Patent ID: 8023312
Filing Date: 2011-09-20
Classification: G11C,H01L

Abstract:
1. A nonvolatile semiconductor memory device comprising: a memory cell having a variable resistive element having two terminals, the variable resistive element having resistive characteristics defined by current-voltage characteristics at both ends, the resistive characteristics transiting between two stably provided resistive characteristics of a low resistance state and a high resistance state when a voltage satisfying a predetermined condition is applied to the both ends; a load circuit connected to one end of the variable resistive element in series, the load circuit having load resistive characteristics defined by the current-voltage characteristics, the load resistive characteristics being switched between first load resistive characteristics and second load resistive characteristics, the second load resistive characteristics having a resistance higher than the first load resistive characteristics; and a voltage generation circuit for applying a voltage to both ends of a serial circuit configured by the variable resistive element and the load circuit, wherein a stored state of the variable resistive element is determined whether the resistive characteristics are in the low resistance state or the high resistance state and written when the resistive characteristics transit between the low resistance state and the high resistance state in response to application of a voltage to the both ends of the variable resistive element, the variable resistive element shows asymmetric characteristics in which when a polarity of an applied voltage to one terminal based on the other terminal is a first polarity, a first threshold voltage is lower than a second threshold voltage, the first threshold voltage being a lower limit value of an absolute value of the applied voltage required for the resistive characteristics to transit from the low resistance state to the high resistance state, the second threshold voltage being a lower limit value of an absolute value of the applied voltage required for the resistive characteristics to transit from the high resistance state to the low resistance state, and when the polarity of the applied voltage is a second polarity different from the first polarity, the first threshold voltage is higher than the second threshold voltage, the load resistive characteristics of the load circuit are switched so as to show the first load resistive characteristics during a first writing operation in which the resistive characteristics of the variable resistive element transit from the low resistance state to the high resistance state, and show the second load resistive characteristics during a second writing operation in which the resistive characteristics of the variable resistive element transit from the high resistance state to the low resistance state, and the voltage generation circuit applies a first writing voltage to the both ends of the serial circuit of the variable resistive element and the load circuit so that a voltage of the first polarity having an absolute value equal to or higher than the first threshold voltage is applied to the both ends of the variable resistive element of the memory cell to be written during the first writing operation, and applies a second writing voltage to the both ends of the serial circuit of the variable resistive element and the load circuit so that the voltage of the first polarity having the absolute value equal to or higher than the second threshold voltage is applied to the both ends of the variable resistive element of the memory cell to be written during the second writing operation.