Semiconductor memory device having dummy cells of divided charge type

Claim 1

A semiconductor memory device comprising: first and second bit lines; at least first and second memory cells respectively connected to said first and second bit lines and to a voltage source; first and second dummy cells and first and second coupling means, said first dummy cell being connected to said first bit line via said first coupling means and said second dummy cell being connected to said second bit line via said second coupling means, an interconnection point between said first dummy cell and said first coupling means forming a first node and an interconnection point between said second dummy cell and said second coupling dummy means forming a second node; a third coupling means connected between said first and second nodes; a precharged circuit connected to said first and second bit lines; a sense circuit connected to said first and second bit lines for sensing the voltage therebetween; said first dummy cell comprises a first dummy capacitor connected in series with a fourth coupling means such that said first dummy capacitor is connected to said voltage source and said fourth coupling means is connected to said first bit line; said second dummy cell comprises a second dummy capacitor connected in series with a fifth coupling means such that said second dummy capacitor is connected to said voltage source and said fifth coupling means is connected to said second bit line; said first memory cell comprising a first memory capacitor connected in series with a sixth coupling means such that said first memory capacitor is connected to said voltage source and said sixth coupling means is connected to said first bit line; said second memory cell comprising a second memory capacitor connected in series with a seventh coupling means such that said second memory capacitor is connected to said voltage source and said seventh coupling means is connected to said second bit line; wherein all of said capacitors have substantially identical capacitances; and wherein said first and second coupling means respectively selectively couple and decouple said first and second nodes to said first and second bit lines and wherein said third coupling means selectively couples and decouples said first node to said second node and said fourth and fifth coupling means respectively selectively couple said first and second dummy capacitors to said first and second nodes and said sixth and seventh coupling means respectively selectively connect said first and second memory capacitors to said first and second bit lines in accordance with the following: during rewriting, said first and second and fourth and fifth coupling means are rendered conductive while said third and sixth and seventh conducting means are rendered nonconductive such that information is written into said first and second dummy capacitors; thereafter, said first and second coupling means are rendered nonconductive and said third coupling means rendered conductive such that said first and second dummy capacitors are electrically decoupled from said first and second bit lines; said third coupling means is rendered conductive such that said first and second dummy capacitors are electrically coupled in series, thereby causing reference charges in said first and second dummy capacitors; simultaneously, said first and second bit lines are precharged to a predetermined potential by said precharged circuit and then, said fourth and fifth coupling means are rendered nonconductive and said first and second coupling means rendered conductive so that first and second nodes are also precharged to said predetermined potential; when information stored in said first memory capacitor is to be read, said sixth and fifth coupling means are rendered conductive and charge stored in said first memory capacitor is read out to said first bit line while the reference charge stored in said second dummy capacitor is read out to said second bit line, thereby causing a potential difference between said first and second bit lines which is sensed by said sense circuit; and when information in said second memory capacitor is to be read, said seventh and fourth coupling means are rendered conductive and charge stored in said second memory capacitor is read out to said second bit line while the reference charge in said second dummy capacitor is read out to said first bit line, thereby causing a potential difference between said first and second bit lines which is sensed by said sense circuit.