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Conference report  IEDM


source voltage of 0.5 V, and the latter can operate in coulomb blockade mode at 4.2 K (see Figure 5). With device scaling, operation in coulomb blockade mode should be possible at room temperature.


The Pennsylvania researchers have also put forward a hybrid logic architecture for sub-250 mV operation, using the pairing of classical and non-classical MuQFETs. “Current complementary logic is not suitable,” explains Liu, “because of the lower current drivability and the low ratio of ‘on-current’ to ‘off-current’ in coulomb blockade mode.” So he and his co-workers used binary decision diagram logic to build logic circuits and harnessed negative differential resistance (NDR) to build static memory systems.


Figure 4.Multi-gate quantum-well FETs have been developed in Suman Datta’s group at The Pennsylvania State University operate at very low voltages


.These transistors can


Using device models that are well calibrated to these experimental efforts, the team found a 50 percent reduction in minimum energy for logic compared to silicon CMOS. When they used this silicon benchmark for memory, they discovered a 75-fold reduction in dynamic power.


the chip. Lu Liu, a graduate student in Suman Datta’s group at The Pennsylvania State University, points out that the obvious applications for low power chips are in battery-powered electronics goods, such as lap-tops, tablets, cell-phones and cameras. However, they can also prevent chip overheating that can lead to premature device failure. Although bolting powerful cooling fans onto CPUs and graphics cards can address this, it’s a workaround that leads to an increase in energy consumption.


A far more attractive option is to build circuits from devices requiring considerably lower operating voltages, such as less than 0.5 V. Such devices require higher ‘on-currents’ and a higher ratio between the ‘on’ and ‘off’ currents. This is possible by replacing silicon with materials with higher mobilities: InGaAs for NMOS and germanium for PMOS. To suppress short-channel effects, multi-gate structures are needed, such as the one used by Purdue and Harvard. In Datta’s group, efforts in this direction have led to the development of classical and non-classical Multi-gate Quantum well FETs (MuQFETs) employing a 14 nm-thick In0.7 well (see Figure 4).


Ga0.3 As


“We consider the classical MuQFET to be a good candidate for sub-14 nm CMOS and beyond,” says Liu, who explains that the non-classical variant promises to play a role at even more extreme length scales, when the number of electrons passing through the transistor starts to approach unity. “Non-classical MuQFETs in Coulomb oscillation mode are used to realize few and ultimately single electron computing with quantum dots.”


Liu and his co-workers have built classical MuQFETs with a 40 nm fin width and non-classical variants with split gates separated by 80 nm. The former can deliver a drive current in excess of 100 µA/µm at a drain-


MOCVD verses MBE If III-V devices are to be used in ICs beyond the 14 nm node, they will have to be formed on large silicon substrates that can be processed in today’s foundries.


Figure 5.Non-classical multi-gate quantum-well FETs developed in Suman Datta’s group at The Pennsylvania State University deliver reconfigurable operation at 4.2K.The three modes of operation of this transistor are: VSG


= 0 V = -1V ,tunnel barriers collapse


and the device behaves as a classical multi-gate quantum well FET gated by the control gate (short mode); VSG


,the split-gates moderately deplete


the one-dimensional multi-gate quantum-well FETs, resulting in 2.5 MΩtunnelling resistance,and the control gate modulates the coulomb island leading to single electron transistor operation (coulomb blockade mode); VSG


= -2V ,the split gates heavily


deplete the multi-gate quantum well FET fin resulting in only background leakage current (open mode)


January/February 2012 www.compoundsemiconductor.net 17


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