Multi-contact Phase Change Toggle Logic Device Utilizing Thermal Crosstalk

Abstract: Phase change memory (PCM) is an emerging high speed, high density, high endurance, and scalable non-volatile memory technology which utilizes the large resistivity contrast between the amorphous and crystalline phases of chalcogenide materials such as Ge2Sb2Te5 (GST). In addition to being used as a standalone memory, there has been a growing interest in integration of PCM devices on top of the CMOS layer for computation in memory and neuromorphic computing. The large CMOS overhead for memory controllers is a limiting factor for this purpose. Transferring functionality like routing, multiplexing, and logic to the memory layer can substantially reduce the CMOS overhead, making it possible to integrate 100s of GB of PCM storage on top of a conventional CPU. In this work, we present computational analysis of a phase change device concept that can perform toggle operations. The toggle functionality is achieved using two physical mechanisms: (i) isolation of different read contacts due to amorphization between different write contact pairs, and (ii) thermal cross-talk between a molten region and a previously amorphized region. Phase-change devices with six contacts can be implemented as toggle flip-flops, multiplexer, or demultiplexer when interfaced with CMOS transistors. Here, we demonstrate the operation of the device as a toggle flip-flop with 5 transistors, requiring ~50% of the footprint compared to conventional CMOS alternatives, with the added advantage of non-volatility.