• Typical RRAM values:
• switching speeds (< 10 ns) [22],
• low power consumption (∼ 1 pJ/transition) [23],
• high endurance (> 10^9 cycles) [24],
• low cost (< 0.02 $/GB) [25,26].

1. Based on electrical impulses needed to
induce

• Unipolar
• Bipolar
• Nonpolar RRAMs
  (the set and reset processes do not depend on the polarity of the applied voltage)
• A new family of RRAM devices exhibits the so-called Threshold Resistive Switching:
  in which a MIM cell can be set by applying an electrical impulse, but when it disappears the MIM cell switches back to the HRS without the need of applying additional impulses
  See references in paper

2. Based on the spatial localization
of the atomic species involved in the RS process

• filamentary RS: local formation/disruption of conductive filaments
• distributed RS: distributed migration of traps at the metal/insulator interface
• molecular RS: molecular rearrangements
  See references in paper

Process:

• the metallic electrodes that provide cations with better mobility in the RS insulator are Ag, Ni, or Cu.
• The metallic ions coming from the active electrode polarized under positive bias can diffuse into the insulator, and when they reach the opposite electrode,
  they get reduced and form CFs that bridge the electrodes triggering a set process.
• By applying negative bias to the same electrode, the metallic atoms that form CF are oxidized and diffuse provoking the disruption of the CF and resetting the
  conductance.

Explains different modelling levels and models: Microscopic, macroscopic and compact

• In general, when the electrodes
  are selected to be active metals, theMIM cell works as
  ECM-RRAM; on the contrary, the use of inert metals enables its use as VCM-RRAM.
• The differences are linked to the origin of the ions:
  In ECM-RRAM, they come from the active electrodes, while in VCM-RRAMs they consist of oxygen ions formed in the insulator when exposed to an electric field. This generates an accumulation of ions close of the top electrode named reservoir

Also explains it in geat detail and why complaince current is used

• When the first dielectric breakdown
  (BD) is reached (a process called forming or electroforming in RRAM), the filament expands laterally due to ion migration (see Fig. 3c) [16,108]. In order to avoid severe damage of the dielectric and irreversible BD, a current limitation is often used [168].

In addition, the ion thermal diffusion from the CF to the surrounding dielectric is taken into account

charge transport in LRS

• modeled assuming ohmic behavior.
• When the CFs achieve the operation size, electrons can flow through them as a common resistor.
• linear behavior of the I–V curves
• However, it is possible to find RRAM devices that show nonlinear LRS behavior eg. Ni/HfO2/Si−n+ (explained how this can be modelled )