Deep Sub-60 mV/decade Subthreshold Swing in AlGaN/GaN FinMISHFETs with M-Plane Sidewall Channel

Subthreshold Characteristics of AlGaN/GaN MIS-FinFETs with Controlling Threshold Voltages

“…This high positive oxide charge density in the sloped device lowers V TH of the sidewall MOS channel to increase I OFF . The SS of steep sidewall device is as low as 37 mV/dec due to the simultaneous turning on of 2DEG channel and MOS channel and the effective channel width modulation as discussed before [17]. On the other hand, the sloped sidewall device exhibits a relatively larger SS of 63 mV/dec, which could be explained by the existence of non-negligible C dep caused by the undepleted part at the wide fin bottom and relatively large C it, SiO 2 /GaN due to a rough sidewall surface [25], which increases both SS con and SS W(V G ) , while the C dep can be ignored for the device with a steep sidewall because the entire fin is narrow and completely depleted from the electric field of the sidewall gate at off-state.…”

“…On the other hand, the narrow FinFET with W fin of 45 nm has a sharper and higher g m peak as can be seen in Figure 2c, which means that the V TH of the 2DEG channel and MOS channel are almost the same and hence both channels simultaneously turn on/off and the effective channel width of the device can be modulated with the gate voltage, which results in very small SS as low as 56 mV/dec, smaller than the theoretical Boltzmann limit of 60 mV/dec. As discussed in our previous work [17], the 2DEG channel will be generated at the center of the 2DEG channel and laterally spread until occupying the whole 2DEG channel as V G increases from the V TH of the 2DEG channel to just above it. Then, the MOS channel will instantaneously turn on because there is only a tiny V TH difference between the top 2DEG channel and the sidewall MOS channel.…”

“…In other words, the channel width first spread laterally within the 2DEG channel and then immediately spread vertically into the MOS channel, which makes the concept of gate-dependent effective channel width modulation reasonable. This sub-60 mV/dec SS can be understood by considering the expression for new SS W(V G ) which includes the gate voltage-dependent channel width modulation [17] as shown below,…”

“…Figure 1a shows the schematic image of AlGaN/GaN MIS-FinFET. The fabrication processes of the FinFETs were similar to our previous work [17]. Figure 1b,c exhibit the cross-sectional TEM images for the fin with sloped and steep sidewall surface, respectively.…”

“…Our previous work demonstrated that AlGaN/GaN MIS-FinFETs with W fin of around 30 nm can show not only normally-off operation, but also extremely low I OFF as well as small SS (smaller than theoretical Boltzmann limit of < 60 mV/decade) [17]. These excellent performances of the AlGaN/GaN MIS-FinFETs suggest that the GaN-based materials, combined with novel nano-structure such as fin or nanowire, can offer an opportunity for a new possible low power logic device application [18][19][20], in addition to conventional efficient power switching device application which requires a relatively large positive V TH to ensure safe device operation as well as low standby power consumption.…”