- Selector (access) devices for x-point array
‘Cross-point’ array is suggested to be the ultimate packing structure of RRAMs for high density memory application.
However, in such large memory array, a sneak path current, which is the current that flow through the unselected (half-selected) cell, causes disturbance problem and unnecessary power consumption during read operation.
Selector devices are candidates to overcome such a barrier by integrating it with RRAM in a 1S-1R structure.
- Insulator-to-Metal Transition (IMT) based selectors
The uniform switching and excellent bidirectional selectivity makes IMT based devices as a best candidate for cross-point selector application.
VO2 and NbO2 has been studied due to there abrupt and fast insulating-to-metallic phase transformation when external bias is applied. NbO2 has better thermal stability since the transition temperature is high enough, 1070K, compared to VO2 which is only ~340K.
However, the high off current renders the applicability of IMT based selectors for low power operation. Device size reduction and eliminating leakage paths in the oxide layer can effectively reduce the off current.
- Tunnel-barrier Type Selector
Tunnel-barrier (exponential) type selectors are an other class of access devices which works by the mechanism of band barrier controlling.
Different oxide stacks has been proposed as a tunnel-barrier selector and multi-layer Ta2O5/TaOx/TiO2 structure, thin Ta2O5 as a tunnel barrier, is one of them.
Optimized thickness and the oxygen profile of tunnel barrier can effectively reduce electron transfer at low voltage level with direct tunneling suppression, whereas this tunnel barrier can exhibit high current flowing at high voltage level with its Fowler-Nordheim (FN) tunneling of barrier lowering.
- Ovonic Threshold Switch (OTS)
The Ovonic threshold switching from insulating state to metallic state is typical characteristics of chalcogenide materials.
OTS based selectors are very attractive due to there excellent performance and simple material structure.
They exhibit field-dependent, abrupt and fast switching from high resistance to low resistance state at the threshold voltage.
The transition occurs in a purely electronic nature without any atomic rearrangement.
Simple and room temperature fabrication process with high thermal/electrical stability makes OTS as one of the best candidate material for x-point array selection application.
- Volatile CBRAM type switch
Two terminal devices with active metal (Cu/Ag) as a top electrode are intensively studied for non-volatile memory applications.
The switching mechanism is simple formation and dissolution of metal filaments within the electrolyte due to electrochemical reactions. This kind of device usually exhibit volatile nature at lower operating current.
Maximizing the instability of filament is, therefore, the central idea for realizing threshold type switching for selector application.
Controlling of interaction between metal filament and oxide electrolyte or reduced metal ion supply can minimize the stability of metal filament which in-turn exhibit threshold switching characteristics.
By utilizing this concept abrupt HRS to LRS transition with very high on/off ratio can be achieved.
Despite its uniformity problem volatile CBRAM type threshold switching devices are regarded as a promising selector for cross-point application.
- E. Cha et al., "Nanoscale (~10nm) 3D vertical ReRAM and NbO2 threshold selector with TiN electrode." Electron Devices Meeting (IEDM), 2013 IEEE International. IEEE, 2013.
- Y. Koo et al., "Te-based amorphous binary OTS device with excellent selector characteristics for x-point memory applications." VLSI Technology, 2016 IEEE Symposium on. IEEE, 2016.
- J. Song et al., "Threshold selector with high selectivity and steep slope for cross-point memory array." IEEE Electron Device Letters 36.7 (2015): 681-683.