Research objectives and approaches: The objective of this research is materials, device and circuit based co-exploration of mixed-anion and mixed-cation compound semiconductor based transistors for energy-efficient computing. The approach is a) experimental investigation of n-channel mixed anion (InAsxSb1-x) quantum-well transistors and p-channel mixed cation (InyGa1-ySb) transistors to address dynamic power consumption in complementary logic and RF circuits; b) experimental investigation of mixed-anion and cation based tunnel transistors to address stand-by power consumption in logic and embedded memory circuits, and c) development of design toolkit to enable heterogeneous circuit implementation with emerging devices. Intellectual merit: The key scientific merits of this proposal are: i) Harnessing the excellent electron and hole transport properties in mixed-anion and mixed-cation antimonide material system to provide ultra-low power transistor solutions. We investigate mixed-anion material, InAsxSb1-x with varying As and Sb mole fraction, to achieve high electron mobility (>13,000 cm2V-1s-1) to demonstrate n-channel quantum-well FETs (QWFETs). We explore mixed-cation materials, InyGa1-ySb to maximize hole mobility (>2,000 cm2V-1s-1) by varying In and Ga mole fractions to enable band-gap engineered p-channel QWFETs; Device layer design is done with the primary goal of achieving a common high-k dielectric gate solution for both n-channel and p-channel QWFETs; ii) Harnessing the availability and tunability of staggered band-edge lineup in the mixed anion-cation antimonide material system to explore tunnel transistor (TFET) architecture with steep switching characteristics to address stand-by energy consumption; iii) Exploration of a heterogeneous system via implementation of speed critical, high activity logic circuits using QWFETs and low activity factor circuits using Tunnel FETs. This investigation will expand our fundamental understanding of the material science of mixed-anion and mixed-cation based material systems, novel QWFET and TFET device configurations and implementation of energy efficient logic elements, interconnect fabric and embedded memory. Broader Impact: The proposed research directly addresses the quest in the semiconductor industry for longer term solutions to technology scaling and addressing energy efficiency. The outcome of this research will have a direct impact on the future of ?green? nanoelectronics and many-core processor architecture design. A broader impact of successful development of the underlying materials, novel device architectures and energy efficient circuits with several orders of magnitude reduced energy consumption than today?s available electronics can usher in a new generation of implantable medical electronics needed for health monitoring and nanomedicine applications. Throughout the project, the key results will be disseminated via a dedicated WIKI web portal and via existing Penn State MRSEC-related outreach channels.
Power consumption is a critical bottleneck towards realizing future many-core microprocessors. A key challenge towards high-performance, ultra-low power systems is to address concurrently both dynamic and static power consumption concerns. To address this challenge, this project was a co-exploration of mixed-anion and mixed-cation compound semiconductor based quantum-well transistors targted at high-activity circuits dominated by dynamic power and tunnel transistors for use in leakage dominated low-acitivty parts of the chip. The key outcomes of the project include experiemental demonstration of InAsSb n-channel and InGaSb p-channel quantum-well MOSFETs, as well as GaAsSb/InGaAs n-channel and InGaAs/GaAsSb p-channel heterojunction Tunnel FETs and the development of their circuit level compact models that were used in circuit design. Intellectual merit: The key intellectual outcomes of this research project are: a) demonstration of an enhancement mode n-channel InAsSb QW MOSFET with high effective electron drift mobility of 5,200cm2/Vs and record high short channel electron velocity of 1.8x107 cm/s; b) demonstration of n-channel GaAsSb source and InGaAs channel hetero Tunnel FETs with staggered gap tunnel junctions which resulted in record high switching speed of 20 GHz at a supply voltage of 500mV. Several other intellectual breakthorughs were achieved in this project. For example, compressively strained InSb (s-InSb) and Ge (s-Ge) quantum-well heterostructures were experimentally studied, with emphasis on understanding the hole transport in confined systems for p-channel device applications. Magnetotransport measurements confirmed that 2.5x lower effective mass advantage of s-InSb over s-Ge is negated by higher phonon scattering, degrading hole transport at room temperature and at high carrier concentration. These results suggest that s-Ge quantum well heterostructure is more favorable and promising p-channel candidate compared to s-InSb for future technology node applications. Finally, we also demonstratde InAs source and GaSb channel based heterojunction p-channel Tunnel FETs and achieved a record drive current density for the first time in p-TFETs resulting from the broken gap tunnel junction. Broader Impact: The results from this project fostered new directions at the intersection of electrical engineering and computer engineering extending applications of novel mesoscopic devices to real-life low power circuit design world. In the device-circuit co-design space, we pioneered a new design of heterojunction tunnel FET based neural amplifier employing a telescopic operational tranconductance amplifier (OTA) for multi-channel neural spike recording. Exploiting unique device characteristics of TFET, we demonstrated record noise efficiency factor significantly lower than CMOS based amplifiers which can lead to high resolution, small form factor brain probes in the future. In the space of self-powererd autonomous electronic health and ambient monitoring systems, we demonstrated the benefits of utilizing Tunnel FET characteristics in RF powerered systems. A series of key circuit blocks such as rectifiers, DC-DC converters, low power amplifiers, and Analog-to-Digital converters, were also created with superior performance of power-harvesting efficiency and mixed signal figure-of-merits. Based on our results stemming from this research project, the Emerging Research Device committee of the International Technology Roadmap for Semiconductors (ITRS) has recently identified TFETs as the most promising emerging device technology for future energy efficient nanoelectronics.
