2.3.1 Basic Current-Voltage Characteristics
Figure 2-3(a) and (b) present the typical transfer characteristics of n-channel SB MOSFET and p-channel SB MOSFET with gate length equals to 5 µm and fin width equals to 80 nm, respectively. Clearly, the ambipolar property and the extremely high leakage current in the off-state were observed. Additionally, smaller on-state current, poor sub-threshold swing, and larger threshold voltage are also obtained. In Fig. 2-4(a) and (b) which show the typical output characteristics of n-channel SB MOSFET and p-channel SB MOSFET, respectively. The obvious sub-linear curves indicate large SB height at the source-side. These on/off behaviors can be easily explained by the schematic energy band diagram of n-channel SB MOSFET in Fig. 2-5 and p-channel SB MOSFET in Fig. 2-6. While operating in the on-state, owing to the thick SB existed at the source-side to block carriers flow, the electrons (n-channel) and holes (p-channel) are required to pass through the SB by the thermionic-emission and tunneling mechanisms. The SB at source-side dominates the carrier transportation, as shown in Fig. 2-5(a) and Fig. 2-6(a). Moreover, the existed SB not only affects the driving capability but also degrades the performance of the sub-threshold swing [5].
Therefore, comparing to the conventional pn junction MOSFETs, lower on-state current and poor subthreshold swing are expected. On the other hand, when biasing in the off-state, large number holes (n-channel) and electrons (p-channel) will tunnel
through the drain junction and induce abnormal high leakage current, as shown in Fig.
2-5(b) and Fig. 2-6(b). Hence, large off-state current and ambipolar behavior are obtained in SB MOSFETs.
For n-channel MSB MOSFET, since an ultra-thin n+ SDE exists in the interface of NiSi/Si channel, the SB at source-side is thinned and the effective SB height is reduced to let the electrons tunnel through the SB more easily and enhance driving ability in the on-state. Furthermore, while biasing in the off-state, the SB at drain-side is thicken and the effective SB height is increased to effectively block holes tunneling and suppresses the leakage current. Figure 2-7(a) and (b) show the schematic energy band diagrams of the n-channel MSB MOSFET in the on-state and off-state, respectively. Figure 2-8(a) and (b) show the transfer characteristics and output characteristics of the n-channel double-gate MSB MOSFET with gate length equals to 5 µm and fin width equals to 80 nm. The properties of the on-state and off-state are clearly enhanced. Higher driving current, better subtheshold swing, lower leakage current, and vanished sub-linear phenomenon are observed. Therefore, the MSB MOSFET can overcome the drawbacks and keep the advantages of SB MOSFET.
Similarly, for the p-channel MSB MOSFET, the ultra-thin p+ SDE not only lowers the effective SB height at source-side to enhance hole tunneling in the on-state but also broads the SB at drain-side to block electron tunneling in the off-state. The schematic energy band diagrams of the p-MSB MOSFET are displayed in Fig. 9. Figure 2-10(a) and (b) show the transfer characteristics and output characteristics of the p-channel tri-gate MSB MOSFETs with the gate length equals to 0.5 µm and the fin width equals to 0.3 µm. No sub-linear phenomenon and ambipolar behavior can be observed.
2.3.2 Fundamental Theory
The current transportation of the conventional pn junction MOSFETs has been described by drift-diffusion model successfully. The driving current decreases as temperature increases due to the phonon scattering induced mobility degradation. At the same time, the threshold voltage reduction and the subthreshold swing degradation occur. For the SB junction, the total current is composed of thermionic emission component (Jthermionic) and tunneling component (Jtunneling) [15], that is,
tunneling constant, q is the unit charge, φb is the Schottky barrier height, ∆φ is the image force lowering, ζ is the energy measured upward from the potential maximum, and η is the energy measured downward from the potential maximum. Fm and Fs are the Fermi-Dirac distribution functions of the metal and the semiconductor, respectively. Q(ζ) and Q(η) are the transmission coefficients above and below the potential maximum, respectively.
Equations (2) and (3) show that both the thermionic emission component and the tunneling component exhibit positive temperature dependence. Moreover, the thermionic emission component has stronger temperature dependence than the tunneling component. Therefore, the temperature dependence of the SB MOSFET should be in reverse trend of the conventional pn junction MOSFETs, that is, the higher temperature is, the higher current can be obtained. The dominant component
depends on the barrier height and barrier thickness. These barrier properties could be affected by doping concentration and electrical field. Moreover, for MSB junctions, the carrier transportation mechanism is similar to the SB junction since it still can be fundamentally considered as a SB junction with varied SB height and thickness.
Hence, the current transportation of MSB MOSFETs would be expected to consist of the drife-diffusion, thermionic emission, and tunneling. Therefore, we can use the temperature dependence of these mechanisms to identify the current transportation mechanism in different bias conditions by measuring the temperature effect of device.
2.3.3 SB MOSFETs
Figure 2-11 shows the typical transfer characteristics of the SB MOSFET with gate length equals to 5 µm and fin width equals to 80 nm, which is biased as an n-channel device. In this case, the electrons from the source-side are dominated in the on-state and the holes from the drain-side are dominated in the off-state. The expected ambipolar phenomenon is clearly observed at each chuck temperature from 300 K to 400 K. These transfer characteristics of the SB MOSFET shown in Fig. 2-11 is redrawn in linear scale at VDS=0.05 V in Fig. 2-12(a). In the both on-state and off-state, the current increases as the temperature increases. It should be noted that the temperature dependence in the off-state is much stronger than that in the on-state. It implies that electron-injection from the source to the inverted channel tends to be dominated by the tunneling mechanism in the on-state while the hole-injection from drain to the p-type body in the off-state tends to be dominated by the thermionic emission mechanism. This inference is consistent with the fact that the barrier for electron is higher than the barrier for hole at the NiSi/Si SB junction [16]. Figure 2-12(b) shows the transfer characteristics of the SB MOSFET shown in Fig. 2-11 in
linear scale at VDS=1 V. The on-state current is still dominated by the tunneling mechanism. However, in the off-state, the positive temperature dependence changes to negative temperature dependence as the VG decreases to be lower than -2 V. This phenomenon implies that the current transportation mechanism changes from thermionic emission or tunneling to drift-diffusion as increasing the gate bias. In addition, when the drain bias varies from +0.05 V to +1 V, the increasing electric field induces more band bending and the SB at drain-side is getting much thinner. Hence, the holes at drain electrode can more easily tunnel through the SB and inject into the p-type body by the tunneling mechanism and so the smaller positive temperature dependence is observed. Moreover, it is believed that the strong negative gate voltage and positive drain voltage induce more barrier thickness thinning and barrier height lowering. Therefore, while the gate bias increases, the injection resistance of the SB at drain-side decreases. As the hole-injection resistance from drain contact becomes low enough, the current flow becomes controlled by the drift-diffusion mechanism.
Similarly, Fig. 2-13 shows the typical transfer characteristics of the SB MOSFET with gate length equals to 5 µm and fin width equals to 80 nm and the device is biased as a p-channel device. In this case, holes are injected from source to the inverted channel in the on-state while electrons are injected from drain to body in the off-state.
Fig. 2-14(a) shows the transfer characteristics of the SB MOSFET shown in Fig. 2-13 in linear scale at VDS=-0.05 V. The on-state current (negative VG region) increases with the increase of temperature apparently. Similar to the discussion on Fig. 2-12(a), the hole current is dominated by the thermionic emission current while the electron current is dominated by the tunneling current. Fig. 2-14(b) shows the transfer characteristics of the SB MOSFET shown in Fig. 2-13 in linear scale at VDS=-1 V.
The on-state current becomes dominated by tunneling current due to the strong drain
field. Additionally, the positive temperature dependence decreases as the negative gate bias increases, indicating that the source-side injection resistance is reduced. This hole-injection resistance would become low enough at further high negative gate bias and thus the reverse temperature dependence is expected to be observed at that condition. In the off-state, the strong positive gate voltage and negative drain voltage reduce the electron-injection resistance at drain contact greatly so that the current becomes dominated by the drift-diffusion mechanism.
2.3.4 n-MSB MOSFETs
Figure 2-15 presents the typical transfer characteristics of the n-channel MSB MOSFET with gate length equals to 5 µm and fin width equals to 80 nm. As indicate in Fig. 2-7, due to the thin thickness and high concentration SDE layer, the modified SB at drain contact is wider and higher than the conventional SB and can block hole-tunneling effectively in the off-state. Moreover, the modified SB at source contact is thinner and lower than the conventional SB and thus can improve the electron-tunneling drastically. Therefore, the properties in the on and off-states are enhanced and the ambipolar phenomenon is suppressed effectively. The transfer characteristics of the n-channel MSB MOSFET shown in Fig. 2-15 in linear scale at VDS=0.05 V is redrawn in Fig. 2-16(a). At low VG, current increases with the increase of temperature while at high VG, the temperature dependence is reversed. The intersecting occurs at about VG=1.2 V. This phenomenon indicates that the current transportation mechanism changes from thermionic emission to tunneling and then to drift-diffusion.
It is believed that at low VG, the SB at source contact is still thick and high enough to dominate current flow. As VG increases, the barrier thickness and height are reduced.
Once the source injection resistance becomes lower than the channel resistance,
current transportation becomes dominated by the drift-diffusion mechanism and thus the negative temperature dependence is observed. In addition, the gate bias dependent source injection resistance on the MSB MOSFET would be discussed in the next chapter. A novel modified external method is proposed to extract the bias dependent series resistance. Furthermore, Fig. 2-16(b) shows the transfer characteristics of the n-channel MSB MOSFET shown in Fig. 2-15 in linear scale with VDS=1 V. Similar to that observed in Fig. 2-16(a), positive temperature dependence is obtained at low VG
but it reverses at high gate bias and the current transportation mechanism changes to drift-diffusion mechanism. Comparing with the SB MOSFET, the stronger negative temperature dependence of the on-state current confirms the effect of the MSB junction, indicating the ITS method and the low-temperature post-ITS annealing at 600 °C for 30 min can form the thin and highly doped SDE regions successfully. To evaluate the process effect, Fig. 2-17 shows the linear scale transfer characteristics of the n-channel MSB MOSFET fabricated with a post-ITS annealing at 600 ºC for 30 sec or 30 min and at VDS=0.05 V. The changing point from tunneling mechanism to drift-diffusion mechanism moves from 1.2 V to 1.8 V as the post-ITS annealing time decrease from 30 min to 30 sec. This result indicates that the insufficient post-ITS thermal budget (600 ºC, 30 sec) results in higher source injection resistance so that the current transportation is more likely to be controlled by the tunneling mechanism.
Figure 2-18 compares the transfer characteristics of the n-channel MSB MOSFETs fabricated with two different post-ITS thermal budget. It is observed that the MOSFET with higher thermal budget exhibits higher driving capability, better subthreshold swing, and lower off-state current.
2.3.5 p-MSB MOSFETs
For the p-MSB MOSFETs, the observation is similar to that of the n-MSB MOSFETs, i.e., the highly doped p+ SDE regions are successfully formed in the interface of the Ni-silicide/Si-channel to improve the on/off states properties. Figure 2-19 shows the typical transfer characteristics of the p-channel MSB MOSFET with gate length equals to 0.5 µm and fin width equals to 0.3µm. The ambipolar phenomenon is suppressed effectively owing to the MSB junctions. The transfer characteristics of the p-channel MSB MOSFET shown in Fig. 2-19 in linear scale at VDS =-0.05 V and -1 V are redrawn in Fig. 2-20(a) and (b), respectively. The trend of the temperature dependence is quiet alike to that of the n-channel MSB MOSFET, as shown in Fig. 2-16. Hence, the same tendency of temperature dependence and dominated current transportation mechanism are obtained. At low negative VG, due to the thick source-side SB, current is controlled by thermionic emission. Then, as the negative VG increases, the SB is thinner and lower and then the current transportation mechanism changes from tunneling and then to drift-diffusion. The intersection occurs at about VG=-1.6 V. Comparing with the SB MOSFET, the stronger negative temperature dependence of the on-state current confirms the effect of the MSB junction.