off due to which the actual channel length reduced to effective channel length. Hence effective channel length become function of Drain bias voltage, Hence saturation current become dependent on drain bias voltage after saturation point.


What is Substrate Bias Effect or Body effect?.
For N-MOS
To keep the source and drain junctions remain reverse biased, we apply bulk voltage VB<0.due to which the depletion region become larger in the bulk at source, drain hence we have to apply larger gate voltage to break the channel, hence threshold voltage become higher.
where Y is the body coefficient and VSB is bulk to source voltage.

MOSFET Scaling

There are two types of scaling, Constant field scaling and constant voltage scaling.
Scaling of the device does not mean only the reduction of channel length but proper scaling of all other device dimensions. 

Constant field scaling:-In constant- field scaling, the magnitude of internal electric field in MOSFET is preserved while the dimensions are scaled down.
it yields the reduction in the power-delay product of the transistor. Hence, it requires the reduction in power supply for reduced feature size.

Constant voltage scaling:-in constant voltage scaling, all dimensions of the MOSFET are reduced by a factor S. the power supply voltage and the terminal voltages remain unchanged. This is preferred scaling technique as it provides voltage compatibility with other technologies.

Short Channel Effects
Short channel effect In MOSFET occurs when the channel length is comparable to the depletion length of source and drain at substrate junction.
                                                                                                                   







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