206 IEEE ELECTRON DEVICE LETTERS, VOL. 22, NO. 5, MAY 2001

A Novel High Performance SiGe ChannelHeterostructure Dynamic Threshold pMOSFET

(HDTMOS)T. Takagi, A. Inoue, Y. Hara, Y. Kanzawa, and M. Kubo

Abstract—In this letter, we propose a novel SiGe channelheterostructure dynamic threshold metal oxide semiconductor(DTMOS) and demonstrate its superiority over conventionalSi-DTMOS. The introduction of a SiGe layer for the channelis very effective for reducing the threshold voltage in spite ofkeeping impurity doping level at the body region. Therefore,a low threshold voltage and a large body effect factor can beachieved simultaneously. The SiGe HDTMOS with highly dopedbody exhibits 2 times higher transconductance, 1.4 times highersaturation current, and better short channel immunity thanthat of the control Si-DTMOS with lightly doped body of whichthreshold voltage is nearly the same.

Index Terms— , body effect factor, DTMOS, parasitic channel,short channel effect, SiGe.

I. INTRODUCTION

DYNAMIC threshold metal oxide semiconductor(DTMOS) field effect transistor (FET) where a gate

electrode is directly connected to a body is one of the mostpromising devices for high speed and low power applications[1]. The DTMOS has an ideal subthreshold swing, and itoperates with high threshold voltage at off-state to reducestandby-power consumption while operating with reduced

in on-state to enhance drive current. In order to realize ahigh-performance DTMOS, there are two important designissues. One is the low threshold voltage. In the DTMOSoperation, a power supply voltage is limited to about 0.6 Vor less because of a body current induced by forward biasedPN junction. Since the body current is one of the static powerdissipation components, it should be low enough for low-powervery large scale integration (VLSI) circuits [1]. Therefore, thelow threshold voltage is strongly required for obtaining a higherdrive current under the low power supply voltage. The other ishigh body effect factor defined as , where

is a back-bias voltage. Since, in the DTMOS, the body istied to the gate electrode, decreases due to the body effectas the gate voltage increases. When the gate voltageis asupply voltage , shift is given by, , andthe gate drive increases by [2]. Therefore, is desiredas high as possible. However, there is a fundamental trade-offbetween and . Namely, a high body impurity doping is the

Manuscript received December 13, 2000; revised February 2, 2001. The re-view of this paper was arranged by Editor K. De Meyer.

The authors are with the Advanced Technology Research Laboratory,Matsushita Electric Industrial Company, Ltd., Osaka 570-8501, Japan (e-mail:ttakagi@crl.mei.co.jp).

Publisher Item Identifier S 0741-3106(01)03704-1.

Fig. 1. Cross sectional view of p-SiGe channel HDTMOS.

most effective approach for enhancement, while, it inducesan increase of .

In this letter, to overcome this trade-off, we propose anovel heterostructure p-channel DTMOS using a strainedSiGe channel (SiGe HDTMOS). The superiority of the SiGeHDTMOS over the Si-DTMOS is demonstrated for the firsttime.

II. SiGe HDTMOS

The p-channel SiGe HDTMOS has the following advan-tages over conventional Si-DTMOS. By introducing a narrowbandgap SiGe layer for the channel region, a reduction ofcan be achieved while keeping high impurity doping level atthe body region. Such a highly doped body is effective forsuppressing the short channel effect and reducing the bodyresistance that is a cause of propagation delay in the SOIDTMOS [3]. A higher drive current can be also expectedutilizing the enhanced hole mobility in the SiGe channel. Asuper-steep-retrograde channel profile that is effective forincreasing can be easily obtained by utilizing an epitaxialchannel. As a result, high performance HDTMOS can berealized, which is accompanied with low , higher drivingcapability, good short channel immunity, and small propagationdelay.

III. EXPERIMENTAL

The p-SiGe channel HDTMOS shown in Fig. 1 was fabri-cated as follows. After substrate (body) implantation with threedifferent conditions ( cm , cm and

0741–3106/01$10.00 © 2001 IEEE

TAKAGI et al.: NOVEL HIGH PERFORMANCE SiGe CHANNEL 207

Fig. 2. Relation between body effect factor and V in SiGeHDTMOS. The value of was extracted using following equation,S = 60(1 + ) mV=decade, whereS is a subthreshold swing of MOSFETs.This figure shows that both high Ge content and high body doping are effectivefor increasing .

cm ), 10 nm-thick Si buffer, 15 nm-thick Si Ge channeland 20 nm-thick Si cap layers were grown by UHV-CVD.After entire device processing, the final Si cap thickness isdesigned to be 5 nm. The Ge content x was varied from 0 to0.3. All epitaxial layers were not intentionally doped, thusthere exhibit a super-steep retrograde channel profile. Such aretrograde channel profile is the useful way to enhancewithmaintaining low . In fact, an enhanced drain current owingto large was realized in the Si DTMOS with super-steep-ret-rograde indium-channel profile [4]. After epitaxial growthand isolation processes, wet thermal oxidation of the Si caplayer at 750 C was carried out to form 8 nm-thick gate oxide.Then p -poly-Si gate implanted with B was formed, followedby source and drain BFimplantation. Finally, wafers wereannealed by rapid thermal processing (RTP) at 900C for 15s. The gate contact and the body contact were provided sepa-rately. Therefore, we have measured the electrical propertiesunder DTMOS mode operation by wiring each contact.

IV. RESULTS AND DISCUSSION

First, we have examined the relationship betweenandshown in Fig. 2 for all devices with various Ge content and var-ious body doping. In this figure, represents the thresholdvoltage of DTMOS. The value of was extracted using fol-lowing equation [2]:

mV/decade

where, is the subthreshold swing of MOSFETs.Regarding to the body doping dependence for same Ge con-

tent devices, there exists a conventional trade-off inandas mentioned above. With increasing Ge content bothre-duction and enhancement are achieved simultaneously. Thisincrease of due to Ge is attributed to the buried channel struc-ture, because the channel of the SiGe HDTMOS is located atSi cap/SiGe heterointerface apart from the gate oxide by about5 nm. Therefore the channel potential is not sufficiently con-trolled by the gate voltage and it can be easily affected by thebody bias. It is obvious that the introduction of a SiGe channel

Fig. 3. Electrical properties (I � V characteristics) of Si MOS ( ),Si Ge MOS (�), Si DTMOS ( ) and Si Ge HDTMOS ( ). The bodycurrents(I ) under DTMOS mode operation are also shown in the figure. [SiDTMOS (4) and Si Ge HDTMOS ( ).] The body impurity concentrationis 2 � 10 cm for Si devices and1� 10 cm for Si Ge devices,respectively. Corresponding transconductance(g ) characteristics are alsoshown in the inset.

with highly doped body is very effective for realizing a high per-formance DTMOS with large at low .

Next, we have compared the properties of the Si DTMOSwith lightly doped body ( cm ) and the Si GeHDTMOS with heavily doped body ( cm ) ofwhich are almost same. Fig. 3 shows their char-acteristics under both conventional MOSFET mode operationand DTMOS mode operation. The body currents underDTMOS mode operation are also plotted. Correspondingtransconductances are shown in the inset of Fig. 3. Thegate length is 2 m and the drain voltage is 0.5 V. Thisdrain voltage is the supposed value to the power supply voltageof the DTMOS. The subthreshold slopes for both DTMOSshow nearly ideal value (61–62 mV/decade). Though thethreshold voltage of the SiGe HDTMOS is slightly higher thanthat of the Si DTMOS, the SiGe HDTMOS exhibits the largerdrive current in the high region ( V). However,there is no noticeable difference in the body current betweenthese two DTMOS. The peak gm of the SiGe HDTMOS isabout two times larger than that of the Si DTMOS, while instandard MOSFET operation is lower for a SiGe than for aSi channel. The factor is also degraded in the SiGe MOSFET.These deteriorations in the SiGe MOSFET are due to the highbody doping and parasitic channel conduction, which appearsin the interface between the gate oxide and the Si cap. The largevertical electric field in the SiGe MOSFET induces parasiticchannel conduction and spoils the advantages of the high mo-bility SiGe channel [5]. In the SiGe HDTMOS, however, suchan undesirable parasitic channel can be eliminated because thevertical electric field is reduced by the unique body-tied-to-gateconfiguration. Consequently, we can take full advantage of thehigh mobility SiGe channel in the DTMOS operation. More-over, since the factor under the DTMOS mode operation isalways ideal at any body doping concentration, the increasedin the SiGe HDTMOS with highly doped body contributes toenhance the drive current.

208 IEEE ELECTRON DEVICE LETTERS, VOL. 22, NO. 5, MAY 2001

Fig. 4. Threshold voltage of Si DTMOS () and Si Ge HDTMOS ( )as a function of gate lengthL . I � V characteristics of Si DTMOS () andSi Ge DTMOS ( ) of which gate length is 0.5�m are also shown in theinset.

Fig. 4 shows the gate length dependence of for bothDTMOS. The characteristics of which gate length is0.5 m are also shown in the inset. The short channel effect ismuch suppressed in the SiGe HDTMOS because of the highbody doping. The slight decrease of below 0.4 m gatelength is not essential and is due to not optimized processconditions. This result indicates that the HDTMOS with highlydoped body potentially has better short channel immunity com-pared to the conventional DTMOS. The short channel effectsshould be greatly improved by employing shallow source/drainjunction and thin gate oxide. The saturation current of the SiGeHDTMOS is about 1.4 times larger than that of the Si DTMOS.Moreover, a clear pinch-off characteristic is observed in theSiGe HDTMOS, but in the Si DTMOS, the drain current doesnot saturate and it increases gradually with increasing drainvoltage. This arises from the drain induced barrier loweringrelated to the lightly doped body.

The excellent performances obtained in the SiGe HDTMOSare due to both the very largeand the higher hole mobility inthe SiGe channel. The SiGe HDTMOS with heavily doped bodywill offer a superior performance in future scaled devices.

V. CONCLUSION

A novel p-channel SiGe heterostructure DTMOS is proposedand its superiority over the Si-DTMOS is demonstrated. In thisstudy, we have used the SiGe for the p-channel devices utilizinglarge valence band offset. If an n-channel DTMOS is fabricatedusing same epitaxial layers as p-channel devices, it would op-erate as a surface channel Si-DTMOS because there is no bandoffset at the conduction band. However, we can still take ad-vantage due to the super-steep-retrograde channel profile by theepitaxial channel technology. Therefore, this technology can beeasily extended to the complementary HDTMOS of which per-formances are greatly enhanced, by integrating both channelsusing same epitaxial layers. Ultra-low-power and high-speedVLSI circuits can be realized by using this technology.

ACKNOWLEDGMENT

The authors would like to thank K. Ohnaka, T. Ohnishi, A.Asai, T. Saitoh, and K. Yuki for their fruitful discussions, andthe members of Semiconductor Group for their contributions tofabrication processes.

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