LOW FREQUENCY NOISE IN JUNCTION FIELD EFFECT TRANSISTORS

Transcription

1 Sld-State Electrncs Vl. 21, pp / /$02.00/0 Pergamn Press Ltd., Prnted n Great Brtan LOW FREQUENCY NOSE N JUNCTON FELD EFFECT TRANSSTORS K. KANDAH and F. B. WHTNG nstrumentatn and Appled Physcs Dvsn AERE, Harwell, Ddct, Oxn OX11 ORA, England (Receved 10 January 1978; n revsed[tt 7 Aprl 1978) Abstract--Lw frequency nse n fur-termnal JFETs has been measured as a functn f substrate (secnd gate) bas wth temperature and dran current as addtnal varables. Sharp peaks f nse have been bserved at sme values f gate bas. The mechansm f lw frequency nse caused by Schckley-Read-Hall (SRH) centres and the sgnfcance f charge capture prcesses n and near the channel are dscussed. The expermental results shw that mst f the excess lw frequency nse s caused by SRH centres stuated n the transtn regn between the channel and the fully depleted regn f the gate-channel junctns. n JFETs wth gate-wdths f 1000/zm r less the nse caused by unt electrnc charge fluctuatns at nvdual SRH centres can be readly bserved. l. NTRODUCTON The nse n junctn feld effect transstrs (JFETs) can be represented by tw cmpnents n an equvalent crcut. One s equal t a vltage generatr n seres wth the surce f sgnal feedng the gate and the ther s a current generatr n shunt. The current generatr nse wll nt be dscussed here. The vltage generatr accunts fr the surces f nse whch mdulate the dran current drectly and s the mprtant factr n many applcatns. t has been shwn by Van der Zel[l] that the nse due t thermal agtatn f the carrers n the cnductng channel s equvalent t that f a resstr f magntude slg n seres wth the gate where g s the transcnductance f the JFET and s s a cnstant lyng n the range 0.7- dependng n devce gemetry. Owng t addtnal nse surces at lw frequences the equvalent nse resstance ncreases accrdng t a pwer law f the frm,4[-" where [ s the frequency and n s usually clse t 1. The factr A vares rather wdely and s strngly dependent n materal and prcessng parameters as well as devce gemetry and temperature. Ths paper presents sme results f an nvestgatn f nse n fur-termnal JFETs. Lw frequency nse s seen t arse manly frm Shckley-Read-Hall (SRH) centres as suggested by Sah[2] and Laurtzen[3] but the detaled mechansm they prpsed des nt fully descrbe the behavur f carefully prcessed devces f small gemetry especally at lw temperatures. The results suggest that mst f the excess nse s due t SRH centres stuated very clse t and at a crtcal dstance frm the cnductng channel. A smple mdel whch als fts ther publshed data and explans sme f ther ncnsstences s presented and the effect f devce gemetry s dscussed. 2. EXPER]~E~TAL ~.CH~Q{~ Ths study s amed at mprvng the perfrmance f JFETs n amplfers used fr spectrmetry wth nuclear radatn detectrs'where the requrements f lw nse are very exactng. n rder t btan the best sgnal-t- rat these spectrmeters emply tme varant flters [4, 5] whch are smewhat cmplex when ther respnse s analysed n the frequency dman. Therefre, n rder t avd the errrs and the tedum nvlved n spt frequency nse measurements f the JFET fllwed by ntegratn ver the pass band f the flter, we have measured the ttal JFET nse wth the gate cnnected t the surce thrugh a lw a.c. mpedance after passng thrugh flters dentcal t thse n nuclear spectrmetry. These tme-varant flters have a characterstc sgnal prcessng tme ~" whch s related t the centre frequency f f the passband by the apprxmate relatnshp /=(1/2~- ) and the effectve bandwdth s equvalent t a tuned crcut wth a Q factr f abut 0.5. The man dsadvantage f usng such a wde band flter s that the dervatn f the equvalent nput nse vltage at the centre frequency requres a crrectn factr f up t 1.2 when the nse n the passband s partly r whlly ll related. The results presented here have nt been crrected n ths respect but t wll be seen that ths des nt serusly dstrt them nr have an apprecable effect n the cnclusns. On the ther hand the understandng f the nse mechansms s easer wth ths type f presentatn. The JFETs were manufactured n an n-type eptaxal layer f nmnally 0.5~-cm resstvty n a p+ substrate. The p+ substrate frmng the bttm gate was used slely t determne the bas cndtns and the tp gate (Gt) was used as the sgnal gate. The relatve cntrl characterstc f the tw gates fr a fxed dran current and vltage s shwn n Fg. 1. The bas Vss n the substrate determnes the pstn f the cnductng channel n the eptaxal layer. Allwng fr the bult-n vltages f the gates t s estmated that the channel mves abut 2500 ]k/v fr values f Vss near zer and 750/~/V at the value f V,s crrespndng t V, = 0. The JFETs were assembled n a package desgned prmarly fr use n an amplfer wth a radatn detectr. Ths requres extremely hgh nsulatn and lw capactance at the gate electrde and the ablty t perate at lw temperatures. The equvalent nse vltage 1079

2 1080 K. KANDAH and F. B. WHTNG - G (v) 7-- l l r Vs ;{V) Fg. 1. Gate cntrl characterstc f JFET 52#1 at 2 ma dran current. measurements reprted here can be crrelated wth the nse perfrmance f the same JFET wth a detectr. The cmprmses n the desgn f the package has led t an uncertanty n the abslute temperature f the JFET f abut 10 K. Hwever the reprducblty f the nse measurements was better than % even when the JFET was transferred frm ne crystat t anther. Anther mprtant aspect f the measurng equpment s that the electrcal peratng cndtns f the JFET culd als be reprduced t a hgh accuracy. t wll be seen frm the results that the gate vltages have t be mantaned wthn abut 10 mv f specfed values n rder t repeat nse readngs near the hgh peaks. 3. RESULTS Fur types f JFETs whch dffered manly n the gate wdths as shwn n Table 1 were used n the experments. The square f the equvalent nse vltages (e~ 2) referred t G~ was measured as a functn f V, n ncrements f 0.1 V fr a fxed dran vltage and current at varus temperatures. All the plts shwn belw were btaned by jnng the expermental pnts wth straght lnes. Fgure 2(a) shws the measured values f e, 2 at rm temperature f devce 55#1 fr 4 values f the prcessng tme. Table 1. Devce type Gate wdth (~m) The general reductn f e, 2 as V, s ncreased s purely due t the reduced dstance f the channel frm G1. Fr mst devces the nse vared smthly wth Vs~ and wth temperature and was ndependent f ~" fr values less than abut 2 #s and ths s taken t be the thermal nse f the channel. n rder t emphasse the lw frequency effects the excess ver the thermal nse wll be pltted as n Fg. 2(b) n the fllwng results fr "r rangng frm 20 tzs t 1 ms and the zer ffset f the vertcal axs shuld be nted. The effect f changng dran current n the excess nse f ths devce at z = 200 ~s s shwn n Fg. 2(c) and t s bserved that the value f Vs, at the excess nse peak s nvarant at -5.0 V. n all devces there s a temperature n the regn f K at whch the nse, partcularly at lw frequences, s mnmum and the perfrmance f ths devce at 132 K s shwn n Fg. 2(d). The general effects f temperature n ne f the smaller devces are seen n Fg. 3. The man pnt t nte n relatn t ths devce s that hgh values f nse ccur at mst temperatures at Vss =-2.6 and -4.5 V. The peaks n Fgs. 2 and 3 suggest that they culd be due t defects at defned lcatns n the eptaxal layer--the term "defect" beng synnymus wth a SRH centre. The bserved appearance and dsappearance f peaks as a functn f temperature at a gven z s a characterstc f the energy levels f the asscated defects as descrbed by Haslett and Kendall[6]. The brad band f hgh nse between Vss f 2.5 and 4.5 V at the temperature f 110 K n Fg. 3(d) just verlappng tw sgnfcant peaks at rm temperature n Fg. 3(a) may be nted. f the defect s f small sze and les near the mddle f the eptaxal layer and nt t dstant frm the pnch-ff regn f the channel t shuld be pssble t sweep the channel frm a pstn belw the defect (nearer the substrate) t a pstn abve the defect (nearer the tp gate) by varyng Vss frm 0 t maxmum. On clse examnatn f many results t was apparent that sme f the peaks ccurred n pars n accrdance wth the abve predctn. Ths stuatn can be verfed by takng measurements f the excess nse vs Vss at the same temperature but at dfferent dran currents and a gd example f ths s shwn n Fg. 4. Ths devce has tw

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4 1082 K. KANDAH and F. B. WHTNG ! r f r [ r (a) T = 293 K 350 T 300 (b) T = 190 K ~ 250 "{ (# s} > c 200~ c rl/~sl /Vj-O00 #A p / ;--~.. / 2,. / ', > c S c ~00 %10 r " l "u" 50 'X " 200--/'\ J~ / ~ 7 -vss (v)! 1 1 L_ ~ 0 l 2 3 A Vss(V] O a; z > 200 c u 50 1 rl/xs] : 17,/'00 1 (el T = 130OK t! / A~ Pl '- ~.... '1" "~').1 \!"... {#'~X... :7:~.'.'~:,~...,, :r./.,~.."px'~/"~'...~,_..~ [ vss[v s, A80 -~ 400~ c ~ z4 ~ f6 x w 80 Defects bve chnne[ A B 1AO l > 80C - c v x 200 '1 r J t \ \ (d} r = 110 K "r (,~ sl \ ~-1000 J -~ ~200,', './ -.. r \... ~'7 \ q... ~20 ~ "~ ~ ~: "::>~"~' '~' A V~,s (V Fg. 3. Excess nse f JFET 52#2 at varus temperatures. = 2 ma. ~- = 20, 200, 1000/zs = 2mA ~ = Area Defects be(w chnnel, O Fg. 4. Excess nse f JFET 05#13 at dran currents f 4mA at ~' #s, T = 140K. A 6 -Vs(V and man defects A "and B causng excess nse at ths temperature wth a large magntude nly when the defect s just abve r just belw the channel. The larger change n V,, requred t sweep the channel thrugh each defect at the hgher dran current s a measure f the ncrease n channel thckness wth dran current. Referrng back t Fg. 2(c) where the pstn f the prmnent nse peaks at V, = -5.0 V s ndependent f dran current we nfer that ths bas crrespnds t the channel beng abve the asscated defect. The examples shwn abve were chsen t llustrate specfc pnts. t wuld be wrng t assume that all stuatns are f such smplcty n the nse behavur as a functn f V,,, r, T and. We shw n Fg. 5 the excess nse f fur devces f 620/,m gate wdth at the same dran vltage and current at a temperature f 145 K whch s nearly ptmum fr mst devces. The number f nse peaks fr each devce ndcates the number f defects wthn the senstve vlume and the range f energy levels apprprate t the values f z and T used

5 d-'g "ON 'Z "lea 3SS Excess nse (nv21hz} Excess nse (nv21hz) ~ca ca t t ca,! w P },', m ~ ~ ca L J N N Excess nse {nvzlhzl Excess nse (nvzlhz)!., N,,, }; " ~.~.~,..2,> f..._._ 2/. ) "n m /.,, <~ ~ [801 sals!su~z~ ~a~ plg u!lun.f u! slu/~aunblj A~ 3

6 1084 K. KANDAH and F. B. WHTNG n these measurements. Takng these examples as cverng the nrmal dstrbutn fr devces f ths type the number f defects wuld appear t vary between abut 3 and 10 fr devces f ths sze. Ths may be cntrasted wth the characterstcs f a partcularly gd devce f three tmes the gate wdth n Fg. 2 shwng fewer peaks and nearly flat curves f excess nse aganst V, at ptmum temperature. The excess nse n a JFET wth a gate wdth f 13,600/zm s shwn n Fg, 6. Ths s typcal f devces f ths sze wth a typcal e, f 0.7nV/x/(Hz) at medum frequences n that the mean excess nse at r = 1000/~s remans relatvely hgh but there are n sharp peaks at any temperature r substrate bas. f we assume that the defect densty s apprxmately the same n all ur expermental JFETs the larger devces (Type 12) wll be expected t have well ver 100 defects n the senstve vlume and the hgh nse level at all bas vltages and the absence f sharp peaks s t be expected. ~'- 8~ lol --T-, ~-, ~ - - ~ - x /X! \ 1000 r (tzs} uj, ,... 2O t / Vss(V) Fg. 6. Excess nse f JFET 12#1 wth gate wdth f v.m at T = 155 K. 4. NOSE MECHANSMS The thery presented by Sah[2] attrbuted excess lwfrequency nse t alternate emssn f electrns and hles by SRH centres stuated n the depletn regn f the gate-channel junctn. Gld and neutrn damage centres have energy levels near the ntrnsc level E~ and Laurtzen[3] and Kern and McKenze[7] have shwn expermentally that these defects prduce excess nse at r near rm temperature. t can be establshed that nse due t changes f charge state restrcted t emssn prcesses wll be present nly ver a lmted temperature range and caused by a narrw range f energy levels f defects. The characterstc emssn tme cnstants ~-, fr electrns and ~' fr hles are prprtnal t exp((e,.-e,)/kt) and exp((e,-e)/kt) respectvely where Et s the energy level f the defect. Let us frst cnsder the effect f temperature. The emssn tme cnstant fr gld wth Ec- E, = 0.54 ev ncreases frm abut 6msec at rm temperature t abut 6 sec at 200 K. The nse due t gld centres wll therefre be neglgble at temperatures f 200 K r belw ver nrmally useful frequences. Fr an arbtrary energy level the dran current fluctuatns resemble an asymmetrc randm telegraph sgnal wth mark-t-space rat r,/~'p. We have r. 2(E, - E,) -- = exp - - r, kt As E, mves away frm E the value f r,/zp rapdly becmes very large r very small cmpared t 1 and the mean square value f the fluctuatns wll fall sharply. The nse wll be neglgble when the energy level f the defect s mre than abut 3 kt away frm El snce r,/zp wll then be greater than 1000 r less than Nse due t SRH centres wth shallw levels whch becmes sgnfcant at lw temperatures has been reprted by Haslett and Kendall[6] and Wang et al.[8]. Fr shallw levels the average tme fr the centre beng n ne f the tw charge states wll be extremely lng and the nse wll be neglgble f emssn s the nly mechansm. f free charge s present near the centre the mean fluctuatn tme wll be reduced by charge capture and the nse can becme apprecable. Wang et al.[8] attrbuted the nse they measured n JFETs whch had been subjected t a hgh level f neutrn damage t SRH centres wth energy levels rangng frm 0.17 t 1.04eV stuated near the edge f the channel where a hgh cncentratn f free carrers s present. Our results ndcate that mst f the excess nse s due t defects n the transtn regn between the channel and the fully depleted regn f the gate-channel junctn. The physcal mdel f a JFET s shwn n Fg. 7 where defects at arbtrary lcatns are ndcated. The regn f partcular nterest s the transtn regn, whch s apprxmately tw Debye lengths n thckness, between the neutral channel and the fully depleted regn. Let us frst cnsder the nse due t md-band SRH centres stuated at dfferent lcatns. f the defect s cntaned wthn the channel the nse wll be very small except at very JSOURCE ~,~ OATE J /~ DRAN ~ /'~, DEPLETED ~,Y TRANST0 N~,~ ~---- D...~ REO,O. \,;, m F SUBST RATE -Vtl= V Fg. 7. Physcal mdel f JFET n an eptaxal substrate shwng defects at arbtrary lcatns A, B, C, D, E, F.

7 Lw frequency nse n junctn feld effect transstrs 1085 lw temperatures as ndcated by Sah[2]. When the defect s n the depletn regn the nse wll have a fxed fluctuatn tme cnstant prprtnal t exp (E~/2 kt) and ts magntude wll be prprtnal t dla where a s the dstance between the defect and the channel and d s the dstance between the gate and the defect. At a very lw frequency the nse wll have the hghest value when the defect s lcated at the edge n the depleted sde f the transtn regn and prgressvely fall t zer as ts pstn falls deeper nt the transtn regn wng t a reductn n the mean ccupancy tme n ne f the charge states. The defects A, B, C and D shwn n Fg. 7 can be made t experence ths range f stuatns by varyng the substrate bas and the resultng equvalent nse vltage as a functn f V,, wll have the characterstcs shwn n Fg. 8. Cnsderng the effect f the pstn f the defect n the drectn frm the surce t the dran t s clear that the nse wll have a maxmum value when the defect s near the pnch-ff regn as at A r C and be smaller when the defect s at B say. We next cnsder the characterstc fr defect A as a functn f frequency and temperature. The curve shwn n Fg. 8 wll apply fr frequences less than 1/(2~'z) where r s the expected fluctuatn tme cnstant f a SRH centre when nly emssn prcesses are used n the calculatn. The actual fluctuatn tme cnstant becmes smaller n the transtn regn and the channel wng t the effect f capture prcesses. Sah[2] has shwn that the tme cnstant f gld centres s abut 6 x 10-3 sec n the depletn regn and <10-7 sec n the channel at rm temperature. At a gven temperature the nse pwer densty at frequences >(l/(2~r)) wll be prprtnal t f-2 when the defect s n the depletn regn. As the defect s mved frm the depletn regn nt the transtn regn by varyng V,, the nse at a fxed hgh frequency f wll reach a peak when the defect s at a lcatn at whch the fluctuatn tme cnstant appraches l/(2~rf) and fall rapdly as the defect enters the channel. Snce the fluctuatn tme cnstant wll be nversely prprtnal t the capture prbablty the exact pstn f the defect n the transtn regn at whch the peak ccurs wll depend n the nature f the defect. The nse as a functn f V,s n these stuatns s llustrated n Fg. 9 where At s the characterstc when f< (l/(2~'r)) and A2 and A3 are at prgressvely hgher frequences. At a fxed frequency the effect f reducng temperature wll be t change the shapes f the characterstc frm A~ t A3 but the peak pstns wll mve apart. The nse characterstc f gld centres at _. = " ~ / \ / k! -.\ k J -k:... ~_,,_,.._._,- E - - ~ -vss v l Fg. 8. Excess nse vltage referred t G~ as a functn f substrate bas at a frequency <(lh') fr the defect lcatns shwn n Fg. 7..-, / ll /' llt! _2 0 -vss v) Fg. 9. Excess nse vltage as a functn f substrate bas fr a md-band defect at lcatn A shwng effects f temperature and frequency.

8 1086 K. KANnlAH and F. B. WHTNG temperatures just belw rm temperature and frequences hgher than abut 30 Hz wll be smlar t A2 r A3. 5. ANALYSS OF RKSULrS Extendng the abve arguments t shallwer energy levels and lwer temperatures ne wuld expect the nse characterstc fr such cndtns t shw peaks as n A3 f Fg. 9 when the number f defects n the eptaxal regn f the devce s small. Ths agrees wth all ur measurements except at the very lwest temperatures as n Fg. 3(d) whch wll be dscussed later. A pnt t nte abut mst f the results n Fgs. 2-5 s that the peaks are nearly f the same wdth wth a tendency t get brader at hgher values f V,s and at hgher temperatures as expected wng t the fact that Debye length s prprtnal t ~/T. The wdths f these peaks agree wth the estmated transtn regn f abut 1000/k allwng fr the gate cntrl characterstcs shwn n Fg. 1. Assumng a dnar densty f 1016/cm3 and a drft velcty f 10 6 cm/sec n the pnch-ff regn f the channel the cntrbutn f ne average rw f carrers t the dran current s s A. f a SRH centre mdulates the dran current by ths magntude the wde-band r.m.s, dran current nse wll be A whch wll be equvalent t a gate nse vltage f V assumng a mutual cnductance f A/V whch s typcal f the JFETs type 05 and 52. Our results ndcate a wde-band nse nearly ne rder f magntude less than ths fgure even at the hghest peaks s that we assume that unt electrnc charge fluctuatns are respnsble fr these nse peaks. Rgrus treatment s beynd the scpe f the present wrk but further arguments wll be publshed elsewhere [9]. Sme f the peaks are seen t merge at certan sgnal prcessng tmes and temperatures f the asscated defects are clse tgether as n Fg. 3(b). We have nt fund much evdence ndcatng the presence f md-band centres wth nse aganst V~ characterstcs f the type shwn n Fg. 8. There have been a few devces wth rm temperature characterstcs smlar t that f defect D n Fg. 8 but ccurrng at lw values f Vss ndcatng a md-band defect clse t the substrate. An example s at P n Fg. 3(a) and t s seen that ths dsappears at the lwer temperatures as n Fgs. 3(b) and 3(c). n general the excess nse characterstc at rm temperatures was ether flat r shwed a few peaks as n Fgs. 2(c) and 3(a). A flat characterstc wth a hgh mean level culd ndcate a large number f defects unfrmly dstrbuted thrughut the devce as analysed by Sah [2] but the mean values n the flat regn are t lw t accunt fr ths. t s suggested that the flat prtns are due t SRH centres at pnts such as E r F n Fg. 7 n the transtn regn adjacent t the heavly dped gate and substrate. Snce these transtn regns are thnner than thse adjacent t the channel wng t the larger n + na the prbablty f fndng a defect n these regns s smaller f the defects ccur wth unfrm densty dstrbutn. Hwever, the densty f defects s knwn t be generally hgher nsde and near heavly dped regns and cnsequently the nse due t defects clse t the gates may be apprecable. There s a great dea f evdence cnfrmng the varatn f nse characterstcs as a functn f temperature and frequency as descrbed n Sectn 4 and an example f such data s seen n Fg. 10 fr devce 05#01 at a dran current f 1 ma. n ths devce there s nly ne defect exhbtng sgnfcant nse levels n the temperature range K. The tw peaks n Fg. 10 at a gven temperature cgrrespnd t the defect beng n the transtn regns abve and belw the channel as dscussed earler and seen als n Fg. 4 n devce 05#13. The dstance between the peaks suggest that the channel thckness sweepng the defect as V.~s s vared s greater n the case f devce 05#01 than devce 05#13. We nte n Fg.10 that the tw peaks get clser t each ther at hgher frequences and hgher temperatures as predcted n Sectn 4 n relatn t Fg. 9. t may be nted that the excess nse n Fg. 10(a) exhbts a scatter f nearly 0.2 nv2/hz whch s cnsstent wth a standard devatn f abut 0.7% n the measurements f the ttal nse whch n ths case was 6.25 nv2/hz at a bas f 4 V. We have nt attempted t estmate the energy levels f the defects wng t the dffculty n decdng n the value f Vss at whch the nse s t be measured as a functn f frequency and temperature. f the devces are cnnected as three-termnal JFETs the peratng pnt culd well have fallen at ne f the steep prtns f the characterstcs shwn n Fgs Ths wuld have led t unrelable nse readngs and the changes n peratng pnt due t the temperature changes culd lead t msleadng fgures fr the energy level. Ths culd be ne f the reasns fr the scatter n the energy levels reprted n sme f the lterature. Hwever measurements n JFETs wth very wde gates r after a hgh level f radatn damage culd yeld accurate and meanngful values fr the energy levels f the defects f they are nt numerus. An nterestng effect bserved n a number f devces s a crrelatn between the values f Vss at whch nse peaks ccur at wdely dfferent temperatures at the same sgnal prcessng tme. Fr example t s seen that nse peaks ccur at Vss = 2.6 and 4.5 V at temperatures f 293 and 130 K n Fgs. 3(a) and 3(c). The tw vltages are taken t be thse at whch a defect sts n the transtn regns abve and belw the channel. The appearance f nse at the tw wdely dfferent temperatures ndcates tw energy levels asscated wth the same defect. One pssblty s the presence under sutable cndtns f the tw fluctuatn tme cnstants crrespndng t capture and emssn f an electrn r capture and emssn f a hle fr a defect wth [E-Et]>0. Ths requres dfferent spatal dstrbutn f denstes and velctes f hles and electrns as mght exst n the transtn regn. A mre lkely explanatn f the tw energy levels s that the defect has multple charge states. Anther nterestng effect seen n a number f devces s the crrelatn between the values f Vss at the edges f the hgh nse band at very lw temperatures as n Fg. 3(d) and thse crrespndng t the pstns f the pars f peaks at hgher temperatures as n Fgs. 3(a) and 3(c). Our nterpretatn s that the tw nse peaks at the

9 Lw frequency nse n juncjtn feld effect transstrs ' ' NO. TEMP K "K / "~ / K 6 /, 213 K 8 -- / \ / K / x, 5 6 2,',3 K -- //-,, y (a),=20)us j,' \Q\ - 0 ~ ~ ""...~...~.j~..:~,~::- ~ ~ ~....,~:~-.-~ 80 > 60 l'_ (b) r =200ps ~... ~ "" "'~'" "~-"~ ~l 30C L 30 t /,0 t 5.0 t 6.0 -Vss (V) Fg. 10. Excess nse f devce 05#01 at a dran current f 1 ma n the temperature range K. hgher temperatures are caused by feld-effect mdulatn f the channel when a defect s n ne r. the ther f tw transtn regns as stated abve wng t the presence f a deep level. At ntermedate values f V,s the defect wll be n the channel and, at suffcently lw temperatures, wll capture charge carrers wng t a shallwer energy level and enhance the nse due t carrer densty fluctuatns as predcted by Sah[2] and bserved by Churchll and Laurtzen[10]. Ths s n agreement wth the bservatn by Sah[2] that at a gven lw temperature the large capture crss sectn f centres such as gld wll lead t mre sgnfcant levels f nse due t fluctuatns n the channel than that due t capture and emssn by dnrs f the channel dpant as prpsed by Van der Zel[ll]. n the restrcted range f sgnal prcessng tmes (<1 msec) used n ur measurements we have bserved that the nse n mst devces has a lw general level at a temperature f abut 145 K and a very hgh value at abut K n agreement wth mst prevus bservatns. The nse n sme devces falls slghtly at temperatures belw 110 K befre ncreasng ~'ery rapdly as the temperature falls belw 100 K. The ncrease belw 100 K s clearly due t carrer densty fluctuatns caused by capture and emssn at the phsphrus dnr centres. We beleve that the peak at K s due t the enhancement f the carrer densty fluctuatns n the channel by the presence f a defect wth a shallw level and a hgh capture crss sectn. Ths peak has been bserved by many wrkers and t culd be a cmmn defect n slcn such as vacances, ntersttals r Frenkel pars.

10 1088 K. KANDAH and F. B. WHTNG 6. CORRELATON BETWEEN NOSE AND GATE LEAKAGE CURRENF Sme publshed results have ndcated a crrelatn between lw frequency nse and gate leakage current. Accrdng t the thery f Sah[2] and Laurtzen[3] the nse pwer spectral densty due t mdband SRH centres s prprtnal t Nr/(l + w2r 2) where N s the number f centres n the depletn regn and r s ther fluctuatn tme cnstant. The leakage current due t these centres wll be prprtnal t Nt s that a true crrelatn between the magntudes f leakage and nse can exst nly fr hgh frequences when ~r ~> 1. n the case f centres wth energy levels away frm mdband the mean ccupancy tmes r,, and ~-,, n the tw charge states wll be wdely dfferent. f fr example r, "~ rp the crrelatn between leakage and nse wll then exst nly at frequences where tt,,> 1. Mst f the evdence supprtng ths thery relates t devces wth a very hgh densty f defects artfcally ntrduced by a hgh cncentratn f gld r neutrn rradatn[3,7,8]. n such devces the nse levels were ne r tw rders f magntude greater than the nse levels reprted here. n any case there has been n evdence f the leakage current shwng the strng temperature dependence as the nse n nrmal devces. Accrdng t ur mdel the lw frequency nse n nrmal devces s largely due t charge exchange between the SRH centres n the transtn regn and the channel r the gate and the resultng mean current wll be zer. We have nt fund any evdence f hgh gate leakage currents at the values f Vss whch gave very hgh nse levels. n general there s n evdence fr a crrelatn between nse and leakage at any temperature. t has been fund[12] that the gate leakage s determned mre by the perpheral length f the gate than by the depleted vlume f slcn adjacent t the gate. The gate leakage n mst nrmal devces s prbably caused by surface generatn-recmbnatn prcesses at the xde-slcn nterface surrundng the gate. There culd be a crrelatn between the gate leakage current and the nse current flwng t the gate but ths nse current wll be expected t be greater than that due t the sht nse f the mean current. 7. CONCLUSONS Snce the nse n a devce wth a gate wdth f abut 1000 ~m s caused by a few slated defects t shuld be pssble t chse an peratng cndtn f temperatture, dran current and substrate bas vltage whch mnmses the nse ver a useful range f frequences. Wth devces havng a larger gate wdth the prbablty f fndng such a favurable cndtn wll becme smaller. Wth three-termnal JFETs lke the 2N4416 the reductn n the degrees f freedm n chsng the peratng cndtns leads t a large scatter n the lw4requency nse perfrmance frm sample t sample especally at lw temperatures. Hwever the rewards f fndng devces whch shw very few defects lcated n the transtn regn are s great that yelds f a few percent are smetmes cnsdered satsfactry n energy dspersve X-ray spectrmetry. n the feld f gamma-ray spectrmetry wth large detectrs where JFETs wth much wder gates are generally used ths selectn prcess has nt been fund t be necessary. Hwever the stuatn wll be transfrmed f defect denstes f the rder f 101 cm -3 r lwer becme feasble r f the defect densty s nt unfrm n the eptaxal layer. The lw-frequency nse f a JFET s caused by SRH centres stuated n the transtn regn whch les between the channel and the depletn regdn f the gatechannel junctn. The effect f md-band centres s shwn t be neglgble except at temperatures well abve 200 K. The SRH centres make the maxmum cntrbutn t the lw-frequency nse when ther pstn n the transtn regn results n a fluctuatn tme cnstant whch s related t the measurement frequency. The number f such actve centres n gd JFETs wth a channel wdth n the regn f 1000 ~m and a channel length f abut 2/zm s n the regn f 3-10 s that the nse due t fluctuatns f ne unt f electrnc charge at ndvdual centres can be bserved n such devces when the substrate bas f a fur-termnal JFET s used as a varable. A rgrus theretcal treatment f the dran current mdulatn caused by SRH centres n the transtn regn appears t be a frmdable prblem. The presence f multple charge states at sme f the SRH centres has been bserved and the excess nse at temperatures between 100 and 120K s attrbuted t enhanced carrer densty fluctuatns due t the presence f centres wth a hgh capture crss sectn nsde the channel. Acknwledgements--t s a pleasure t acknwledge the assstance gven by D. Clman f Texas nstruments, Bedfrd, UK. fr supplyng the expermental JFETs and fr nvaluable dscussns and the DCVD, Mnstry f Defence fr havng made the wrk pssble. The relablty f the large amunts f data btaned n the curse f ths wrk wed a great deal t the test equpment prvded by G. Whte and A. Stflng f AERE, Harwell. REFERENCES 1. A. Van der Zel, Thermal nse n feld-effect transstrs. Prc. RE. 50, (Aug. 1%2). 2. C. T. Sah, Thery f lw-frequency generatn nse n junctn-gate feld effect transstrs. Prc. EEE 52, (July 1964). 3. P. O. Laurtzen, Lw frequency generatn nse n junctn feld effect transstrs. Sld-St. Electrncs, 8, (1%5). 4. K. Kandah, Semcnductr nuclear partcle detectrs and crcuts. Nat. Acad. Sc. Publ p. 544 (1969). 5. V. Radeka, Trapezdal flterng f sgnals frm large germanum detectrs at hgh rates. Natl. nst. Meth. 99, (Mar. 1972). 6. J. W. Haslett and J. M. Kendall, Temperature dependeance f lw-frequency excess nse n junctn-gate FETs. EEE Trans. Electrn Devces, ED-19, (Aug. 1972). 7. H. E. Kern and J. M. McKenze, Nse frm neutrn nduced defects n junctn feld-effect transstrs. EEE Trans. Nucl. Sc. NS-17, (Dec. 1970). 8. K. K. Wang, A. Van der Zel and E. R. Chenett, Neutrnnduced nse n junctn feld effect transstrs. EEE Trans. Electrn Dev. F_.,D-22, (Aug. 1975). 9. K. Kandah and F. B. Whtng. T be publshed. 10. M. J. Churchll and P. O. Laurtzen, Carrer densty fluctuatn nse n slcn junctn feld effect transstrs at lw temperatures. Sld-St. Electrncs 14, (1971). 11. A. van der Zel, Carrer densty fluctuatn nse n feld effect transstrs. Prc. EEE (Crrespndence) 51, 1670 (Nv. 1%3). 12. D. Clman. Persnal cmmuncatn.