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J. Low Power Electron. Appl. 2014, 4(1), 15-25; doi:10.3390/jlpea4010015
Published: 10 January 2014
Abstract: This paper describes the performance prospect of scaled cross-current tetrode (XCT) CMOS devices and demonstrates the outstanding low-energy aspects of sub-30-nm-long gate XCT-SOI CMOS by analyzing device operations. The energy efficiency improvement of such scaled XCT CMOS circuits (two orders higher) stems from the “source potential floating effect”, which offers the dynamic reduction of effective gate capacitance. It is expected that this feature will be very important in many medical implant applications that demand a long device lifetime without recharging the battery.
Since the 1990s, various families of fully-depleted SOI MOSFET have been proposed and extensively studied [1,2] due to their various merits in terms of device scaling (high drivability, steep swing, less short-channel effects, smaller foot print, etc.). One of the authors (Omura) proposed the cross-current tetrode SOI MOSFET (XCT-SOI MOSFET) for analog applications (see Figure 1), in 1986, on the basis of partially-depleted SOI MOSFET technology , and conducted experiments to evaluate the fundamental aspects of the device. While the scaling feasibility of XCT-like devices has been studied recently [4,5], we expect that XCT-SOI devices will yield new applications, such as medical implants, which demand low-energy operation with high noise margin. In order to discuss those applications in detail, device models have already been proposed to perform circuit simulations [6,7]. However, modeling an XCT-SOI device is not so easy due to the three-dimensionality of its operations [3,5]. Recently, our laboratory group examined the device model proposed in , and its usefulness was comprehensively revealed . In addition, a mechanism analysis demonstrated the potential of scaled XCT CMOS devices for extremely-low-energy operation .
This paper considers the dynamic and standby power dissipation characteristics of the sub-30-nm-long gate XCT-SOI MOSFET. We start by analyzing the low-energy operation of XCT-SOI CMOS circuits; the model proposed here strongly suggests that the “source potential floating effect (SPFE)” substantially reduces the operation power consumption. The great advantages of the design methodology are also elucidated. In addition, this study addresses the reality of sub-20-nm-long gate XCT-SOI devices and a scaling scheme to suppress the standby power consumption for future low-energy applications.
2. Device Structure and Assumptions for Modeling
The schematic device structure is shown in Figure 1a. In an n-channel XCT-SOI device, the n-channel SOI MOSFET and p-channel JFET are self-merged and the electron current of nMOSFET is relayed to the hole current of pJFET in series; the broken arrows reveal the current flow by the node connection of the MOSFET source and the JFET source.
As an example, we show the fundamental ID-VD characteristics of a 0.1-μm-long gate partially-depleted nMOSFET without the parasitic pJFET, see Figure 2a, following 3-D device simulations . Device parameters are summarized in Table 1. As the XCT-SOI device has two active body contacts (B1 and B2) of the SOI MOSFET, the conventional “quasi-static body-floating effect”  is eliminated automatically. Details of the device operation are described in a previous paper . The nMOSFET shows slight short-channel effects due to channel length modulation. Figure 2b shows the ID-VD characteristics of an n-channel XCT-SOI MOSFET as determined from 3-D device simulations. It is seen that the XCT-SOI device has lower ID than the MOSFET because of the series resistance of the parasitic pJFET. In addition, the XCT-SOI device shows the negative-differential conductance (NDC) in the saturation region of drain current [3,6,7]. On the other hand, the short-channel effects (SCEs) of the original MOSFET are sufficiently suppressed in the XCT device ; subthreshold swing is 76 mV/dec at VD = 1 V. This is one of great advantages of XCT devices.
|Table 1. Scaling scheme and device parameters for sub-100-nm-long gate XCT-SOI MOSFET.|
|Parameters||NA (cm−3)||LG (nm)||EOT (nm)||tSOI (nm)||tBOX (nm)||VD (V)||VTH (*) (V)|
|LG = 100 nm||2.0 × 1018||100||2.0||129||111||1.00||0.20|
|LG = 75 nm||4.0 × 1018||75||1.5||117||100||0.97||0.16|
|LG = 20 nm||2.3 × 1019||20||0.4||75||65||0.50||−0.06|
|LG = 15 nm||3.4 × 1019||15||0.3||69||59||0.43||−0.10|
(*) n+-poly-Si gate is assumed.
In support of XCT-SOI MOSFET modeling, we have already proposed the equivalent circuit (for quasi-static analysis), shown in Figure 1b. The basic feasibility of this model has been examined by circuit simulations [5,7]. In addition, mechanisms of the low-energy operation were examined by a simplified analysis, based on the model shown in Figure 1c , where CGn,MOS, CSn,BOX, CDn,BOX and Rch,pJFET denote the gate-to-source capacitance of SOI MOSFET, the source-to-substrate capacitance of SOI MOSFET, the drain-to-substrate capacitance of SOI MOSFET, and the source-to-drain resistance of the parasitic pJFET, respectively. In the case of p-channel XCT-SOI MOSFET, we label them as CGp,MOS, CSp,BOX, CDp,BOX, and Rch,nJFET, respectively.
3. Circuit Simulation Results of SOI CMOS and XCT-SOI CMOS
Here, we advance the discussion to better understand XCT-CMOS EXOR circuit features. We assume the OR-NAND type EXOR circuit that consists of four CMOS inverters (standard layout). We concentrate the discussion on the energy ratio of CMOS-EXOR circuits, where energy ratio (ER) is defined as the energy dissipated by the XCT-CMOS EXOR over that of the comparable conventional SOI-CMOS EXOR. Calculation results, based on HSPICE  simulation results, are shown in Figure 3.
First, it is seen that the ER value of 1-μm-long gate devices is almost unity regardless of the VDD value. This behavior is reasonable for the following reasons. The energy dissipation of conventional devices, evaluated by the Pd-td product, is not a function of VDD due to the simple recognition of the MOS gate capacitor’s charging and discharging operations. It is anticipated that the XCT-CMOS, with a 1-μm-long gate, follows this principle. In the case of 0.1-μm-long gate devices, on the other hand, it is seen that the ER value rapidly falls as VDD rises. As this is a very interesting result and somewhat mysterious, we discuss below a possible mechanism based on physics.
Frequency-dependent energy ratio (ER) is defined by :
At the low-frequency limit (ω→0), CGn,XCT and CGp,XCT are reduced to CGn,MOS and CGp,MOS, respectively, as expected. At the high-frequency limit, however, we have:
Generally speaking, reducing the gate length (LG) raises the operation frequency at the same supply voltage. The ER rises when the scaling is enhanced; the scaling scheme assumed here [5,8] slightly reduces CSn,BOXRch,pJFET and CSp,BOXRch,nJFET as the scaling is advanced. For f < 1/(CSn,BOXRch,pJFET) and f < 1/(CSp,BOXRch,nJFET), the roles of CSn,BOX and CSp,BOX are lost; i.e., we have CGn,XCT~ CGn,MOS and CGp,XCT~ CGp,MOS. This is equivalent to the low-frequency limit. As a result, the ER value approaches unity; the intrinsic advantage of the XCT-SOI CMOS is lost. For f > 1/(CSn,BOXRch,pJFET) and f > 1/(CSp,BOXRch,nJFET), on the other hand, we have CGn,XCT < CGn,MOS and CGp,XCT < CGp,MOS. Device operation approaches the high-frequency limit. In this case, the ER value decreases as the frequency rises. The ER value approaches zero as the supply voltage rises, shown in Figure 3. It is anticipated that the depletion layer beneath the buried oxide layer reduces the parasitic capacitance of source diffusion. In order to achieve a small ER value, therefore, we have to increase the effective channel resistance of the parasitic JFET when scaling is advanced.
Calculation results of the energy ratio (ER), defined by Equations (1) and (2), are shown in Figure 4. The model clearly predicts that high-frequency drive will drastically reduce XCT-SOI CMOS power consumption. This stems from the “source potential floating effect (SPFE)” obtained by the model shown in Figure 1c. Consequently, we can conclude that the SPFE of the source diffusion of the SOI MOSFET plays an important role in reducing the energy dissipated by XCT-SOI CMOS devices.
Figure 5 shows simulation results of the time-dependent through-current of the conventional SOI CMOS EXOR chain circuit and the XCT-SOI CMOS EXOR chain circuit, both under dynamic operation. It is seen that the standby power of the XCT-SOI CMOS EXOR chain circuit is about two orders lower than that of the conventional SOI CMOS EXOR chain circuit. This suggests that logic circuits composed of XCT-SOI CMOS can offer drastically lower standby-energy dissipation as well as lower switching-energy dissipation.
4. Further Scaling Potential of XCT-SOI MOSFET
We now investigate the scaling potential of the XCT-SOI CMOS. The fundamental scaling scheme of the XCT-SOI MOSFET has been already studied in [5,8] and a 100-nm-long gate XCT-SOI CMOS has been realized. However, body doping is apt to rise to 1019 cm−3 as the XCT-SOI MOSFET is inherently a partially-depleted SOI device. We investigated how well the XCT-SOI MOSFET can be scaled down to realize a 15-nm-long gate. Using 3D device simulations , we simulated the performance of 20-nm-long gate and 15-nm-long gate XCT-SOI MOSFETs under the scaled bias condition, where we assumed abrupt source and drain junctions, for simplicity, in the scaling scheme proposed recently . As the previous scheme [5,8] cannot be applied to the sub-100-nm regime, we restructured the scheme (see Table 1) so that devices work well .
Here, simulated ID-VD characteristics of the 15-nm-long gate XCT-SOI MOSFET, with device parameters, shown in Table 1, shown in Figure 6; the device has the abrupt source and drain junctions. The device shows the negative differential conductance in the saturation region, as is expected.
Simulated subthreshold characteristics of the 15-nm-long gate XCT-SOI MOSFET and the 20-nm-long gate XCT-SOIMOSFET are compared in Figure 7; both devices have the abrupt junctions. It is seen in Figure 7 that the subthreshold swing (S) is 77 mV/dec, for the 20-nm-long gate device, and 81 mV/dec, for the 15-nm-long gate device. Short-channel effects are well suppressed. However, the gate-induced drain-leakage (GIDL) current level of the 15-nm-long gate XCT-SOI device is somewhat high (~10−9 A/μm) as the device simulation assumes the abrupt junction for simplicity.
In response, we introduced a 10-nm-long graded doping region for source and drain junctions as a “new” scaling scheme, as shown in Figure 8. Simulated ID-VG characteristics of the 15-nm-long gate XCT-SOI MOSFET are shown in Figure 9. Drain current characteristics of 15-nm-long gate XCT-SOI devices with the abrupt and the graded junctions are compared. It should be noted that the subthreshold swing is 65 mV/dec for the 15-nm-long gate device with the graded junction. Short-channel effects are well suppressed in the 15-nm-long gate XCT-SOI MOSFET. The GIDL current level is lowered to 200 pA/μm. The most noticeable result is the improved drivability of the device with the graded junction; this is due to the remarkable improvement in carrier velocity over the whole device region, as shown in Figure 10 , and the reduction of the channel resistance of the parasitic pJFET .
5. Performance Expected from the Scaled XCT-SOI MOSFET
This section comprehensively examines the potential of the scaled XCT-SOI MOSFET. Calculated values of the on-current (Ion), the off-current (Ioff), the intrinsic switching time (CGVD/Ion), the switching energy (Esw), and the standby power (Pstby), are summarized in Table 2, where it is assumed that the threshold voltage (VTH) is 0.2 V and WG/LG = 5, regardless of scale. Here, we assume that CG,XCT = CG,MOS; in other words, SPFE is not assumed in Table 2. From Table 2, we can identify the following.
(i) The graded source/drain junction improves the drivability;
(ii) The graded source/drain junction does not always reduce the off-current;
(iii) The intrinsic switching time is degraded as the scaling is advanced if an abrupt junction is assumed. However, it can be drastically improved by using a graded junction due to the improved drivability;
(iv) The switching energy will be further reduced when SPFE is taken into account.
|Table 2. Performance perspectives of scaled XCT-SOI MOSFET.|
|LG (nm)/WG (nm)||Ion (A)||Ioff (A)||CGVD/Ion (s)*||Esw (J)*||Pstby (W)*|
|100/500||5.4 × 10−6||2.5 × 10−9||1.7 × 10−10||5.1 × 10−16||2.8 × 10−9|
|75/375||1.0 × 10−5||1.3 × 10−9||1.6 × 10−11||3.0 × 10−16||1.3 × 10−9|
|20/100||abrupt S/D||1.7 × 10−6||7.6 × 10−9||4.9 × 10−11||2.1 × 10−17||3.8 × 10−9|
|graded S/D||2.2 × 10−6||2.4 × 10−9||3.8 × 10−11||2.1 × 10−17||1.9 × 10−9|
|15/75||abrupt S/D||5.2 × 10−7||2.1 × 10−10||1.4 × 10−10||1.2 × 10−17||9.0 × 10−11|
|graded S/D||4.4 × 10−6||1.9 × 10−9||1.6 × 10−11||1.2 × 10−17||8.2 × 10−10|
*It is assumed that CG,XCT = CG,MOS.
In order to evaluate the above aspects of the scaled XCT-SOI MOSFET, we start by referring to the ITRS roadmap 2011 . Figure 10 shows the projected switching performance suggested by the ITRS roadmap 2011; the calculation results of various XCT-SOI devices are also plotted. Simple calculation results reveal that 15-nm-long gate XCT-SOI devices without SPFE are identical to the non-volatile memory device group. However, it is worthwhile to note that the 15-nm-long gate XCT-SOI device is expected to lie in the zone below the non-volatile memory zone; SPFE should provide XCT-SOI devices with much better performance (“expected zone”) than that shown in Figure 11.
We demonstrated the low-energy operation of XCT-SOI CMOS devices scaled down to 15 nm and analyzed the key underlying mechanism. It was shown that the source-follower like operation of the XCT-SOI MOSFET dynamically reduces the effective input capacitance, and, thus, the energy dissipated by XCT-SOI devices. This operation should be called the “source potential floating effect (SPFE)”. It is predicted, based on a physics-based model, that realizing SPFE in XCT-CMOS circuits will significantly suppress standby power dissipation.
It was also suggested that such aspects are still available in the sub-30-nm-long gate regime. Therefore, we can state that XCT-SOI CMOS devices are very promising for future extremely-low-energy circuits that suit medical implant applications.
A part of this study was supported by “Strategic Project to Support the Formation of Research Bases at Private Universities” in Japan: Matching Fund Subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology) in Japan.
Conflicts of Interest
The authors declare no conflict of interest.
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