Considering the operating power, the maximum conductance of the synaptic device is one of the key governing factors in determining and boosting the neuromorphic hardware performance. This is because if the conductance is significantly high, the size of the transistor of the 1T–1R and peripheral circuit (e.g., multiplexer) should be increased to avoid voltage drop.
[42] Significant area overhead occurs and the systems operate slowly, resulting in longer latency and reduced throughput. Accordingly, the noticeable advantage of the RRAM over the PCM is a lower operating current due to non-Joule heating-related switching mechanism, implying synaptic weights in a lower conductance range. However, in practice, non-negligible parasitic components such as line resistance are involved in the cross-point array.
[66] The voltage drop due to the line resistance is spontaneously increased when the feature size of the interconnect line is scaled. In the column of the array nearest to the voltage source, most of the applied read voltages are delivered properly to the synaptic devices without any noticeable loss, accurately executing the multiplication. However, the read voltage decays along the line, and the voltage is significantly lowered in the farthest cell. The weighted sum current is thus lower than expected because the lowered read voltage is multiplied even though the given weight remains unchanged. It has been reported that the operating current of the RRAM can be reduced to ≈1 μA.
[67] Note that the low current operation in the RRAM indicates that the filament weakly comprises a lesser number of vacancies and no longer ensures metallic behavior exhibited by the stronger filament clustered from denser vacancies. Consequently, in the current–voltage (
I–
V) curve, the current at the LRS started to get distorted nonlinearly with respect to the voltage. It caused the conductance measured at the reduced read voltage to be lowered exponentially, and deviation of the actual computed weighted sum results became pronounceable. Therefore, studies have been conducted to carefully design electrode materials that can modify the conical shape of the filament to dissipate heat appropriately
[68] or to compensate the nonlinearity with circuits.
[69] Strengthening the
I–
V trace of the RRAM linearly allows the achievement of constant conductance, which can be less affected by the voltage drop.
Nonideal factors such as nonlinearity, asymmetry, and limited conductance range have been intensively studied in device and system-level analysis,
[70] but reliability concerns such as data retention, cycling endurance, variability, and failure have been less discussed and explored.
[71, 72] The conductance states can be affected in unexpected ways by various reliability issues. For simplicity, the conductance degradation trends were categorized in two major ways by considering whether the weighted sum current was consistently changed toward a certain direction or not. When the external parasitic components such as line resistance or conductance drift were considered at a specific BL, the output current was always changed uniformly due to the lowered input voltage or changed conductance with respect to the time, causing accuracy deterioration. Due to the consideration of the variation of the RRAM as a true stochastic behavior due to an inherent working principle, each weight could either be lower or higher than the criterion (e.g., the median). Thus, the lowest weight was compensated by the highest weight at the selected BL. This result indicated that the total weighted sum at the end of the BL was near the expected value. This explains the reduced effect of the non-uniformity of the individual devices on the accuracy of the recognition. The non-uniformity can introduce advantages that can help overcome the challenge depending on neural networks.
[73] Learning with a gradient descent scheme allows finding the optimum value defined by the global minimum; however, the learning process can converge to the local minimum level and be stuck while finding a route. The variation in the weights caused by non-uniformity can act as the driving force to escape the minimum area.
To minimize spatial and temporal variations in the filamentary RRAMs that affect accuracy, a novel material engineering technique was proposed. Instead of filaments formed randomly during device operation, dislocations in the material were deliberately threaded to confine the path.
[74] Thus, ions preferred to travel through the 1D channels, significantly improving uniformity. In the early stage of the RRAM-based synaptic element, an electrical barrier, such as Schottky barrier modulation, which is smoothly controlled by the movement of ions in the entire active area along the electric field, was used as a more uniform switching mechanism.
[64, 65, 75] The gradual conductance update that is proportional to the number of identical pulses was available, but the conductance increase (or decrease) was substantial at the very first step from the initial HRS (or LRS), respectively. It resulted in a highly asymmetric conductance response versus the pulse number. A slow speed of a few ms to drive the ions over the entire area was also another critical problem (see Table
1). Therefore, the exploration on the interfacial mechanism has been rarely studied currently in two-terminal device structures, but a similar ion movement mechanism has regained substantial attention and expanded by using a three-terminal structure and new materials, as will be discussed below.
ECRAM
The ECRAM, which was only designed for ultimate linear and symmetric synaptic characteristics, has been proposed due to the need to improve the controllability of ion transport. By using the traditional three-terminal transistor structure, the channel conductance between the source and drain is precisely tuned by the number of mobile ions provided by the gate. Motivated by the principle of a solid-state ion battery in which mobile Li ions stored at the cathode are transported to the anode,
[76] a channel material of LiCoO
2, which is capable of providing Li ions due to weak bonding, was used as the channel material.
[37] To promote effective migration of the Li ions, LiPON material was used as an electrolyte. When a negative voltage was applied to the gate and source (
VGS), the intercalated Li ions in the LiCoO
2 channel were pulled to the gate, which was a write operation. In the empty position where the Li ion was released from the LiCoO
2, the valence of Co ion was changed from 3
+ to 4
+ to maintain charge balance, generating positive charge. When the n-type oxide semiconductor MoO
3 was used as the channel, due to the formation of the electrons at the positions where Li ions were removed, an increase in conductance was observed by applying a positive gate voltage.
[78] The read path was decoupled with the write operation by applying voltage to the drain and grounding it to the source with a zero
VGS signal source. The current (
ISD) flowing between the source and drain separated by a long channel distance of 2 μm can thus be analogously adjusted by the proportionally modulated quantity of the Li ions moved under the number of applied
VGS signals. The conductance continued to increase when
VGS was simultaneously provided. A steady and constant current was observed when the gate voltage was removed to identify the altered channel state. In general, the changed conductance lasted for several weeks, and it was expected to continue to last for several months.
Other mobile cations such as H ion that emulates the role of the Li ions have also been examined, as shown in Table
2.
[77, 79-82] Unlike the Li ions embedded in the host material, the gate voltage pushed the cations (e.g., H ion) toward the bottom of the electrolyte of WO
3 while attracting the electrons to the top of the channel.
[82] It was discovered that the film quality and physical properties of each layer played a crucial role in determining the dynamic range of conductance. Recently, a fully CMOS compatible ECRAM device was reported by exploiting fab-friendly oxygen anions and metal oxides as the mobile source and electrolyte/channel, respectively.
[77] The ECRAM satisfied the requirements of the basic synaptic characteristics, and it was also experimentally demonstrated in small-sized arrays.
[83]