Comparative Study of Cross-Point MRAM Array with Exponential and Threshold Selectors for Read Operation

We investigate a tunnel magnetoresistance (TMR) ratio of magnetoresistive random access memory (MRAM) devices with selectors classified as exponential and threshold types for cross-point array applications. Through array-level SPICE simulation, we analyze how the TMR ratio, which can vary with the type of the selector, affects a read-out current ratio in the array. In MRAM arrays with exponential selectors, the read-out current ratio close to 2× at high read voltage range is achieved only when the selector exhibits high current drivability with minimal external resistance. Instead, using the threshold selector that can provide sufficient current without external constraints due to switching mechanism enables to obtain the ratio greater than 2× in a small array, which can be distinguished with the help of sensing circuitry. However, the aggressive scaling of the technology node leads to inevitable wire resistance, thereby degrading the ratio. Thus, we also discuss optimizing the resistances of the MRAM, which is considered one of the ways to mitigate the degradation.