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IEEE TKANSACTI3NS ON ELECTRONDEVICES, VOL. ED-26, NO. 11 NOVEMBER 1978 6

Briefs

Field Induced Reemissionof Electrons Trapped inSO z

L FORBES, E. SUN, R. ALDERS, AND J MOLL

Abstract-A new form of ho t electron njection and rapping in n-chan nel IGFET’s is descr ibed, a necessary and sufficien t conditi on far

the emission of these trapped electrons is a reset pulse of negative gate

and positive drain voltages. The effectof this trapping on device

characteristics, reliability, and the proposed low-temperature operation

of IGFET’s is discussed.

Electron injection and trapping in silicon dioxide is a topic

of considerable mportance n ntegrated-circuit development

and reliability. Abbas and Dockerty [ l [ 2 ] were among the

first to docu ment the changes in IGFET characteristics cauaed

byhotelectron njection nto heoxideandconsequentlyshiftsn device haracteristics. Ning [3] , 4 ] has ecently

reported on not only such electron injection but in addition

the hermal reemission of these lectrons rom heoxide,

which he has characterized by low-temperature measurements.Low-temperature operation hasalso beenproposedfor nte-

grated circuits to ake advantage of the increase n mobility

and improved performance [5 ] - 71 . As Keyes e t al [ 5 ] E~US-

pected and postulated, new reliability imits would be found

forow-temperatureperation ; we will demon stratehat

this is in fact imposed in the case of IGFET’s by electron in-

jection into the oxide which results in permanent change:; in

conduct ivity under normal operating conditions and whic.h is

greatly ccelerated t ow emperatures.These changes are

so large and fast that in reality no advantage in performance

for MOSFET’s is likely to be achievedby low-temperature

operation.

Like Ning, we have observed electron injection into the oxide

andbot h he her mal reemission and ermanentrapping

which will not thermally reemit at room temperatures. How-

ever, on good-quali ty ox ides such as used in current integratedcircuitshere is nohermalemission,atherhiseport

demonstra tes that a reset pulse of negative gate voltages and

positive drain voltages is a necessary and sufficient condition

to cause field induced reemission on n-channel IGFET’s.

A striking illustration of these effects is given by the set of

curve-tracercharacteristics nFig. 1. Fig. l( a) shows nn-

channel GFETat77Kbefore tress; Fig. l( b) shows he

same device afte r he drainvoltagehas been increased to

allow for avalanche mult iplication of elec trons earhe

drain and subsequent njection nto he oxide. The effect of

stress has been to cause a arge shift in the threshold voltage

at the drainend of thechannel, so large n fact hat he

transistor will not urnonwith henorma l ange of gatevoltages unti l there s sufficient drain voltage t o punch through

this ormally ffegion.Therapped lectrons will no t

Manuscript received January 3, 197 9; revised May 2 9, 19 79.L Forbes was with he Integrated CircuitsLaboratory,Hewlett-

Packard, Palo Alto,CA, on leave rom heUniversity of California,

California, Davis, CA 9 56 16.Davis, CA 95 616 . He is with the College of Engineering, University of

E. Sun, R. Alders, andJ Moll are with the Integrated Circuits Labora-tory, Hewlett-Packard,Palo Alto, CA 94304.

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Shortn-channel GFETdeviceJMSCl: W / L = 4 um/3 urn

k F E T characterist ics at 77 K after successive stress, reset and stresvoltage conditions. a) FET characteristics at 77 K. b) FE T characteristics after stress at 77 K c) FET characteristics after reset withnegative gate and p ositive drain voltages. d) FE T ch aracteristics aftehigh stress at 77 K

~ .

thermally eemitand hesecharacteristics willpersist if the

device is warmed to room tempe rature. They will in fact only

bake out by extended times athigh temperatures like +200° Cand are unlike he results reported by Ning where most are

thermally eemittedat ow emperatures.The rapped elec

trons can, however, be removed by field induced reemissionby applying negative gate voltage and positive drain voltage.

This sets up a large reverse electric field in the oxid e and wil

remove th e electrons rapped n heoxide near th e drain

Fig. l(c) sh ows the FET haracteristics after such a reset cycl

where it has reverted to its original characteristics as shown in

Fig. l(a). Fig. l( d) shows he FET’s characteristicsat 77 Kafter another stresscycle.

The trapped electrons can in fact be reemitted by applying

only a negative gate voltage, however, the rat e at which th e

device will reset is extremely slowndicating tha t aarge

reverse field in the ox ide is required for reemission. In similar

measurements on p-channel devices to be reported at a later

date , we have found no evidence of hole injection but rather

under the most severe stress conditions only residual electron

injection.Thuswedonot believe th e reset sdue to hole

injection [SI [91,but ather sdue o he field-inducedreemission of the trappe d electrons.

The natureof these conductance changes havebeen character

ized by using an automated calculator-controlled measuremen

system t o apply imed sequences of stress and measurement

voltage conditions [ 101. One of these is shown n Fig. 2whe re the small signal ac drain conductance is measured first

at time “ A ” the device is stressed by a large drain voltage, and

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1817EEE T RANSA CTION S ON ELECTRON DEVICES VOL. ED-26, NO. 1 1 , NOVEMBER 1979

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1818 IEEE TRANSACTIONS ON ELECTRONEVICES,OL. ED-26 , NO. 1 1 , NOVEMBER 197

the conducta nce gain measured at time“ ” and the difference

in these measurementslottedsemperature is varied.

Between hesecyclesandwhile the empera ture is changing

the reset condition of negative gate and positive drain oltages

are applied. The time diagram is shown at the bot tomof Fig.

2. Clearly there is a large acceleration in the changes at low

temperatures.This igure also shows he esults if no reset

pulse is applied t o he device, there is no changebetween

measurement imes ndicating eset is a necessaryconc.ition.

The same system has been used to plot conductance changes

versus time at room emperatu re; he same largechanges in

device characteri stics occur but only over much onger imeperiods.Fig. 3 shows he ime dependence of he stress and

reset conditions at77 KA large number of variables are clearly important factors in

determining the magnitude f these changes [ 11 [2] ; owever,

the following pproximate ccelerationactors ave een

deduced; V n where V is voltageand n is around 10, T - m ,

where T is temperature in Kelvin and r is around 3, and t l t l

where t is imeand 1 is around 4. Largechangescanoccur

in IGFET haracteristics over lon g imeperiods tmom

temperature, there is no apparent saturation in this trapping,

which is a fundamental reliability limit 1 [ 21These acceleration factorsalso clearly preclude any advantage

to be gained from operation of IGFET circuits at low t empera-

tures. Because of the accelerated changes due to electron in-

jecti on, lower voltages must be used. It is not just the mohil ity

which determines IGFET performance but rather the prcductof mobility and voltage. In fact, hese effects might beused

to advantage in constructinga very simple electrically alterable

READONLY memory E A R O M) , considerable rocess nd

power supply simplic ity might be achieved over othe r cu::rent

designs by cooling during programming. Recently very dgnif-

icant advances have been made n Joule-Thomson refrigerators

for low-temperature cooling.

Using the field induced reset conditions of negative gate and

positive drain voltages on n-channel IGFET’s serves to ma ke

the changes aused byelectron njectionveryevidentand

allows a veryconvenient echnique or hecharacterization

of the magnitude, time dependence, and temperature depen-

denceof hese changes.Similar electron njectionhasalso

beenobserved on p-channeldevices,whoseconductance n-

creases after stress.

REFERENCES

S A. Abbas and R. C. Dockerty, “N-Channel IGFET Limitations

due to hot electron trapping,” in Proc.975

nt. Electron DevicMee t . pp. 35-38, 1915.“Hot carrier nstability in IGFETs,” Appl .Phys .Le t t .

V O ~ . 7, pp. 147-148, AUg., 1977.T H. Ning,“Shallow-levelelectron raps in SiOZ,” presentedat 1 9 7 8 ElectronDeviceResearch Conf., SantaBarbara,CA.Abstracts published in IEEE Trans. Electron Devices vol. ED-25p. 1348, Nov. 1978.

Phys. to be published.R. W Keyes, E. P Harris, and K. L. Konnerth, “The role of lowtemperatures in the operation of logic circuitry,” in Proc. IEEEvol. 5 8 , pp. 1914-1932, Dec. 1970.R. W Keyes, “Low emperature high mobility ransistor mate-rials,”Commun.Sol idS ta tePhys . vo l . 8 , n o . 2,pp. 37-46, 1977.

Solidstate Phys. vol. 8, no. 3, pp. 47-53, 1977.H. Hara et al . “A new instability in MOS transistors caused by

hot electronandhole njection romdrainavalancheplasmaintogateoxide,” Jap. J Appl.Phys. vol. 9, pp. 1103-1112,1970.C.Bulucea, Avalanchenjection into oxides in silicongate

‘‘Thermal reemissionof trapped electrons in SiO2, J. Appl

“Semiconductor devices at low temperatures,” Commun.

controlled devices:Theory,” Solid-stateElectron. vol. -18DD 3 6 3 - 3 7 4 . 1 9 7 5 .L. Forbes and U. Kaempf, “Capacitance and conductance deeplevel transient spectroscopy using HP-IB instruments and a desktop computer,” Hewlett-Packard J. vol. 30 no 4 , pp. 29-32,1979.