mosfet and short channel effects
TRANSCRIPT
MOSFET & Its CharacteristicsChannel Length ModulationShort Channel MOSFET & Short Channel Effects
1. Drain induced barrier lowering and Punchthrough2. Surface scattering3. Velocity saturation4. Impact ionization5. Hot electrons
Conclusion
Presentation Outline
MOSFET & Short Channel Effects
Muzafar Ahmad RatherM.Tech Nanotechnology(Ist-Sem)Jamia Millia Islamia, New Delhi
ResistorSource(S) Drain(D)
Gate(G), Changing cross section area
MOSFET Structure
MOSFET is 4-Terminal device with terminals Source, Drain, Gate and Body
Source and Body are generally connected together
The Gate potential controls the formation of channel
Characteristics & Regions of operation
In MOSFET we have various regions of operation
By varying V-GS
By varying V-DSUnder linear distribution of charge
Accumulation region, VG < 0Depletion/Weak Inversion region, VG > 0Strong Inversion region, VG >>0
Linear/Triode region, VDS < (VGS-VTH)Saturation region, VDS > (VGS-VTH)
Under Linear region:
LVVTHVGSWCI DOX
D)(
Resistance R :)(C/1 OX VTHVGS
LWR
Under Saturation region: (can act as current source)
LVTHVGSWC
satI OXD
22)(
,
Transconductance
LVTHVGSW
dVdIgG
Dsatm
)(COX,
Transconductance:
LWV
dVdIg D
G
Dlinm
OX,
C
Triode Mode
Channel Length Modulation & Pinch offThe characteristics are true as long as we have constant distribution of charges under Gate, In practice with increased VDS, the distribution of charge under MOS-capacitor changes causing pinch-off ,we thus have variation in actual channel length of MOSFET. Such variation in channel length is called channel length modulation, and by this we have a variation in Drain current
)1(2
2)(, DOX
D VL
VTHVGSWCsatI
)1()(COX, D
G
Dsatm V
LVTHVGSW
dVdIg
Also Transconductanc modifies to
A MOSFET is considered to be short when the channel length ‘L’ is the same order of magnitude as the depletion-layer widths (xdD, xdS). The potential distribution in the channel now depends upon both, transverse field Ex, due to gate bias and also on the longitudinal field Ey, due to drain bias When the Gate channel length <<1 m, short channel effect becomes important . This leads to many undesirable effects in MOSFET.
Five different physical phenonomena have to be considered in short-channel devices: Drain induced barrier lowering and Punchthrough Surface scattering Velocity saturation Impact ionization Hot electrons
Short-channel devices & Short Channel Effects
Drain induced barrier lowering and PunchthroughThe potential barrier, in small-geometry MOSFETs, is controlled by a two-dimensional electric field vector (in other words by both VGS and VDS).If the drain voltage is increased the potential barrier in the channel decreases(=pn junction band cure with more –ve slop), leading to Drain-Induced Barrier Lowering (DIBL). Under DIBL condition electrons can flow between the source and drain even if VGS < VTH.
Because of charge sharing the Threshold Voltagr also decreases,;Reduction in S and D junction depth xj can reduce the VTH shift,as charge shared by drain and source gets lower.
PunchthroughIncrease in VDS makes depletion width more and more to increase and there is occurrence of punch through once these depletion regions of source and drain touches.It can be decreased by having higher doping levels either in substrate or near source and drain.
For small-geometry MOSFETs, the electrons mobility in the channel depends on a two-dimensional electric field (Ex, Ey). The surface scattering occurs when electrons are accelerated toward the surface by the vertical component of the electric field Ex
Causes a reduction in the mobilityThe average surface mobility is about half as much as that of the bulk mobility
Surface scattering
G
Substrate
S D
Velocity saturationThe electron velocity is related to the electric field through the mobility:
V= μEFor higher fields the velocity does not increase with electric field, we have degradation of mobility because of scattering by vertical field. This leads to earlier saturation of current. i.e.,before VGS-VTH. Net result is reduction in drain current .
The velocity saturation reduces the transconductance of short-channel devices in the saturation condiction.
Impact ionizationThe presence of high longitudinal fields can accelerate electrons that may be able of ionizing Si atoms by impacting against themNormally most of the e- are attracted by the drain, so it is plausible a higher concentration of holes near the sourceIf the holes concentration on the source is able to creates a voltage drop on the source-substrate n-p junction of about 0.6V then
e- may be injected from source to substratee- travel toward the drain, increasing their energy and create new e-h pairse- may escape the drain fields and afect other devices
Hot electronsThe channel Hot Electrons effect is caused by electrons flowing in the channel for large VDS
e- arriving at the Si-SiO2 interface with enough kinetic energy >3.1ev to surmount the surface potential barrier are injected into the oxide
This may degrade permanently the C-V characteristics of a MOSFETs
ConclusionFOR IMPACT IONIZATIONFOR PUNCH THROUGH
Short Channel Effects are governed by complex physical phenomena and mainly Influenced because of both vertical and horizontal electric field components.
To meet the current requirements of Electronic devices, the miniaturization of devices is important. And so is Second Order effects which otherwise degrade the performance of devices.
