Analog storage semiconductor memory that uses plural write voltages and plural read voltages having different voltage levels

• US 6,687,155 B2
• Filed: 05/09/2003
• Issued: 02/03/2004
• Est. Priority Date: 01/11/2001
• Status: Expired due to Fees

• Current Assignee: Oki Semiconductor Company Limited (ROHM Co., Ltd.)
• Original Assignee: OKI Electric Industry Company Limited
• Inventors: Nagasue, Makoto
• Primary Examiner(s): Tran, Andrew Q.
• Application Number:
  US10/434,217
• Publication Number:
  US 20030193825A1
• Time in Patent Office: 270 Days
• Field of Search: 365/185.03, 365/185.33, 365/185.19, 365/185.2, 365/185.22, 365/45, 365/168, 365/189.09, 365/189.11
• US Class Current: 365/185.03
• CPC Class Codes: G11C 27/005 with non-volatile charge storage, e.g. on floating gate or MNOS

Abstract

An analog storage flash memory by which sufficient write accuracy can be obtained even when the write speed of the memory cell transistor disperses due to manufacturing dispersion or other reasons. A read voltage adjustment circuit outputs the read voltage generated by a read voltage generation circuit as is, or drops and outputs the read voltage. A write voltage adjustment circuit outputs the write voltage generated by a write voltage generation circuit as is, or drops and outputs the write voltage. A write control circuit repeats the write operation at the write voltage Vw until the memory cell transistor turns OFF at the read voltage Vr−ΔVr in the first write cycle, and repeats the write operation at the write voltage Vw−ΔVw until the memory transistor turns OFF at the read voltage Vr in the second write cycle.

Claims

1. An analog storage semiconductor memory comprising:

a memory cell transistor that stores analog data;

a write voltage supply circuit for supplying a plurality of write voltages to a control gate of said memory cell transistor;

a read voltage supply circuit for supplying a plurality of read voltages to said control gate of said memory cell transistor; and

a write control circuit for executing an operation of making said read voltage supply circuit supply said read voltages and judging a threshold voltage of said memory cell transistor, and for executing an operation of making said write voltage supply circuit supply said write voltages as having decreasing voltage as said threshold voltage approaches a voltage corresponding to said analog data.

2. The analog storage semiconductor memory according to claim 1, wherein a plurality of said memory cell transistors constitute a flash memory.

3. The analog storage semiconductor memory according to claim 1, wherein said read voltage supply circuit continuously changes a value of the read voltage supplied, without stopping supply of the read voltages.

4. The analog storage semiconductor memory according to claim 3, wherein said read voltage supply circuit provides a read voltage Vr−

Δ

Vr during a first write cycle, and provides a read voltage that continuously changes from Vr−

Δ

Vr to Vr during a second write cycle.

5. An analog storage semiconductor memory comprising:

a write voltage supply circuit for supplying n (n≧

2) write voltages Vw1, . . . , Vwn (Vw1>

. . . >

Vwn) to a control gate of a memory cell transistor;

a read voltage supply circuit for supplying n read voltages Vr1, . . . , Vrn (Vr1 <

. . . <

Vrn) to said control gate of said memory cell transistor; and

a write control circuit for sequentially executing, from m=1 to m=n, an mth write cycle where a write operation at an mth write voltage Vwm and a read operation at an mth read voltage Vrm are repeated until an operation threshold value of said memory cell transistor exceeds the mth read voltage Vrm.

6. The analog storage semiconductor memory according to claim 5, wherein said write voltage supply circuit comprises:

a write voltage generation circuit for generating one of said write voltages Vw1, . . . , Vwn using an analog voltage signal; and

a write voltage adjustment circuit for generating the rest of said write voltages Vw1, . . . , Vwn by dropping or raising an output voltage of said write voltage generation circuit according to a control signal which is input from said write control circuit.

7. The analog storage semiconductor memory according to claim 6, wherein said write voltage generation circuit generates the one of said write voltages Vw1 , . . . , Vwn by shifting a level of a reference voltage according to said analog voltage signal, and then samples and holds the shifted reference voltage.

8. The analog storage semiconductor memory according to claim 6, wherein said write voltage generation circuit generates said write voltage Vw1 and said write voltage adjustment circuit generates said write voltages Vw2, . . . , Vwn by dropping said write voltage Vw1.

9. The analog storage semiconductor memory according to claim 6, wherein said write voltage generation circuit generates said write voltage Vwn and said write voltage adjustment circuit generates said write voltages Vw1, . . . , Vwn−

1 by raising said write voltage Vwn.

10. The analog storage semiconductor memory according to claim 5, wherein said read voltage supply circuit comprises:

a read voltage generation circuit for generating one of said read voltages Vr1, . . . , Vrn using an analog voltage signal; and

a read voltage adjustment circuit for generating the rest of said read voltages Vr1, . . . , Vrn by dropping or raising an output voltage of said read voltage generation circuit according to a control signal which is input from said write control circuit.

11. The analog storage semiconductor memory according to claim 10, wherein said read voltage generation circuit generates one of said read voltages Vr1, . . . , Vrn by shifting a level of a reference voltage according to said analog voltage signal, and then samples and holds said shifted reference voltage.

12. The analog storage semiconductor memory according to claim 10, wherein said read voltage generation circuit generates said read voltage Vrn, and said read voltage adjustment circuit generates said read voltages Vr1, . . . Vrn−

1 by dropping said read voltage Vrn.

13. The analog storage semiconductor memory according to claim 10, wherein said read voltage generation circuit generates said read voltage Vr1, and said read voltage adjustment circuit generates said read voltages Vr2, . . . , Vrn by raising said read voltage Vr1.

14. The analog storage semiconductor memory according to claim 5, wherein said write voltage supply circuit comprises:

a write voltage adjustment circuit for dropping or raising a reference voltage according to a control signal output from said write control circuit to provide an adjusted reference voltage; and

a write voltage generation circuit for generating said write voltages Vw1, . . . , Vwn using said adjusted reference voltage.

15. The analog storage semiconductor memory according to claim 14, wherein said write voltage generation circuit generates said write voltages Vw1, . . . , Vwn by shifting a level of said adjusted reference voltage output from said write voltage adjustment circuit according to an analog voltage signal, and then samples and holds said shifted reference voltage.

16. The analog storage semiconductor memory according to claim 15, wherein said write voltage adjustment circuit generates n adjusted reference voltages Vs1, . . . , Vsn by dropping the reference voltage.

17. The analog storage semiconductor memory according to claim 15, wherein said write voltage adjustment circuit generates n adjusted reference voltages Vs1, . . . , Vsn by raising the reference voltage.

18. The analog storage semiconductor memory according to claim 5, wherein said read voltage supply circuit comprises:

a read voltage adjustment circuit for dropping or raising a reference voltage according to a control signal output from said write control circuit to provide an adjusted reference voltage; and

a read voltage generation circuit for generating said read voltages Vr1, . . . , Vrn using said adjusted reference voltage.

19. The analog storage semiconductor memory according to claim 18, wherein said read voltage generation circuit generates said read voltages Vr1, . . . , Vrn by shifting a level of said adjusted reference voltage output from said read voltage adjustment circuit according to an analog voltage signal, and then samples and holds the shifted reference voltage.

20. The analog storage semiconductor memory according to claim 18, wherein said read voltage adjustment circuit generates n adjusted reference voltages Vs1, . . . , Vsn by dropping the reference voltage.

21. The analog storage semiconductor memory according to claim 18, wherein said read voltage adjustment circuit generates n adjusted reference voltages Vs1, . . . , Vsn by raising the reference voltage.

22. The analog storage semiconductor memory according to claim 5, wherein said write control circuit judges that the operation threshold value of said memory cell transistor exceeds the mth read voltage Vrm by detecting ON/OFF status of said memory cell transistor from a drain potential of said memory cell transistor.

23. The analog storage semiconductor memory according to claim 5, further comprising a switch for selectively supplying said write voltage or said read voltage to said control gate of said memory cell transistor under control of said write control circuit.

24. The analog storage semiconductor memory according to claim 5, wherein a plurality of said memory cell transistors constitute a flash memory.

25. The analog storage semiconductor memory according to claim 5, wherein said read voltage supply circuit continuously changes a value of the read voltage supplied, without stopping output of the read voltages.