FeRAM: An interesting alternative are ferroelectric RAM (FeRAM) cells [1]. Here, data is stored by the bi-stable switching behaviour of the polarisation P of a ferroelectric layer under external voltage pulses. As depicted in Fig. 2, the magnitude of the polarisation charge of the ferroelectric layer influences the charge in the Si channel of the MOSFET transistor. In consequence, different threshold voltages Vt cof the transistor result as a function of the polarization status of the ferroelectric material, presenting the logic “0” and “1” values of the FeRAM memory cell. The low-voltage, low-power and fast-write characteristics, combined with good reliability, make this an ideal memory technology for portable systems and smartcards. Despite these promising characteristics, FeRAM suffers in particular from compatibility problems with mainstream Si technologies, namely etching and contamination issues are difficult to control due to the complicated stoichiometry of ferroelectric materials.
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Figure 2: Schematic cross-section of a FeFET in FeRAM |
MRAM: Magnetic RAM cells are based on the magneto-resistive effects in magnetic materials and structures that exhibit a resistance change when an external magnetic field is applied [3, 4]. In the MRAM, data are stored by applying magnetic fields that cause magnetic materials to be magnetized in one of two possible magnetic states. Reading data is performed by measuring resistance changes in the cell compared to a reference. Passing currents nearby or through the magnetic structure creates the magnetic fields applied to each cell. Two magneto-resistive effects are used in MRAM: giant magneto-resistance (GMR) and tunnelling magneto-resistance (TMR). Correspondingly, two types of MRAM are explored: GMR (pseudo spin valve (PSV) version) MRAM and magnetic tunnel junction (MTJ) MRAM.
The Pseudo Spin Valve cell, providing the GMR effect, is made of two magnetic layers (e.g., Ni, Co, Fe) separated by a thin conductive nonmagnetic layer (e.g., Cu) as shown in Fig. 3. In PSV MRAM devices, the GMR is sensed with the current flowing along the central conducting layer, which is coupled to both magnetic cladding layers. The resistance to current flow in the thin nonmagnetic conducting layer in the center of the structure depends upon the alignment of magnetic spin polarization of the two cladding magnetic metal layers with respect to each other. The main disadvantage of PSV cells is up to nowadays the small signal voltage difference of about ±3 mV during read so that large read times need to be applied.
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Figure 3: Pseudo spin valve trilayer structure |