One-time programable memory with additional programming time to ensure hard breakdown of the gate insulating film
A nonvolatile semiconductor memory device includes a storage element which is programmed with information by breaking an insulating film by application of electrical stress to the storage element, a control switch which controls the application of electrical stress to the storage element, and a control circuit which controls conduction/nonconduction of the control switch. The device further includes a power supply circuit including a voltage generation circuit which generates a first voltage to cause the electrical stress in program operation, a sensing circuit which senses that the insulating film is broken down, and a counter circuit which controls the control circuit to interrupt the application of electrical stress to the storage element when a given period of time elapses after the sensing circuit senses that the insulating film is broken down.
1. A nonvolatile semiconductor memory device comprising:
a storage element of an insulating film breakdown type, which is programmed with information;
a power supply circuit connected to one end of the storage element to apply a high voltage that is not lower than a maximum rating to the storage element;
an open/close switch connected to another end of the storage element to control application of the high voltage to the storage element from the power supply circuit;
a control circuit which controls an open/close operation of the open/close switch;
a sensing circuit which senses that an insulating film of the storage element is broken down by the application of the high voltage from the power supply circuit;
an oscillator which oscillates; and
a counter circuit which counts in response to an output of the sensing circuit and an output of the oscillator,
wherein the device is programmed by controlling the control circuit such that an output of the counter circuit interrupts the application of the high voltage to the storage element from the power supply circuit after a lapse of a given period of time.
2. The nonvolatile semiconductor memory device according to claim 1 , wherein the power supply circuit includes a charge pump circuit which performs a step-up operation in response to the output of the oscillator, and the charge pump circuit performs the step-up operation to replenish the counter circuit with charges corresponding to those consumed by current flowing through the storage element whose insulating film is broken down.
3. The nonvolatile semiconductor memory device according to claim 1 , wherein the sensing circuit is a breakdown sensing circuit which senses a potential difference caused at both ends of the open/close switch.
4. The nonvolatile semiconductor memory device according to claim 1 , wherein the sensing circuit is a breakdown sensing circuit which senses a variation in the amount of current flowing through the storage element.
5. The nonvolatile semiconductor memory device according to claim 1 , wherein the high voltage applied from the power supply circuit is a voltage enough to break the insulating film of the storage element.
6. The nonvolatile semiconductor memory device according to claim 1 , wherein the output of the counter circuit is used as a clock signal of a program counter to write information to the storage element for each bit.
7. The nonvolatile semiconductor memory device according to claim 1 , further comprising a protection transistor between the storage element and the open/close switch.