Transistor structures for advanced nanoscale processes

June 16, 2016 // By Graham Prophet
Nanotech researchers at imec (Leuven, Belgium) report demonstrating junction-less gate-all-around lateral and vertical nanowire FET devices; the GAA-NWFET is, they say, a promising candidate for advanced logic and analogue/RF applications, and scaled SRAM cells.

With their simplified processing, improved reliability, reduced low frequency noise and lower IOFF values, junction-less gate-all-around (GAA) nanowire (NW) FETs built in lateral and vertical configurations are an attractive option for advanced logic, low power circuits and analogue/RF applications, that also may enable a simpler path for considerable SRAM scaling via the stacking of vertical devices.


GAA-NWFETs -with the gate fully wrapped around the device body for optimum electrostatics control- are considered one of the most promising candidates for enabling (sub-)5nm CMOS scaling. Great process simplicity follows from the fact that devices do not require junctions.


Previously, at the 2015 VLSI and IEDM conferences, imec demonstrated the superior reliability behaviour of these devices and their potential for low power circuits. At the 2016 Symposia on VLSI Technology & Circuits , imec thoroughly evaluated key control paramters for junction-less device operation, namely controlling the NW doping vs. NW size to achieve optimum performance. The feasibility of these devices for analogue/RF applications was also studied, concluding that they are a viable option with reported similar speeds and voltage gains as compared to inversion-mode NWFETs.


Imec also addressed the variability of junction-less GAA-NWFETs, showing that whereas the VT mismatch increased with increasing nanowire doping for NMOS devices, less impact was observed for PMOS devices and at smaller nanowire dimensions. Additionally, the junction-less concept was demonstrated in vertical devices integrated on the same 300 mm (wafer) Si platform, also used for lateral devices. Low IOFF and IG values and good electrostatics were achieved over a wide range of vertical nanowire arrays.


Lastly, taking advantage of the junction-less devices’ process simplicity, Imec further explored their potential by proposing a novel SRAM cell design with two vertically stacked junction-less vertical NWFETs with the same channel doping, thus enabling reduction of the SRAM area per bit by 39%.


“Imec’s work has contributed to an increased and more in-depth understanding of junction-less GAA-NWFETs,”