Research article

Analytical models of threshold voltage and drain induced barrier lowering in junctionless cylindrical surrounding gate (JLCSG) MOSFET using stacked high-k oxide

  • Received: 26 December 2021 Revised: 28 February 2022 Accepted: 11 March 2022 Published: 16 March 2022
  • We proposed the analytical models to analyze shifts in threshold voltage and drain induced barrier lowering (DIBL) when the stacked SiO2/high-k dielectric was used as the oxide film of Junctionless Cylindrical Surrounding Gate (JLCSG) MOSFET. As a result of comparing the results of the presented model with those of TCAD, it was a good fit, thus proving the validity of the presented model. It could be found that the threshold voltage increased, but DIBL decreased by these models as the high-k dielectric constant increased. However, the shifts of threshold voltage and DIBL significantly decreased as the high-k dielectric constant increased. As for the degree of reduction, the channel length had a greater effect than the thickness of the high-k dielectric, and the shifts of threshold voltage and DIBL were kept almost constant when the high-k dielectric constant was 20 or higher. Therefore, the use of dielectrics such as HfO2/ZrO2, La2O3, and TiO2 with a dielectric constant of 20 or more for stacked oxide will be advantageous in reducing the short channel effect. In conclusion, these models were able to sufficiently analyze the threshold voltage and DIBL.

    Citation: Hakkee Jung. Analytical models of threshold voltage and drain induced barrier lowering in junctionless cylindrical surrounding gate (JLCSG) MOSFET using stacked high-k oxide[J]. AIMS Electronics and Electrical Engineering, 2022, 6(2): 108-123. doi: 10.3934/electreng.2022007

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  • We proposed the analytical models to analyze shifts in threshold voltage and drain induced barrier lowering (DIBL) when the stacked SiO2/high-k dielectric was used as the oxide film of Junctionless Cylindrical Surrounding Gate (JLCSG) MOSFET. As a result of comparing the results of the presented model with those of TCAD, it was a good fit, thus proving the validity of the presented model. It could be found that the threshold voltage increased, but DIBL decreased by these models as the high-k dielectric constant increased. However, the shifts of threshold voltage and DIBL significantly decreased as the high-k dielectric constant increased. As for the degree of reduction, the channel length had a greater effect than the thickness of the high-k dielectric, and the shifts of threshold voltage and DIBL were kept almost constant when the high-k dielectric constant was 20 or higher. Therefore, the use of dielectrics such as HfO2/ZrO2, La2O3, and TiO2 with a dielectric constant of 20 or more for stacked oxide will be advantageous in reducing the short channel effect. In conclusion, these models were able to sufficiently analyze the threshold voltage and DIBL.



    As the channel length of the transistor decreases to sub-10 nm or less, the conventional transistor structure becomes increasingly difficult to use due to the short channel effects [1,2,3]. For high-speed operation, low power consumption, and improved productivity, the reduction of transistor size is becoming the greatest competitiveness of all integrated circuit manufacturers. Transistors of various structures have been developed and used to reduce the short channel effects that inevitably occurs due to the reduction in transistor size. In particular, FinFET is widely used in Qualcomm's communication chips. FinFET has a tri-gate structure and it has been shown to have excellent control of carriers in the channel even at channel length and fin width of sub-10 nm [4,5]. In order to improve the gate control capability, the four-gate structure called Omega-FET, was developed [6]. Omega-FET is a structure in which the gate is wrapped around a rectangular channel, so that the control capability by the gate terminal is improved than that of FinFET. However, the Omega-FET has a problem of the corner-effect that inevitably occurs in the square structure. The structure developed to solve this problem is a cylindrical surrounding gate (CSG) MOSFET. The CSG MOSFET has been developed to cope with the short channel effects more than the FinFET used in various application ranges [7,8]. The CSG MOSFET is a structure in which the gate terminal completely encloses the cylindrical channel. As it is known as the most effective structure for controlling the carriers in the channel by the gate terminal as well as removing the corner effect that occurs in the Omega-FET, many studies are being conducted [9,10].

    As the channel length is shortened, not only the structural problem of the channel but also the doping technology are causing problems. In the junction-based 3D FET of the inversion type using the PN junction formed between the source/drain and the channel, the reduction of channel length makes the PN junction process difficult, and the channel formation difficult due to the depletion layer effect that occurs in the PN junction. The MOSFET developed to solve this problem is the junctionless MOSFET [11,12]. A lot of research has been carried out since this structure was presented by Colinge et al., and it is a structure that can solve the difficulty of the process because it is not necessary to form a PN junction between the cannel and source/drain region [13]. In particular, in many studies, it has been known that the short-channel effects are more significantly reduced in the accumulation type which is a junctionless MOSFET than in the inversion type MOSFET that is a junction-based MOSFET [14,15].

    In addition to the above problems, there are many problems in the thickness of the gate oxide layer with the reduction of the channel length due to scaling effects. According to the scaling rule, the thickness of the gate oxide film must be reduced in proportion to the channel length. However, when the thickness of the gate oxide is decreased, not only a problem in the process but also another short channel effects occurs due to an increase in gate parasitic current. As the reduction of the gate oxide film hits its limit, many efforts are made to use a high-k oxide film as the gate oxide film to reduce the short channel effects and gate leakage current [16,17,18,19,20,21,22]. However, the high-k oxide film material has a limitation in reducing the gate parasitic current due to a small band offset, generating roughness of the interface with silicon used as a channel. A structure that has emerged to solve this problem is a stacked structure of a gate oxide film. In this structure, SiO2 is used for the part in contact with the silicon channel and the high-k dielectric is used for the part in contact with the gate metal, thereby solving the short channel effect caused by the reduction of the gate oxide thickness. Rasol et al. analyzed the Ioff current for a junctionless cylindrical surrounding gate(JLCSG) MOSFET using stacked high-k oxide by ATLAS, and Darwin et al. used HfO2, HfSiO4, Al2O3, Si3N4, etc. as high-k materials, but the short-channel effects of the junctionless cylindrical MOSFET was analyzed using only the parametric form of the potential distribution [23,24]. Also Kosmani et al. analyzed the short-channel effects by TCAD, using a high-k oxide film in a junction-based double gate and Gate-All-Around MOSFET [25]. S. Gupta et al. obtained the potential distribution of the dual gate metal junctionless cylindrical gate-all-around (JLC-GAA) MOSFET, using HfO2 as a high-k material [26]. As such, many researchers are trying to reduce the short channel effect of JLCSG MOSFETs by using stacked high-k materials. In this study, in order to meet this purpose, the analytical threshold voltage and DIBL model were proposed using the definition of the threshold voltage and DIBL which are a kind of the short channel effects. We will prove the validity of the proposed models and analyze the threshold voltage and DIBL using the proposed models to consider the phenomenon of short-channel effects reduction in the case of using SiO2/high-k gate oxide stacked on the JLCSG MOSFET. As a high-k dielectrics, we will use SiO2 (εs = 3.9), Al2O3 (εs = 9), Y2O (εs = 15), HfO2/ZrO2 (εs = 25), La2O3 (εs = 30), TiO2 (εs = 80).

    Figure 1 shows the schematic diagram of the JLCSG MOSFET used in this paper. The source, drain, and channel were doped with n-type of high concentration, and a metal with a work function of ϕm was used. Lg denotes the length of the channel, R the radius of the silicon. The tSiO2 is the thickness of the SiO2 oxide layer bonded to the silicon channel, and the thk the thickness of the high-k material, which is in contact with the gate metal, and these two dielectrics are stacked. The tSiO2 used 1 nm, and the thk between 1 nm and 5 nm in this paper. The Vgs, Vds, and Vs represent a gate voltage, a drain voltage, and a source voltage, respectively. At this time, the potential distribution of the JLCSG MOSFET was obtained using the following Poisson equation [24,26,27].

    1rr[rrϕ(r,z)]+2ϕ(r,z)z2=qNdεsi (2.1)
    Figure 1.  Schematic cross-sectional diagram of the cylindrical surrounding gate (CSG) MOSFET.

    Here, εsi is the dielectric constant of silicon, and Nd is 1019/cm3 as the channel doping concentration. Using the superposition technique, the potential distributions in channel region are expressed as follows.

    ϕ(r,z)=ϕ1(r)+ϕ2(r,z) (2.2)

    Here, ϕ1(r) is the one dimensional solution to Poisson's equation and ϕ2(r, z) is the two dimensional solution to the homogenous Laplace equation, which is expressed as follows.

    1rr(rϕ1(r)r)=qNdεsi (2.3)
    2ϕ2(r,z)r2+1rϕ2(r,z)r+2ϕ2(r,z)z=0 (2.4)

    To solve (2.3), the following boundary conditions are used [28].

    ϕ1(r)r|r=0=0ϕ1(r)r|r=R=Coxεsi[Vgsϕmsϕ1(R)] (2.5)

    where Vgs is the applied voltage of the gate and ϕms is the work function difference between gate metal and silicon. The Cox is expressed as follows [29].

    Cox=εSiO2Rln(1+toxeff/R)toxeff=tSiO2+(εSiO2/εhk)thk (2.6)

    At this time, the solution to (2.3) is as follows [30].

    ϕ1(r)=qNd4εsir2+Vgsϕms+qNdR2Cox+qNdR24εsi (2.7)

    Since the method of calculating ϕ2(r, z) from (2.4) is independent of the doping distribution, using the variable separation method and Fourier-Bessel series by the method of C. Li et al. [30], ϕ2(r, z) is as follows.

    ϕ2(r,z)=n=1[Cnexp(αnzR)+Dnexp(αnzR)]J0(αnrR) (2.8)

    where an is eigenvalue that satisfy the following equation.

    RJ0(αn)εsiCoxαnJ1(αn)=0 (2.9)

    In (2.8), Cn and Dn are obtained using the following boundary conditions [28].

    ϕ(r,z=0)=VFϕ(r,z=L)=VF+VdsVF=kTqln(Ndni) (2.10)

    At this time, because the first term dominates the series of (2.8) due to rapid decay of the Fourier-Bessel series, only C1 and D1 were used as follows.

    C1=A[exp(λ1)1]B2sinh(λ1)D1=A[1exp(λ1)]+B2sinh(λ1)λ1=α1LRA=2J20(α1)+J21(α1)J1(α1)α1(Vgsϕms+qNDR2Cox+qNDR24εsiVF)+qND2εsi1J20(α1)+J21(α1)α1J1(α1)2J2(α1)(α1/R)2B=2VdsJ20(α1)+J21(α1)J1(α1)α1 (2.11)

    Substituting (2.7), (2.8), and (2.11) into (2.2), the potential distribution of the JLCSG MOSFET in the channel can be obtained.

    The analytical threshold voltage model is derived using the definition of the threshold voltage and the distribution of electrostatic potential in the previous section. The junctionless MOSFET operates in accumulation mode, and the gate voltage is defined as the threshold voltage when the minimum value of the central potential distribution becomes VF [31]. In other words, the gate voltage that satisfies

    ϕmin=ϕ(0,zmin)=VF (2.12)

    must be obtained. At this time, zmin represents the z value at which the central potential distribution becomes the minimum, and is given as [28].

    zmin=(R2α1)ln(D1C1) (2.13)

    S. K. Gupta et al. obtained the threshold voltage by the same method, but ignored the dependence of Vgs to the variables A, B, and zmin [32]. Therefore, in this paper, the following threshold voltage Vth can be obtained as in Appendix A if Vgs that satisfies (2.12) is obtained.

    Vth=12[a1/sinh2(λ1)1]{[a2/sinh2(λ1)2H]+[a2/sinh2(λ1)2H]24[a1/sinh2(λ1)1][a3/sinh2(λ1)H2]} (2.14)

    The constants a1, a2, a3 and H are indicated in Appendix A. In order to prove the validity of (2.14), it was compared in Figure 2 with Hu's model [28].

    Figure 2.  Comparisons of the various threshold voltages with this model (2.14).

    As seen in Figure 2, the threshold voltage obtained by the gm/Id method is overestimated. It can be seen that (2.14) suggested in this paper agrees well with Hu's model. However, it can be found that a difference between the threshold voltages occurs when the channel length is reduced to 10 nm.

    The DIBL is a measure of the change of the threshold voltage to the drain voltage. In the previous paper [33,34,35], the change of the threshold voltage was calculated when the drain voltages are changed from Vds = 0.1 V to Vds = 0.5 V or 1.0 V to obtain the DIBL. However, in this paper, using the definition of the DIBL and (2.14), the following DIBL could be obtained through the same process as in Appendix B.

    DIBL=12[a1sinh2(λ1)]{X2(exp(λ1)+exp(λ1)2)+{(a2sinh2(λ1)2H)(X2(exp(λ1)+exp(λ1)2))2(a1sinh2(λ1)1)(XY(exp(λ1)+exp(λ1)2)2X2Vds)}[a2/sinh2(λ1)2H]24[a1/sinh2(λ1)1][a3/sinh2(λ1)H2]} (2.15)

    In (2.15), X and Y values are indicated in Appendix A. It can be shown in (2.15) that the DIBL changes according to the value of Vds. That is, the DIBL value changes according to the Vds value measuring DIBL. In order to prove the validity of (2.15), the DIBL values of other models in Figure 3 were compared under the same conditions. As a result, it can be observed that it fits well with the results of other models. However, it can be observed that the errors between models increase as the channel length decreases. As above, since the validity of the threshold voltage model (2.14) and DIBL model (2.15) presented in this paper has been verified, we will use these equations to observe the characteristics of the JLCSG MOSFET with the stacked oxide film of SiO2 and high-k dielectrics. Device parameters used in this study are listed in Table 1.

    Figure 3.  Comparison of the various DIBLs with this model (2.15).
    Table 1.  Device parameters used in these models.
    Device parameter Symbol Value
    Channel length Lg 10 - 30 nm
    Silicon radius R 5 nm
    Doping concentration Nd 1019/cm3
    Thickness of SiO2 tSiO2 1 nm
    Thickness of high-k dielectric thk 1 - 5 nm
    Permittivity of high- εhk 3.9 - 80

     | Show Table
    DownLoad: CSV

    In the structure of Figure 1, the change of the threshold voltage with respect to the dielectric constant of the high-k material is calculated with the drain voltage as a parameter as shown in Figure 4. The difference in threshold voltage is indicated by arrows in Figure 4 when the drain voltage increases from 0.1 V to 1.1 V for a specific high-k material. As shown in Figure 4, as the dielectric constant of high-k material increases, the threshold voltage increases, but the threshold voltage is almost constant from the dielectric constant of 20. As shown in Figure 4, DIBL, which is the difference between the threshold voltage at the drain voltage of 0.1 V and the threshold voltage at 1.1 V, shows a larger value as the dielectric constant decreases, but gradually decreases as the dielectric constant of high-k increases. Therefore, the value is an almost constant at the dielectric constant of high-k of above 20.

    Figure 4.  Threshold voltages for dielectric constant of high-k with the drain voltage as a parameter.

    Figure 5 shows the change of the DIBL calculated using (2.15) according to the high-k permittivity. As seen in Figure 5, the DIBL appears large when the high-k dielectric constant is small, and the DIBL decreases as the dielectric constant increases, and it can be observed that the DIBL appears constant when the high-k dielectric constant is 20 or higher. As seen in (2.15) and Figure 5, the DIBL changed according to the drain voltage, and the DIBL increased as the drain voltage increased. If the high-k dielectric constant is as small as 3.9 like SiO2, the rate of change of DIBL for the change of the drain voltage (△DIBL/△Vds) is about 14.6 mV/V2 under the conditions given in Figure 5, but the changing rate of DIBL was maintained with 10 mV/V2 when the high-k dielectric constant is 20 above.

    Figure 5.  DIBLs for dielectric constant of high-k with the drain voltage as a parameter.

    The change of the threshold voltage with respect to the high-k dielectric constant as a parameter of the channel length is shown in Figure 6(a). As seen from Figure 2 and Figure 6(a), a threshold voltage shift occurs and the threshold voltage decreases when the channel length decreases. As the channel length decreased, the degree of reduction increased significantly according to the high-k dielectric constant. As described above, a constant threshold voltage was exhibited regardless of the channel length when the high-k dielectric constant is 20 or more. The shift of the threshold voltage is shown in Figure 6(b) when the channel length decreases from 30 nm to 10 nm. As seen from Figure 6(b), the shift of the threshold voltage decreases as the dielectric constant of high-k increases. In particular, the shift of the threshold voltage is almost constant when the dielectric constant of high-k is 20 or more.

    Figure 6.  (a) Threshold voltages and (b) threshold voltage shift for dielectric constant of high-k with the channel length as a parameter.

    The change of DIBL according to the high-k dielectric constant as a parameter of the channel length is shown in Figure 7(a). The DIBL shows a large change according to the high-k dielectric constant when the channel length is very small such as about 10 nm. However, it can be observed that the change of high-k dielectric constant does not have a significant effect on DIBL when the channel length is increased to 30 nm. The change of DIBL (ΔDIBL) is shown in Figure 7(b) when the channel length changes from 30 nm to 10 nm. As seen from Figure 7(b), the change in DIBL was greatly reduced as the high-k dielectric constant increased. Like the threshold voltage, ΔDIBL was almost constant when the high-k dielectric constant was 20 or higher.

    Figure 7.  (a) DIBLs and (b) shift of DIBL for dielectric constant of high-k with the channel length as a parameter.

    The reason for using high-k dielectric is to solve the difficulty in the process due to the reduction of the oxide film thickness. That is, the effective oxide thickness (EOT) can be increased by the ratio of the high-k dielectric constant and the SiO2 dielectric constant of 3.9 [36]. Therefore, if the high-k oxide film is used, it will be possible to solve the difficulty in the process due to the reduction in the oxide film thickness. Here, we will observe the effect of the thickness of the high-k material on the threshold voltage. In Figure 8, the change of the threshold voltage with respect to the dielectric constant of high-k materials is shown with the thickness of the high-k material as a parameter. As shown in Figure 8(a), the threshold voltage increases as the dielectric constant of high-k material increases, but the rate of increase decreases rapidly, and the threshold voltage is maintained almost constant at a dielectric constant of above 20. In particular, if thk was decreased to 1 nm, even if the dielectric constant was 10 or more, the threshold voltage was maintained constantly regardless of the value of the high-k dielectric constant. Figure 8(a) shows that the threshold voltage changes sensitively to the change of dielectric constant as the thickness of high-k material increases, and the shift of threshold voltage decreases as the dielectric constant increases when the thickness of high-k material is changed from thk = 1 nm to thk = 5 nm as shown in Figure 8(b).

    Figure 8.  (a) Threshold voltages and (b) threshold voltage shift for dielectric constant of high-k with the thickness of high-k dielectric as a parameter.

    Figure 9 shows the change of the DIBL according to the dielectric constant with the thickness of the high-k material as a parameter. Figure 9(a) shows that the DIBL decreased as the dielectric constant of the high-k material increased and the thickness decreased. As explained in Figure 8, it could be observed that if thk is decreased to 1 nm, the change in DIBL according to the dielectric constant is negligibly small even if the high-k dielectric constant is 10 or more. Figure 9(b) shows the change of DIBL according to the dielectric constant when the thickness of high-k is changed from thk = 1 nm to thk = 5 nm. The effect of the thickness change of the high-k material on DIBL is very small as the dielectric constant increases.

    Figure 9.  (a) DIBLs and (b) shift of DIBL for dielectric constant of high-k with the thickness of high-k dielectric as a parameter.

    Comparing Figure 6 and Figure 8, the change in the threshold voltage according to the dielectric constant of the high-k material has a greater influence on the channel length than the change in the thickness of the high-k material. That is, the threshold voltage shift of about 0.26 V appeared when the channel length changed from 30 nm to 10 nm for the high-k dielectric of 20 or more, but the threshold voltage shift of about 0.05 V was only occurring when the thickness of the high-k dielectric changed from 1 nm to 5 nm. As seen from the comparison between Figure 7 and Figure 9, DIBL shift of about 380 mV/V appears when the channel length changes from 30 nm to 10 nm with a high-k dielectric constant of 20 or more. However, only the change of the DIBL of about 20 mV/V occurred when the dielectric thickness changes from 1 nm to 5 nm.

    In this study, the analytical models of threshold voltage and DIBL were proposed to analyze the change in the threshold voltage and DIBL among the short channel effects of JLCSG MOSFETs using a stacked high-k dielectric as an oxide layer. They matched very well as a result of comparison with other papers. As scaling progresses, problems such as difficulty in process and short channel effects occur due to a reduction in the thickness of the oxide film according to the channel length. In this paper, the change of the threshold voltage and DIBL according to the dielectric constant of the high-k materials was observed using the proposed model with the channel length and thickness of the high-k dielectric as parameters. The shifts of threshold voltage and DIBL are 0.39 V and 577 mV/V, respectively, when the channel length decreases from 30 nm to 10 nm if SiO2 is used as a high-k material, whereas it can be seen that those decrease to 0.27 V and 403 mV/V, respectively when the dielectric constant of the stacked high-k material is increased to 20. In addition, considering the changes in the thickness of the high-k dielectric, the shifts of threshold voltage and DIBL are 0.33 V and 97.0 mV/V, respectively when the thickness of the high-k dielectric decreases from 1 nm to 5 nm if SiO2 is used as a high-k material. On the other hand, it was found that those decreased to 0.086 V and 20.8 mV/V, respectively when the dielectric constant of the high-k material increased to 20. As described above, the short-channel effect can be reduced in JLCSG MOSFETs by using a stacked high-k dielectric as the gate oxide. It was found that the proposed models excellently analyzed the threshold voltage and DIBL.

    All authors declare no conflicts of interest in this paper.

    If H=ϕms+qNdR2Cox+qNdR24εsiVF,

    ϕmin=ϕ(0,zmin)=Vgs+H+C1exp(α1zminR)+D1exp(α1zminR)=Vgs+H+C1exp(α1RR2α1ln(D1C1))+D1exp(α1RR2α1ln(D1C1))=Vgs+H+2C1D1=Vgs+H+(1/sinh(λ1))[A(exp(λ1)1)B][A(1exp(λ1))+B]=Vgs+H+(1/sinh(λ1))[(XVgs+Y)(exp(λ1)1)B][(XVgs+Y)(1exp(λ1))+B]=Vgs+H+(1/sinh(λ1))X2(exp(λ1)1)(1exp(λ1))V2gs+(Y(1exp(λ1))+B)(exp(λ1)1)XVgs+(Y(exp(λ1)1)B)(1exp(λ1))XVgs+(Y(exp(λ1)1)B)(Y(1exp(λ1))+B)=Vgs+H+(1/sinh(λ1))a1V2gs+a2Vgs+a3=0 (A-1)
    X=2J20(α1)+J21(α1)J1(α1)α1Y=(ϕMS+qNDR2Cox+qNDR24εsi)+qND2εsi1J20(α1)+J21(α1)α1J1(α1)2J2(α1)(α1/R)2B=2VdsJ20(α1)+J21(α1)J1(α1)α1=XVdsa1=X2(exp(λ1)1)(1exp(λ1))a2=(Y(1exp(λ1))+B)(exp(λ1)1)X+(Y(exp(λ1)1)B)(1exp(λ1))Xa3=(Y(exp(λ1)1)B)(Y(1exp(λ1))+B) (A-2)

    If you find Vgs using the quadratic formula of (A-1), this is Vth of (2.14).

    The DIBL is defined as follows.

    DIBL=dVthdVds

    Therefore, it can be found by differentiating (14) by Vds. In (14), the only variables for Vds are a2 and a3 with B, so

    DIBL=dVthdVds=12[a1sinh2(λ1)1]{(1sinh2(λ1))da2dVds{(a2sinh2(λ1)2H)(1sinh2(λ1))da2dVds2(a1sinh2(λ1)1)(1sinh2(λ1))da3dVds}[a2/sinh2(λ1)2H]24[a1/sinh2(λ1)1][a3/sinh2(λ1)H2]} (B-1)

    In (B-1), the derivative of a2 and a3 with respect to Vds is obtained using (A-2) as followings.

    da2dVds=X2(exp(λ1)+exp(λ1)2)da3dVds=XY(exp(λ1)+exp(λ1)2)2X2Vds (B-2)

    If (B-2) is substituted for (B-1), the DIBL of (B-3) can be obtained.

    DIBL=12[a1sinh2(λ1)]{X2(exp(λ1)+exp(λ1)2)+{(a2sinh2(λ1)2H)(X2(exp(λ1)+exp(λ1)2))2(a1sinh2(λ1)1)(XY(exp(λ1)+exp(λ1)2)2X2Vds)}[a2/sinh2(λ1)2H]24[a1/sinh2(λ1)1][a3/sinh2(λ1)H2]} (B-3)


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