Circuit and method to increase read margin in non-volatile memories using a differential sensing circuit

• US 7,859,906 B1
• Filed: 03/31/2008
• Issued: 12/28/2010
• Est. Priority Date: 03/30/2007
• Status: Active Grant

• Current Assignee: Monterey Research, LLC (Vector Capital Corporation)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Vispute, Hemant
• Primary Examiner(s): Ho, Hoai V
• Assistant Examiner(s): RADKE, JAY W
• Application Number:
  US12/059,560
• Time in Patent Office: 1,002 Days
• Field of Search: 365/189.15, 365/189.07, 365/189.09, 365/207, 365/210.1, 365/185.2, 365/185.21
• US Class Current: 365/185.21
• CPC Class Codes: G11C 16/28 using differential sensing or reference cells, e.g. dummy cells

Abstract

A differential sensing circuit and method for enhancing read margin of a memory device are disclosed. The differential sensing circuit includes a first current-to-voltage converter. The circuit includes a first current subtraction circuit having an erase reference cell. A first input terminal of the first current-to-voltage converter is coupled to the first current subtraction circuit. The circuit includes a second current-to-voltage converter. The circuit also includes a second current subtraction circuit having a program reference cell. A first input terminal of the second current-to-voltage converter is coupled to the second current subtraction circuit. Both the first and second current subtraction circuits are coupled to a memory access bias signal. Outputs of the first and second current-to-voltage converters are compared to generate an enhanced read margin output.

Claims

1. A differential sensing circuit, comprising:

a first current-to-voltage converter;

a first current subtraction circuit having an erase reference cell,

wherein a first input terminal of the first current-to-voltage converter is coupled to the first current subtraction circuit;

a second current-to-voltage converter; and

a second current subtraction circuit having a program reference cell,

wherein a first input terminal of the second current-to-voltage converter is coupled to the second current subtraction circuit,

wherein both the first and second current subtraction circuits are coupled to a memory access bias signal, and

wherein outputs of the first and second current-to-voltage converts are compared to generate an enhanced read margin output.

2. The circuit of claim 1, wherein each of the first current subtraction circuit having an erase reference cell and the second current subtraction circuit having a program reference cell comprises a plurality of series configured transistor devices.

3. The circuit of claim 2, wherein the plurality of series configured transistor devices comprises:

a first transistor coupled to the memory access bias signal at a gate terminal;

a second transistor coupled to a fixed biasing signal at a gate terminal; and

a third Silicon-Oxygen-Nitrogen-Oxygen-Silicon (SONOS) transistor device coupled to a power supply signal at a gate terminal.

4. The circuit of claim 1, wherein each current-to-voltage converter is coupled to the memory access bias signal at a respective second input terminal.

5. The circuit of claim 1, wherein each current-to-voltage converter comprises an operational amplifier coupled to a feedback resistor.

6. The circuit of claim 1, wherein both the erase reference cell and the program reference cell comprise Silicon-Oxygen-Nitrogen-Oxygen-Silicon (SONOS) cells placed along with a plurality of matrix memory cells in the same array, and

wherein both the SONOS cells and the plurality of matrix memory cells are arranged in an array to undergo memory program and erase operations.

7. The circuit of claim 1, wherein the memory access bias signal is generated through a bias transistor of a matrix memory cell.

8. The circuit of claim 7, wherein the matrix memory cell comprises a series configuration of transistors, further including:

a diode-connected P-channel Metal Oxide Semiconductor (PMOS) bias transistor for generating the memory access bias signal;

a first N-channel Metal Oxide Semiconductor (NMOS) transistor having a gate terminal coupled to a fixed biasing signal;

a second NMOS transistor having a gate terminal coupled to a column access signal; and

a Silicon-Oxygen-Nitrogen-Oxygen-Silicon (SONOS) transistor having a gate terminal coupled to a power supply signal.

9. A memory device with an enhanced read margin, comprising:

a first current subtraction stack having an erase reference cell coupled to a first current-to-voltage converter;

a second current subtraction stack having a program reference cell coupled to a second current-to-voltage converter; and

a matrix cell stack coupled to both the first and second current subtraction stacks and coupled to provide a memory access bias signal,

wherein output terminals of both the first and second current-to-voltage converters are coupled to a comparator circuit for generating an enhanced read margin output.

10. The memory device of claim 9, wherein each of the first current subtraction stack having an erase reference cell and the second current subtraction stack having a program reference cell comprises:

a P-channel Metal Oxide Semiconductor (PMOS) transistor having a gate terminal coupled to the memory access bias signal;

a N-channel Metal Oxide Semiconductor (NMOS) transistor having a gate terminal coupled to a fixed biasing signal,

wherein the NMOS transistor is configured as a common gate amplifier and a bit line voltage limiter; and

a Silicon-Oxygen-Nitrogen-Silicon (SONOS) transistor device having a gate terminal coupled to a power supply signal.

11. The memory device of claim 9, wherein the matrix cell stack comprises:

a diode-connected bias transistor for generating the memory access bias signal;

a first transistor having a gate terminal coupled to a fixed biasing signal;

a second transistor having a gate terminal coupled to a column access signal; and

a SONOS transistor having a gate terminal coupled to a power supply signal.

12. A method for improving read margin of a memory device, comprising:

generating a memory access bias signal;

generating an erase current in response to the memory access bias signal;

mirroring the erase current through an erase reference cell stack;

mirroring the erase current through a program reference cell stack;

generating a program current in response to the memory access bias signal;

mirroring a program current through a program reference cell stack; and

mirroring the program current through the erase reference cell stack.

13. The method of claim 12, wherein each of the steps of mirroring the erase current through a program reference cell stack and mirroring the program current through the erase reference cell stack comprises:

generating a first differential current through the erase reference cell stack; and

generating a second differential current through the program reference cell stack.

14. The method of claim 13, wherein a read margin output is proportional to a difference between an erase reference cell stack current and a program reference cell stack current.

15. The method of claim 12, further comprising:

arranging the program reference cell stack, the erase reference cell stack, and a plurality of matrix memory cells in a scalable common memory array.

16. The method of claim 15, wherein the program reference cell, the erase reference cell and a plurality of matrix memory cells are arranged to minimize process and geometric mismatches.

17. A method for enhancing read margin of a memory device, comprising:

subtracting a first current component from a second current component to generate a first differential current,

wherein the first differential current is generated in accordance with an erase reference operation of the memory device;

subtracting a third current component from a fourth current component to generate a second differential current,

wherein the second differential current is generated in accordance with a program reference operation of the memory device;

generating a first voltage component substantially proportional to the first differential current corresponding to a memory access bias signal;

generating a second voltage component substantially proportional to the second differential current corresponding to the memory access bias signal; and

comparing the first and second voltage components to generate an enhanced read margin output.

18. The method of claim 17, wherein one of the first voltage component and the second voltage component is proportional to a difference between an erase current component and a program current component.