M odulation-doped field effect transistor

A m odulation doped FET(MODFET) basically operates on th e sam e principles as th e Si-based M OSFET w ith th e exception th a t th e channel is b u ilt in th e stru c tu re in th e form of a q u an tu m well an d th e c arriers originate from a m odulation doped supply layer.

In Si-based M OSFET stru ctu re, a th in lay er of oxide is grown on top of th e Si to in su late th e m etal gate from th e inversion layer th a t builds up directly u n d e rn e a th th e oxide a t th e surface of th e Si. For n-channel M O SFETs th e su b stra te is h g h tly p -doped, w hereas th e source an d d ra in regions are heavily n^-doped. T here are two types of channel w hich can be form ed betw een th e source an d d ra in regions. If a t zero gate bias th e channel conductance is very low, a positive voltage needs to be applied to th e gate to form th e n- channel, an d hence it is called th e norm ally-off or en h an cem en t mode n- channel M OSFET. In th e case w here th e channel conductance alread y exists a t zero bias, a negative gate voltage will deplete th e c arriers in th e channel to reduce its conductance. T his type is called th e norm ally-on or depletion mode n-channel M OSFET. Sim ilarly, th ere exists th e concept of th e p-channel en h an cem en t an d depletion mode M OSFET. The basic device p a ra m eters to describe a M O SFET are th e channel length, th e channel w idth, th e insulator(oxide) thickness, th e junction depth an d th e su b stra te doping, while its perform ance is ch aracterised by th e values of th e transconductance, the cut-off an d m axim um operating frequency, an d th e gate voltage swing(logic swing) w hich defines th e a b ru p tn e ss betw een th e on an d off sta te w hen used as a sw itch in digital logic circuits. O th er p a ra m e te rs such as th e leakage cu rren t, sub th resh o ld ch aracteristic or “ off ” c u rre n t are also very im p o rtan t an d should be considered in order to be able to fully ch aracterise a M OSFET device [8].

The n-channel an d p-channel mode described for th e M O SFET have been realised in M ODFET stru c tu re s too, an d th e ir perform ance h a s been stu d ied for m any years. A schem atic view of th e M ODFET stru c tu re an d its b an d ah g n m en t is show n in figure 2.18.

Source Gate Drain Source Gate Drain

SLGeojCap Si^7Gegj supply Sio.7Geoj spacer

Si channel Sio.7Geoj buffer

SiGe(5-30%) graded buffer

S i " Si supply Si spacer Sii .Ge. channel

relaxed Si,.^Ge, buffer

Si-substrate

(a) n-type MODFETs (b) p-type MODFETs

F ig u r e 2.18 Schematic diagram of the layer structure for a SiGe n-MODFET and a SiGe p-MODFET grown on Si Substrate [43].

n -C h a n n e l M O D FE T s; The first reported Si-channel n-MODFET devices[44] were based on a single step buffer layers th a t led to quite m oderate mobilities. These devices employed a Pt/Ti/Au Schottky gate w ith a b a rrie r height of around 0.9eV. The well-behaved dc characteristics of the devices have been observed, a m axim um extrinsic transconductance value of 50mS mm^ at a gate length of 1.6pm h as been reported. D rastic im provem ents resulted from the introduction of a graded SiGe buffer layer, giving significantly enhanced electron mobilities. Ism ail et al. [45] reported on Schottky-gated n-MODFETs w ith a layer sequence grown in two successive UHV-CVD processes. They fabricated a 0.25pm gate length n-channel MODFET which gave mobilities of 1500cm V *s'^(300K) and 9500cm ^'^s‘^(77K) w ith corresponding sheet densities of 2.5 X lO^^cm ^ and 1.5 x lO^^cm In comparison, the room tem p eratu re mobility for bulk Si doped to the sam e level is only one th ird of the MODFET value, and the 77K mobility for Si MOSFET is approxim ately one th ird as well.

A lthough some p arallel conduction w as p resen t a t room tem p era tu re , the depletion-type (norm ally on) tra n s isto rs behaved quite well. Im pressive m axim um extrinsic transconductance values of 330 an d GOOmS m m^ have been obtained a t 300 an d 77K respectively. Konig e t al. [46] reported a sim ilarly successful approach. They employed M BE-grown n-M ODFETs. The m axim um transconductance obtained from th ese sam ples are quoted to be 340mS m m^ a t room tem p era tu re an d 670mS mm^ a t 77K, an d they dem o n strated excellent H all m obilities of 2200 (300K) an d 15700 cm ^'^s'^ (77K).

p - c h a n n e l M O D F E T s; P e a rsall e t a l.[47] reported th e first p-MODFETs, w hich em ployed a pseudom orphic SiggGeog channel. These devices exhibited well-behaved tra n s is to r characteristics, b u t th e ir dc perform ance were found to be m odest because of th e low room tem p era tu re m obihties of pseudom orphic SiGe channels. L ittle progress h a s been m ade over th e y ears reg ard in g th e p- type devices.

A lthough p-type M ODFETs can be im plem ented w ith pseudom orphic SiGe channels, th e low m obilities caused by alloy scatterin g in such layers suggested th e use of pure Ge as th e channel m aterial, w hich ag ain requires a relaxed SiGe buffer layer.

M obilities a t 300K(77K) of 1300 c m ^ ^ s \1 4 0 0 0 cm ^'^s'^) a t c arrier concentration of 1.5 X 10^^cm ^(1.0 x lO^^cm ^ ) for th e Ge ch an n el p-MODFET have been reported [48]. T his high m obihty a t room tem p era tu re indicated prom ising figure of m erit reg ard in g th e dc perform ance. T ransconductance values for 1.2pm gate-length devices a t 300K(77K) have been found to be

125mS m m \2 9 0 m S m m^ ).

possible choice in te rm s of high-speed perform ance, th ey are not well suited w hen combined w ith n-channel FETs employing a p u re Si ch an n el for CMOS. For th is reason m ost resea rch on p-channel M ODFETs rev erted to th e m ethod of using a SiGe alloy channel. In co n trast to th e aforem entioned pseudom orphic SiGe channels, w hich have show n low hole m obilities, new efforts have been focused on SiGe channel w ith th e composition x significantly larg e r th a n 50%. In creased hole m obihties in th ese p-channel sam ples have been observed due to th e reduction in th e effective m ass. A rafa e t al. [49] inv estig ated Sij.^Ge^ ch an n el p-M ODFETs w ith x aro u n d 50% used a n inverted supply layer. These stru c tu re s showed o u tstan d in g hole m obilities of 800-1000 cm ^'^s'^ a t room te m p e ra tu re and of 3300-3500 c m ^ ^ s ^ a t 77K. They obtained rem ark ab le re su lts in bo th the dc perform ance an d th e high-firequency behaviour of th e p-M ODFETs. A m axim um extrinsic transconductance of 230mS mm^ h a s been m easu red a t room tem p era tu re for a gate len g th of 0.25pm an d a gate/source distance of 0.2pm. This hig h transconductance value led to a tra n s it frequency of 24 GHz and th e m axim um oscillation frequency of 37GHz. P re h m in a ry re s u lts on 0.1pm devices indicated a tra n s it firequency as high as 70GHz[50].

In conclusion, th e perform ance d em o n strated experim entally by SiGe M ODFETs is a n extrem ely im pressive one an d one can safely sta te th a t these devices have th e ability to compete a t le a st in term s of speed w ith some of th e III-V technologies. However, u n til high quality, low defect density v irtu a l su b stra te s or b u lk SiGe su b stra te s become available, th e technology will be confined to th e laboratories. O ther perform ance re la te d issu es such as noise have y et to be addressed.

R eferences

[1] E.G. D ism ukes, L. Ekstrom , an d J. P aff R., J. Phys. Chem. 68, 3021(1964)

[2] R. Fabbri, F. Cemboli, M. Servidori an d A. Zani, J . Appl. Phys. 74, 2359 (1993)

[3] J . H. Herzog, “ P roperties of S train ed an d R elaxed Silicon G erm anium ” edited by E. K asper, 49(1994)

[4] F.C. F ran k , an d J.H . van der Merwe, Proc. R. Soc. A 198 , 216(1949) [5] J. H. v an der Merwe, J. Appl. Phys., 34, 117(1963)

[6] E. K asper, an d H. J. Herzog, T hin Sohds Film s, 44, 357(1977)

[7] W.A. Jesser, an d J.H . van der Merwe, J. Appl. Phys., 63, 1928(1988) [8] J. Singh, “Physics of Sem iconductor an d th e ir H e tero stru ctu re ” McGraw-

Hill, Inc., 219(1993)

[9] R. People, an d J . C. Bean, Appl. Phys. L ett., 47, 322(1985) [10] C.J. Bean, Proc. of IEEE, 80, 569(1992)

[11] J.S ingh, “Sem iconductor Devices: An introduction” M cGraw-Hill, Inc., 79(1994)

[12] L andholt-B ornstein, “ N um erical D ata an d F u n d a m en tal R elationships in Science an d Technology”, III/17a, Springer Verlag(1982)

[13] R. B rau n stein , A.R. Moore, an d F. H erm an, Phys. Rev. 109, 695(1958) [14] R. People, IE E E J. Q u an tu m Electronic, QE-22, 1696(1986)

[15] D.V. Lang, R. People, J.C . Bean, an d A.M. Sergent, Appl, Phys. L ett. 47, 1333(1985)

[16] G. A bstreiter, H. Brugger, T. Wolf, H. Jo rk e an d H -J Herzog, Phys. Rev. L ett. 54, 2441(1985)

[17] E.A. Fitzgerald, Y.H Xie, M.L. G reen, D. B rasen, A.R. K ortan, J. Michel, Y .J M ii an d B.E. Weir, Appl. Phys, L ett. 59, 811(1991)

[18] F.K. LeGoues, B.S. Meyerson, an d J.F . M orar, Phys. Rev. L ett. 66, 2903 (1991)

[19] F. Schaffler, D. Tobben, H .J. Herzog, G. A bstreiter, an d B. H ollander, Semicond. Sci. Tecbnol. 7, 260(1992)

[20] F .S tern, an d S.E.Laux, Appl. Pbys, L ett. 61, 1110(1992) [21] F. Schaffler, Solid-St. Electron. 37, 765(1994)

[22] R. Dingle, H. L. Storm er, A C. Gossard, an d W. W eigm ann, Appl. Pbys. L ett. 33, 665(1978)

[23] N. Ashcroft, an d N.D. M erm in, “ Sobd S tate Physics” CBS Pub. Asia LTD(1988)

[24] Don Monroe, Y.H. Xie, E.A. Fitzgerald, P .J. Silverm an, an d G.P. W atson, J. Vac. Sci. Tecbnol. E l i , 1731(1993)

[25] K. Ism ail, M. A rafa, K.L. Saeger, J.C . Cbu, an d B.S. M eyerson, A ppl.Pbys. L ett. 66, 3021(1995)

[26] D .J. Paul, N. Griffin, D.D. Arnone, M. Pepper , C.J. E m eleus, P .J. Phillips, an d T.E. W ball, Appl. Pbys, L ett. 69, 2704(1996)

[27] J .J . H arris, J . A. Pals, an d R. W oltjer, Rep.Pro. Pbys. 52, 1217(1989) [28] L. L andau, an d Lifsbifts, “ Q u an tu m M echanics” Pergam m on P ress L td

(1981).

[29] A .Isibara, an d L.Sm rcka, J.P b y s. C: Solid S ta te Pbys. 19, p6777 [30] P. Coleridge, Semicon. Sci. Tecbnol. 12, p22(1997)

[31] D. Monroe, Y.H. Xie, E.A. Fitzgerald, and P .J. Silverm an, Pbys. Rev. B46, p7935(1992)

[32]K. von K litzing, G. Dorda, an d M. Pepper, Pbys. Rev. L ett. 45, 494 (1980)

[34] R.E. F range, Phys. Rev. B29, 3303(1981) [35] R.H. L aughlin, Phys. Rev. B23, 5623(1981) [36] B.I. H alperin, Phys. Rev. B25, 2185(1982)

[37] E.F. Shubert, K. Ploog, H. Dam bkes, an d K. Heime, Appl. Phys. A33 , 63 (1984)

[38] M .J. K ane, N. Apsley, D A. Anderson, L.L. Taylor, an d T. K err, J. Phys. C: Solid S ta te Phys. 18, 5629(1985)

[39] A. G ruhl, an d A. Shuppen, T hin Sohd Film s, 294, 246(1997)

[40] A. Schüppen, A. G ruhle, H. Kibbel, U. E rben, an d U. Konig, Electronic L ett. 30, 1187(1994)

[41] A. Schüppen, A. G ruhle, U E rben,. H. Kibbel, an d U. Konig, lED M Tech, Dig., 377(1994)

[42] T. Uchino, T. Shiba, T.K. Kuchi, Y. T anaki, A. W atanabe, Y. Kiyota, and M. H onda, lED M Tech. Dig., 67(1993)

[43] O.K. M aiti, L.K. B era, and S. C hattopadhyay, Semicond. Sci. Tecbnol. 13, 1225(1998)

[44] H. D aem bkes, H. Herzog, H -J Jorke, H. Kibbel, an d E. K asper, IE E E T rans. E letron. Devices ED-33, 633(1986)

[45] Ism ail K., M eyerson B.S., R ishton S., C bu J., N elson S., an d Nocera J., IE E E T rans. E lectron Device L ett. ED L-13, 229(1992)

[46] U. Konig, A .J. Boers, F. Schaffler, an d E. K asper, E lectronic L ett. 28, 160(1992)

[47] T. P. P earsall, an d J.C . Bean, IE E E E lectron Device L ett. EDL-7, 308 (1986)

[48] U. Konig, a n d F. Schaffler, IE E E E lectron Device L ett. 14, 205(1993) [49] M. Arafa, P. Fay, K. Ism ail, J.C . Cbu, B.S. M eyerson, an d I. Adesid,

IEEE Electron Device Lett. 17, 124(1996)

[50] M. A rafa, P. Fay, K. Ism ail, J.O . Chu, B.S. M eyerson, an d I. Adesid IE E E E lectron Device L ett. 17, 586(1996)

Chapter 3