Memory cell with three states, two programmable and one non-programmed state, comprises pre-read stages and programming stages with resistors of polycrystalline silicon

摘要

The memory cell (1) comprises at least one, in particular two branches (2,3) connected between two terminals (4,5) where the read voltage (Vr) is applied; each branch comprises two stages connected in series, that is a pre-read stage (6,7) each with two switchable resistors (Rg1,Rg2;Rg3,Rg4) connected in parallel, and a programming stage (8,9) containing a programmable resistor (Rp1,Rp2) of polycrystalline silicon, where the programmable resistors terminals (14,15) are accessible to a proper programming circuit to implement an irreversible decrease of each programmable resistance. The decrease (delta)Rp of the value of the programmable resistance (Rp1,Rp2) is predetermined and chosen to be greater than the difference (delta)Rg between two resistances (Rg1,Rg2;Rg3,Rg4) in the pair of each pre-read stage (6,7). The memory cell also comprises interrupters (K10,K11) for isolating the pre-read stages (6,7) from the programming stages (8,9). The programming stages comprise switches (K14,K15) for aplying the programming voltage (Vp) which is higher than the read voltage (Vr) to the terminals of the programmable resistors (Rp1,Rp2). The reading of the cell state is effected in two successive steps in the course of which the switchable resistors (Rg1,Rg2,Rg3,Rg4) of the pre-read stages are alternatingly selected. Each programmable resistor (Rp1,Rp2) is connected to the lower supply voltage terminal, in particular the ground (5) by a transistor (MN1,MN2) connected as a bistable with the transistor of the other branch. The switchable resistors (Rg1,Rg2;Rg3,Rg4) of the two branches (2,3) are simultaneously controlled so that the values of the selected resistances in each branch are inverted. The irreversible decrease (delta) Rp of the programmable resistances is greater than the difference (E) of the nominal values of the programmable resistances in the non-programmed state. In a variant of the memory cell, the terminal of the programmable resistor is the read terminal which is connectable to the first input of an amplifier whose second input is connected to a reference potential which is chosen at a level intermediate between the voltage levels at the read terminal in the two read phases when the programmable resistance is in the non-programmed state. A method (claimed) for reading the memory cell (claimed) consists in effecting two successive read steps in the course of which the switchable resistances of the pre-read stage are selected.