The ferroelectric capacitor alternatively denoted as ferroelectric tunnel junction (FTJ) was also used as a stand-alone memory.
[92, 93, 96] Unlike the FeFET-based synapse, where the conductance from the source to drain is adjusted by polarization change occurring in the ferroelectric oxide between the gate and silicon substrate, conductance through the FTJ is directly affected by up or down direction of the dipole. The conductance of the FTJ is transmitted smoothly by the identical pulse. Due to the simple structure, the FTJ-based synapse can be integrated into a 3D vertical NAND structure, where the FTJ is formed on the sidewall.
[92] Inherently low conductance range from 1 to 3 nS of the usual FTJ can slow down the system during the read operation. However, the neuromorphic systems usually sense the weighted sum of the multiple FTJs. Thus, the weight mapping and array size must be carefully designed to calculate the proper output current that does not affect the speed read by peripheral circuitries.
Meanwhile, MRAM utilizes the orientation of the spin rotated by the direct voltage or magnetic field of magnetic metal electrodes placed on either side of a thin tunneling oxide, which is a magnetic tunnel junction (MTJ) structure. Due to the achievement of only two HRS and LRS in the MRAM driven by spin-transfer torque (STT), implementation is expected for limited neuromorphic systems that routinely perform inference on the small-sized input data by adopting binary neural network (BNN) algorithms, where the weights were quantized and binary.
[97, 98] The digital state can be further extended while the multiple MTJs are stacked.
[99] Recently, a new writing mechanism called spin-orbit torque (SOT) has been suggested by passing the write current through an additional in-plane SOT layer, typically composed of heavy metals such as Pt and Ta.
[100-102] The current flowing via the SOT layer creates a spin current in the vertical direction, where the MTJ is located, due to spin Hall effect. The resistance state of the MTJ can be fine-tuned because the write current flows through the low-resistance heavy metal to generate spin-orbit coupling (see Table
4). However, as the range of achievable resistance is small, the MRAM has been studied primarily as memory applications so far. It is a preliminary stage for the synaptic applications, so a lot of parts need to be further studied.