A selector is a two-terminal device which turns on above a certain voltage and stays off otherwise. High density memory architectures (e.g. cross-point arrays) can be realized by using 1S1R structure (one selector paired with one memory) as a building block. When such a memory array is properly biased to operate a selected memory cell, the sneak current from non-selected memory cells can be eliminated by the selectors connected in series to each memory cell. To achieve high performance, it is critical to develop selectors that match the characteristics of specific non-volatile memories. Key requirements for the selector include on-state to off-state current ratio (non-linearity), high on-state current density, fast switching speed, high endurance cycles, high thermal stability, ease of process integration, and operational compatibility with the memory element. There are four major types of selectors being studied worldwide: Ovonic threshold switch (OTS), metal-ion threshold switch, insulator-metal transition, and tunneling barrier type. Cross-point memory arrays using OTS selector and PCRAM are already in production as storage-class memory but there is still a lot of room for improvement. High operating voltage is one of the key issues. In order to work more efficiently with the logic platform, the total operating voltage of the selector and non-volatile memory cell should be compatible with the logic platform supply voltage (e.g. 1.5V for advanced nodes). TSMC is exploring new selector materials, devices, and processes for high performance and energy efficient high density non-volatile memories.

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  • Engineering defects in pristine amorphous chalcogenides for forming-free low voltage selectors

    Amorphous chalcogenide-based threshold selectors are among the most promising two-terminal technologies for high density non-volatile memories. However, the necessity of a high voltage forming operation makes their implementation in low voltage logic chips a key challenge. This work reports a new approach towards forming-free chalcogenide selectors, where extra defects are introduced to assist the forming process and reduce the forming voltage. The added defects are shown to increase the conductivity of the pristine chalcogenide and can be annihilated after the first switching pulse operation. Forming-free low voltage selectors based on SiNGeCTe (SNGCT) chalcogenide are demonstrated along with excellent endurance characteristics over 1010 cycles.
  • First Fire-free, Low-voltage (~1.2 V), and Low Off-current (~3 nA) SiOxTey Selectors

    Operating voltage compatibility and low power consumption are crucial for on-chip integration of high-density one-selector-one-resistor (1S/1R) arrays. However, traditional chalcogenide-based threshold selectors require a one-time first fire operation with voltage higher than the threshold voltage. Here, we introduce a novel SiOTe selector based on a stable silicon oxide matrix, with tunable first fire voltage and ultimately first fire-free characteristics. These selectors achieve low threshold voltages (V th = 1.1 V – 1.5 V) and low off-current (I off ~ 3 nA at 0.5 V for V th = 1.2 V). SiOTe selectors show promising thermal stability (300 °C, 30 min in air) and endurance of >108 cycles.
  • Low-Voltage (~1.3V), Arsenic Free Threshold Type Selector with Ultra High Endurance (> 1011) for High Density 1S1R Memory Array

    Low voltage selectors are critical for low power operation of high density non-volatile memories. In this work, selectors based on arsenic free chalcogenide materials are demonstrated with record high endurance over 1011 cycles together with threshold voltage ~1.3V and leakage current ~5nA. The enhanced endurance is attributed to suppression of phase separation with more stable amorphous network by proper dopants.
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