As semiconductor integrated circuit (IC) technologies are developed, the reconfigu-rable ICs have been widely used due to the easy modification of logic and neuromorphic functions with small hardware resources and low costs. Nevertheless, the reconfigurable IC technology still has several issues about device area, delay, dynamic and static power, and flexibility of the systems.
A resonant tunneling diode (RTD) has been considered as one of the most promising beyond-CMOS nano-scale quantum effect devices. Due to the unique negative differential resistance (NDR) and pico-second switching characteristics at room temperature, RTD has been considered as a practical More-than-Moore device to possibly overcome the limitations of the mainstream silicon CMOS technology. For programmable logic, reconfigurable IC, and neuromorphic systems, the RTD based architecture has the advantages of reduced device count, high density integration, low power consumption, and fast switching speed. Therefore, the issues of the reconfigurable ICs can be improved by implementing RTDs as data pro-cessing devices.
Meanwhile, a resistive switching device such as a memristor has the attention for ap-plications to the reconfigurable ICs, based on their non-volatile memory and variable re-sistance characteristics. Most of the resistive switching devices are constructed based on the crossbar array structure for high density integration. Therefore, there has been an approach to implement the reconfigurable ICs with the crossbar array-based resistive switches using the advantageous potential of RTD based threshold gates. Recently, the RTD-memristor reconfigu-rable ICs have been theoretically demonstrated in various programmable logic and neuromor-phic system operations achieving a low operation voltage and low power consumption with significantly reduced circuit complexity.
In this research, the RTD-Resistor threshold gates have been designed and fabricated as a basic research for the reconfigurable IC applications based on the RTDs and memristors. The reconfigurable IC is used as a novel terminology, which refers the novel structure consists of NDR devices and resistive switches to perform reconfigurable system functions such as programmable logic and future neuromorphic system. At first, the programmable logic array has been simulated using the RTDs for the logic operation and resistors for simulating the re-sistive switches. The AND, OR, and NOT functions are the basic components of the general programmable logic array. Meanwhile, the programmable logic array can be composed with fixed resistance and RTD gates for various logic operations. Therefore, AND, OR, and NOT gates have been designed and fabricated. In addition, inverting and non-inverting threshold gates are the basic cell of the RTD-memristor neuromorphic system. Therefore, the threshold gates have also been selected as the fabrication target.
Afterwards, the RTD-memristor reconfigurable IC systems have been designed and simulated. For the RTD-resistor programmable logic array, a simulation of the full-adder func-tion has been demonstrated with two different design methodologies. In addition, a future ap-plication of neuromorphic system has also been designed for the function for the image pat-tern recognition and simulated using RTD-memristor models. The simulation results exhibit a full-adder and a pattern recognition operations with potential of decreased power and delay due to the possibility of reduced device count, low power consumption, and fast switching characteristics of the RTD-memristor reconfigurable ICs.
Consequently, the RTD-resistor threshold gates have been designed and fabricated as a fundamental research of the RTD-memristor reconfigurable ICs. The simulated program-mable logic and neuromorphic functions based on the gates potentially shows the possibility of the RTD-memristor reconfigurable IC applications.
본 연구에서는 기존에 이론적으로만 발표되었던 RTD 및 memristor 를 이용한 reconfigurable IC 의 구현을 위한 과정에서, 석사 연구를 통해 이에 필요한 기본 회로를 RTD 및thin-film resistor로 실제 제작 및 실험적으로 동작을 검증하였다. 제작된 RTD-resistor 기반 basic gate는 RTD-memristor reconfigurable system의 기본이 되는 basic cell의 기능을 구현하기 위해 기존의 architecture를 개선하여 실제 제작 가능하도록 설계되었으며, 공정의 편의성을 고려하여 자체 제작 기술로 가장 안정적으로 구현 가능한 $10 \times 10 \mu m^{2}$ RTD emitter size를 사용해 제작 및 측정하였다. 측정 결과 threshold gate및 inverting threshold gate의 동작을 확인하였고 이를 바탕으로 bias를 조절하여 모든 basic gate function에 대한 동작을 확인할 수 있었다.
최종적으로 RTD-memristor 기반 reconfigurable system의 대표적 기능을 simula-tion으로 확인하였다. RTD 기반 reconfigurable IC의 대표적인 예로는 RTD-resistor 구조의 programmable logic array와, RTD-memristor 구조의 차세대 neuromorphic IC를 살펴보았다. 특히 기존 Boolean 방식의 정보 처리 효율을 뛰어넘는 신경세포 모방 정보처리 회로인 neuromorphic architecture는, 회로의 가변성을 극도로 필요로 하므로 RTD-memristor architecture 의 장점을 가장 잘 보여줄 수 있는 system application이라 할 수 있다.
먼저 programmable logic으로 구현 가능한 무수한 logic 기능 중 하나로 full-adder를 예시로 선정하였다. 또한 full-adder의 구현 방식으로 single gate 기반 full-adder와, 전형적인 programmable logic array 의 AND-OR 구조를 이용한 AND-OR array 기반 full adder 두 가지를 선정하였다. 한편, neuromorphic system application에 대응하는 func-tion의 예시로는, 최근 거의 대부분의 neuromorphic분야에서 구현 기능으로 삼고 있는 image pattern recognition을 선정하였다. Simulation 결과 확인된 특성을 통해, 추후 RTD-memristor reconfigurable IC 공정을 통해 system을 실제로 구현할 경우 RTD-memristor 응용 architecture 고유의 특성에 의해 낮은 전력 소모와 delay 특성 등 우수한 성능을 보일 것을 예상할 수 있었다.
