Chapter 3 Results and Discussion
3.1 Basic Electrical Characteristics…
Results and Discussion
3.1 Basic Electrical Characteristics 3.1.1 Transfer Curves
Figure 3.1 shows the transfer curves of an IM device. Due to the existence of un-gated regions which contribute extra parasitic resistances in the SG-1 mode [31], the electrical performance of the SG-1 mode is basically worse than that of the SG-2 mode. For the IM devices, the drain current conducts when the inversion layer is formed near the interface between the channel and gate dielectric. From the plot, we can see the drain currents in the three operation modes rise up at almost the same gate voltage. From the TEM images in Chapter 2, the channel thickness for our fabricated device is quite thin (down to about 20nm), and hence the gate coupling effect [32] is pretty strong so that the gate controllability and electrical performance are obviously enhanced in the DG mode [26]. As is evident in the current-drive of a single device, the Ion of the DG mode is larger than the sum of those of the two SG modes. Also, owing to the effect of volume-inversion on account of the thin channel thickness [33], the SS under the DG mode is better than that in the SG mode.
The transfer curves of the AcM device are given in Fig. 3.2. There are some differences in electrical characteristics from those of the IM device. First, it can be seen that the curves of the AcM device apparently shift to the right (more positive value of Vth), showing the normally-on characteristics. Moreover, due to
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the different conduction mechanisms for the AcM devices [34], the characteristics of Id-Vg curves are different from those of the IM devices. In the AcM case, the key point is whether the device can be effectively turned off. When the AcM device is turned on, the channel flowed through by the carriers contained in the accumulation region at the surface and the quasi-neutral region in the film center. On the other hand, for the PMOS devices, if the gate bias isn’t sufficiently positive to deplete the free holes in the channel, a high leakage current will flow and the device cannot be effectively turned off. Thanks to the ultra-thin channel thickness in our fabricated devices, the AcM device can still be successfully turned off in all operation modes despite the very high channel doping (>1018/cm-3). Moreover, for the 0.7µm channel length, the drain-to-source leakage current in each operation modes is significantly larger than that for the device with 2µm channel length. It may be due the severe SCE and the large bulk leakage current for the AcM devices, which will be discussed in the next section.
For the AcM devices, the gate coupling effect is also obvious under the DG mode. However, its influence is quite different from that of the IM device. For the DG mode, the two gates are simultaneously used to deplete the channel and can therefore more effectively turn off the device, leading to a smaller SS and a smaller Vth.
Figure 3.3 shows the threshold voltage (Vth) of the three operation modes for the two types of devices with 0.7µm channel length. For the IM devices, the conduction is through inversion carriers induced near the channel interface by the gate bias. The Vth is more positive for the DG mode than the SG-2 mode and SG-1 mode, indicating the IM device can be more effectively switched with the switching mode [27]. However, for the AcM device, the device is normally on and its switching depends on the ability of depleting the carriers in the body. As
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shown in Fig. 3.3, the order in the magnitude of Vth among various modes for the AcM device is opposite to that of the IM one, and the Vth of the DG mode, which has the strongest gate controllability over the channel, is the smallest. This is reasonable since the DG mode depletes the channel from the two opposite channel interfaces and the two depletion regions would merge at around the center of the channel as the device is turned off. For the SG modes, the depletion starts from only one of the two channel interfaces where the driving bias is applied and it needs to deplete the whole channel for effectively switching off the device, thus the SS is worse while the Vth is larger.
Figure 3.4 shows the Ion characteristics as a function of channel length for both devices. In the AcM devices, the carriers flow through the whole channel, thus their current level is significantly larger than that of the IM ones in which the conduction is mainly through the channel interface. For the two types of devices, the current of SG-1 mode is smaller than that of SG-2 mode. As explained in [31], the current path of the SG-1 mode contains un-gated regions, as shown in Fig. 3.5, which drastically increase the series resistance. Regarding this problem which leads to the degraded performance, the Ion of the SG-1 mode is obviously lower than that of the SG-2 mode for the IM devices, and under the DG mode, the Ion is almost dominated by that of the SG-2 mode. This issue is slightly ameliorated when adopting the AcM scheme, and due to the high doping concentration in the channel, impact of the un-gated regions is relieved, as evidenced by the smaller difference in Ion between the two SG modes.
3.1.2 Output Characteristics
Figure 3.6 shows the output characteristics for both devices. For the AcM devices, the current conduction occurs through the whole channel layer rather
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than the surface, and it is apparent that the output current is enhanced significantly for the AcM devices, which can provide about 328% enhancement of saturation current at Vg-Vth = -4V and VD = -5V for the 2µm channel length, and 198% enhancement for the 0.7µm channel length over the IM devices.
The S/D resistance can be extracted from the Id-Vg curves in Fig. 3.6 by using the linear regression method. Fig. 3.7 shows the total resistance (Rtot, = ) as a function of channel length. The S/D resistance can be extracted from the intercept of y-axis of the plots. Because we had performed an extra implantation on the AcM devices to reduce the S/D resistance [35], the extracted S/D resistances are almost the same for the two devices. The value of the AcM devices is about 12.4kΩ, just a little smaller than 13.8kΩ of the IM devices, presumably due to the lower-resistivity path in the AcM device between the channel and the S/D as explained in [36]. The channel resistance can be obtained by extracting the S/D resistance from the Rtot, and the results for both devices are shown in Fig.
3.8. Due to the high channel doping for the AcM devices, the channel resistance is much smaller than that for the IM devices with undoped channel. For the long channel length, the difference of channel resistance between the two devices is more obvious, and the difference diminishes with decreasing channel length. The outcome is consistent with the results shown in Fig. 3.6 that the enhancement of the output characteristics is more prominent for the long-channel device.
3.1.3 Simulation Results
Here we use the TCAD simulation to analyze the differences in conduction mechanisms between the two types of devices in the subthreshold region. The simulated structures have a uniform doping concentration in the channel and S/D regions for the AcM devices. The channel has boron doping at a
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concentration of 5 × 1018 cm-3, and the channel thickness is 20nm while the gate oxide thickness is 10nm. The work function of gate electrode is 4.15eV, and here we ignore the quantum effect to simplify the condition. The Vth is defined as the value of Vg when Id equals 10nA. Figure 3.9 shows the simulation results along the channel depth for the IM devices. As the absolute value of gate overdrive increases in Fig 3.9(a), the energy bands near the interfaces gradually bend and invert the surface region. Then we can see from Fig. 3.9(b) that the concentration of the inverted holes induced near the two interfaces increases with increasing gate overdrive.
The simulation results for the AcM device are shown in Fig. 3.10. In Fig 3.10(a), when the absolute value of gate overdrive increases, the surface bend bending is relieved while the width of a flat-band region (zero electric field) in the central Si channel increases with increasing gate overdrive. It can be noted from Fig. 3.10(b) that the carriers are concentrated at the center of the Si bulk [37]. Unlike the IM device whose channel body is nearly depleted when the device is turned on, the quasi-neutral region of the AcM device gradually expands from the channel center to the interfaces with increasing gate overdrive.
In conclusion, from the simulation results, we can define the differences of conduction mechanisms between the two devices. For the IM devices, the conduction carriers are from the surface charges induced by the gate bias, and for the AcM devices with doped channel, the conduction is mainly through the center of the Si bulk opened by the gate bias.