*Silicon on Insulator (SOI) Based Devices Amitava DasGuptaDepartment of Electrical EngineeringI.I.T. MadrasChennai 600037adg@ee.iitm.ac.in

*Why SOI ?Why SOI ?CMOS circuits on SOI has several advantages resulting in lower power and higher speedIdeal for radiation hard devicesIdeal substrate for high temperature devices, MEMS based sensors

SOI devices will allow us to keep up with Moores Law for a longer period

*SOI MOSFET Device of future

*SOI MOSFET StructureG1DSG2N+N+PSubstrateBuried Oxide

*CMOS - Bulk vs. SOIBulk and SOI CMOS structures

*Advantages of SOI CMOS TechnologySimpler technology with no wells or trenchesDevice performanceBetter dielectric isolation in both vertical and horizontal directionsNo latch up Better radiation tolerance

*Advantages of SOI CMOSDevice performanceReduced capacitance and leakage currents as the area of the source drain junctions are reducedReduced short channel effects allows enhanced scaling

DrainField oxideField ImplantSubstrateBulkDrainSubstrateSOIBuried oxide

*Advantages of SOI CMOSDevice performanceReduced subthreshold swing allows lower voltage operation

*SOI Wafer TypesSOS for Silicon on Sapphire (almost obsolete)SIMOX Separation by IMplantation of OxygenBonded Wafer SOIBESOI stands Bonded and Etch-back SOISmart-Cut processEltran process

*SmartCut processSOI film thickness set by H2 implant energy and BOX thicknessHandle wafer B can be low-grade and cheapHigh grade wafer A can be reused many timesSmart Cut SOI wafer can be cheaper than Bulk Silicon Wafer !!!Hydrogen implantationthrough thermal oxidedose ~1-5e16 cm-2Handle wafer B is bondedAt ~400-600C wafer A separates from B at H2 peak

*SOI-like structures

*Common types of SOI MOSFET

*Energy Band diagrams of Bulk, Partially Depleted (PD) and Fully Depleted (FD) SOI MOSFETs Shaded regions are depleted

Ef

Ec

Ev

Gate oxide

Ef

Ec

Ev

Ef

Ec

Ev

Front

gate

oxide

Back

gate

oxide

Front

gate

oxide

Back

gate

oxide

A

B

C

*FD MOSFET: Operation Modes

Front Inversion

Back Accumulation

Front Depletion

Back Accumulation

Front Accumulation

Back Accumulation

Front Inversion

Back Depletion

Front Accum.

Back Depl.

Front Depl.

Back Depl.

Front Inversion

Back Inversion

Front Depletion

Back Inversion

Front Accumulation

Back Inversion

VG1

VG2

*SOI Device structure

*0EElectric field and Potential distributionFront surface is always in inversionA Back surface is in accumulationB Back surface is in depletion C Back surface is in inversion2B2B00

*Channel Coupling Equations in FD SOISolving Poissons equation in the SOI film under fully depleted condition, we have two equations which describe the coupling between the two channelsWhen back channel is in accumulation, sb= 0. Therefore

*VTh versus Vgb in FD SOIWhen back channel is in inversion, sb= 2B. ThereforeHenceWhen toxb>>tSi , Cs>>Coxb ,

*Effect of Coupling on Threshold Voltage of FD SOI

*VTh versus Vgb in FD SOIVTh is independent of toxbHowever, VTh becomes less sensitive to Vgb variation when toxb becomes much larger than toxf

*Transconductance Characteristics of FD SOIThe plateau in the characteristics indicates the back channel turning on before the front channel

Front gate voltage, VG1

Transconductance, gm

VG2

More positive

More negative

Plateau

-30

-20

-10

0

10

20

30

40

50

-6

-4

-2

0

2

Threshold voltage (V)

D

C

Back gate voltage (V)

A

B

VGb1

VGb2

VGb3

EMBED Word.Picture.8

VGf (V)

_1052982597.doc

-30

-20

-10

0

10

20

30

40

50

-6

-4

-2

0

2

Threshold voltage (V)

D

C

Back gate voltage (V)

A

B

VGb1

VGb2

VGb3

*Subthreshold swing (S) S = change in VGS for unit change in log(ID) ORS = change in VGS for one decade change in IDFor sharp transition between ON and OFF state, S should be smallIn subthreshold region of operation

*Subthreshold swing (S)(contd.) How well does the gate control the channel potential?When control is idealWhen control is badIn short channel devices, gate control is less due to effect of drain n>1Minimum possible value of S = 2.3VT = 60 mV at 300 K

*Subthreshold swing (S) in PD SOINote: Interface trapped charges have been neglected Including Interface trapped charges

*Subthreshold swing (S) in FD SOIReplace CD in the analysis for PD SOI with series combination of CS and CoxbAssuming CSCDSo n is much closer to 1 in FD SOI than in PD SOIA thicker back oxide helps in reducing Subthreshold swing

*Subthreshold slope vs. SOI ThicknessThe subthreshold slope changes drastically in the transition region from PD to FD SOI

*Current-Voltage relationsSPICE Level-3 modelFor FD SOI, n is less than in Bulk or PD SOI (refer to discuusion on Subthreshold slope)Current drive is more for FD SOI

*Electric field (Mobility): FD vs. Bulk Since transverse electric fields are smaller in FD devices, mobilities are higherAlso reflected in higher current drive

*Drive current: FD vs. Bulk and PD SOI

0

10

20

30

VG1-VTH (V)

5

4

3

2

1

0

SOI

Bulk

IDsat

m

A

(

)

*Floating Body Effects (FBEs) in SOIThese effects are due to- The creation of body charge QB in PD SOI- The source (n+) - body (p) - drain (n+) parasitic bipolar transistor (PBT) in both PD and FD SOI

*DC Current Kink in a PD SOI

*Explanation of DC current KinkWith increase in VD, e-h pairs may be generated near the drain by impact ionization. The electrons flow in to the drain (and become part of drain current).In FD SOI, the holes flow to the source In PD SOI, the holes accumulate in the floating body due to the higher barrier at source-substrate junction. This increases substrate potential, causing the threshold voltage to decrease

*Floating vs. grounded body VTCBody:The nMOS turns on while Vout is still high Floating body charges up Its threshold voltage decreases The VTC stretches out

*FBE due to PBT (both PD and FD)Holes generated by impact ionization flow into the neutral body. This hole current, which is the base current of the parasitic bipolar transistor (PBT), gets amplified by the PBT beta (b).This leads to: abnormal subthreshold slope/increased leakage early breakdown/ single transistor latch hysterisis in the operation of devices/circuits

Substrate (back gate)

Front gate

Floating body

Drain

*PBJT induced single transistor latch-up(a) Low VD - normal subthreshold slope(b) Higher VD - Slope increases gradually to infinite. Hysteresis appears due to reduction in threshold voltage.(c) Very high VD - Device "latches-up, i.e. does not switch off due to a large reduction in threshold voltage

*Single transistor latch-up examplePD SOI

Chart2

-15.2653585608-14.8181478405-14.7530093008

-15.3138590731-14.7699472336-14.7473945146

-15.3180346377-15.1252764595-14.7506695393

-15.2918434965-15.0193381826-14.7518623708

-15.2077624423-14.9164006422-14.742842411

-15.1023311305-14.828051338-14.7202805971

-14.868965492-14.7376723339-14.6913246261

-15.1888568224-14.9805721267-14.6506777823

-14.87397255-14.8179393191-14.5998420766

-14.9404176508-14.6850631194-14.7843386475

-14.9820072622-14.8280279543-14.6928314109

-14.9849128122-14.9798915769-14.601454139

-14.9574835525-14.7526479489-14.7302162899

-14.8923491739-14.8397356082-14.6034275641

-14.8060604726-14.8927730661-14.6993740861

-14.8689301635-14.6392002793-14.5481491157

-14.7374587383-14.6460722342-14.4349991729

-14.6282544282-14.4491200825-14.2677630971

-14.4385539424-14.1700796452-14.0640214127

-13.9250263937-13.7084158319-13.5650209453

-12.984284068-12.809416204-12.6912755565

-11.6006291791-11.4348986761-6.6575595529

-10.1929191214-10.0000886051-5.9504680029

-8.8025622971-8.1824498343-5.9288878912

-7.46669705-6.5914022285-6.0339132031

-6.4025351828-6.0426039262-5.9732380046

-5.7512471758-5.6723856734-5.6389921788

-5.2869120168-5.2724619961-5.2522185388

-4.985269505-4.9545283693-4.9449467465

-4.7665216081-4.7272953075-4.7152445128

-4.5984064823-4.5610133003-4.5449875499

-4.4621217442-4.4322551122-4.415025727

-4.3462136317-4.3250810184-4.3101798803

-4.2446473735-4.2295471338-4.2191968092

-4.1545385456-4.141071392-4.1351129305

-4.0742275528-4.0589897361-4.0553387116

-4.0023563483-3.9841751782-3.9804558598

-3.9376248719-3.9172110405-3.9119986521

-3.8788375381-3.8578483971-3.8509595701

-3.8250480857-3.8051749643-3.7971930266

-3.937263878-3.9156310891-3.9092659306

-4.0777396989-4.0538322812-4.0482813049

-4.2480015181-4.2228313659-4.2167726749

-4.4715949892-4.4464033624-4.4376559751

-4.795833876-4.769170599-4.7541124215

-5.3060164184-5.2679554246-5.2340015663

-6.3611175601-6.1115408049-5.9112580944

-8.8025126842-7.1359062082-6.4735586237

-11.6004422212-8.2019629094-6.5130795337

-14.1676546172-9.0647815201-6.124102223

-14.6019279859-9.4522671267-5.8791892455

-14.8319889393-9.2018011453-5.8484983796

-14.9020073385-7.8624496774-5.8653257473

-14.9884422429-6.4436987056-5.8613910348

-14.9357741957-6.3525700817-5.8466364268

-14.987280852-6.4492801326-5.8348328623

-14.9400278486-6.518722234-5.8258209449

-15.2645009037-6.5196681768-5.8168212965

-15.2841854485-6.4844891027-5.8080657735

-15.1703077878-6.4547064941-5.8019783272

VD=3VVD=3.3VVD=3.4V

VG1 (V)

ID (A)

Sheet1

VG1VD=3VVD=3.35VVD=3.45V

-1.9-15.2653585608-14.8181478405-14.7530093008

-1.8-15.3138590731-14.7699472336-14.7473945146

-1.7-15.3180346377-15.1252764595-14.7506695393

-1.6-15.2918434965-15.0193381826-14.7518623708

-1.5-15.2077624423-14.9164006422-14.742842411

-1.4-15.1023311305-14.828051338-14.7202805971

-1.3-14.868965492-14.7376723339-14.6913246261

-1.2-15.1888568224-14.9805721267-14.6506777823

-1.1-14.87397255-14.8179393191-14.5998420766

-1-14.9404176508-14.6850631194-14.7843386475

-0.9-14.9820072622-14.8280279543-14.6928314109

-0.8-14.9849128122-14.9798915769-14.601454139

-0.7-14.9574835525-14.7526479489-14.7302162899

-0.6-14.8923491739-14.8397356082-14.6034275641

-0.5-14.8060604726-14.8927730661-14.6993740861

-0.4-14.8689301635-14.6392002793-14.5481491157

-0.3-14.7374587383-14.6460722342-14.4349991729

-0.2-14.6282544282-14.4491200825-14.2677630971

-0.1-14.4385539424-14.1700796452-14.0640214127

0.0000000298-13.9250263937-13.7084158319-13.5650209453

0.1-12.984284068-12.809416204-12.6912755565

0.2-11.6006291791-11.4348986761-6.6575595529

0.3-10.1929191214-10.0000886051-5.9504680029

0.4-8.8025622971-8.1824498343-5.9288878912

0.5-7.46669705-6.5914022285-6.0339132031

0.6-6.4025351828-6.0426039262-5.9732380046

0.7-5.7512471758-5.6723856734-5.6389921788

0.8-5.2869120168-5.2724619961-5.2522185388

0.9-4.985269505-4.9545283693-4.9449467465

1-4.7665216081-4.7272953075-4.7152445128

1.1-4.5984064823-4.5610133003-4.5449875499

1.2-4.4621217442-4.4322551122-4.415025727

1.3-4.3462136317-4.3250810184-4.3101798803

1.4-4.2446473735-4.2295471338-4.2191968092

1.5-4.1545385456-4.141071392-4.1351129305

1.6-4.0742275528-4.0589897361-4.0553387116

1.7-4.0023563483-3.9841751782-3.9804558598

1.8-3.9376248719-3.9172110405-3.9119986521

1.9-3.8788375381-3.8578483971-3.8509595701

2-3.8250480857-3.8051749643-3.7971930266

1.8-3.937263878-3.9156310891-3.9092659306

1.6-4.0777396989-4.0538322812-4.0482813049

1.4-4.2480015181-4.2228313659-4.2167726749

1.2-4.4715949892-4.4464033624-4.4376559751

1-4.795833876-4.769170599-4.7541124215

0.8-5.3060164184-5.2679554246-5.2340015663

0.6-6.3611175601-6.1115408049-5.9112580944

0.4-8.8025126842-7.1359062082-6.4735586237

0.2-11.6004422212-8.2019629094-6.5130795337

0.0000000298-14.1676546172-9.0647815201-6.124102223

-0.2-14.6019279859-9.4522671267-5.8791892455

-0.4-14.8319889393-9.2018011453-5.8484983796

-0.6-14.9020073385-7.8624496774-5.8653257473

-0.8-14.9884422429-6.4436987056-5.8613910348

-1-14.9357741957-6.3525700817-5.8466364268

-1.2-14.987280852-6.4492801326-5.8348328623

-1.4-14.9400278486-6.518722234-5.8258209449

-1.6-15.2645009037-6.5196681768-5.8168212965

-1.8-15.2841854485-6.4844891027-5.8080657735

-2-15.1703077878-6.4547064941-5.8019783272

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10-4

10-8

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10-16

VD=3VVD=3.3VVD=3.4V

VG1 (V)

ID (A)

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Sheet3

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Sheet4

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Sheet5

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Sheet6

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Sheet7

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Sheet8

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Sheet9

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Sheet10

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Sheet11

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Sheet12

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Sheet13

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Sheet14

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Sheet15

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Sheet16

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