Method and circuit for simultaneously programming and verifying the programming of selected EEPROM cells

• US 5,495,442 A
• Filed: 05/04/1995
• Issued: 02/27/1996
• Est. Priority Date: 07/08/1993
• Status: Expired due to Term

• Current Assignee: SanDisk Technologies LLC (Western Digital Corporation)
• Original Assignee: SanDisk Corporation (Western Digital Corporation)
• Inventors: Cernea, Raul-Adrian, Lee, Douglas J., Mehrotra, Sanjay
• Primary Examiner(s): Zarahian, A.
• Application Number:
  US08/434,970
• Time in Patent Office: 299 Days
• Field of Search: 365/185, 365/210, 365/208, 365/205, 365/189.07, 365/189.09, 365/168, 327/51
• US Class Current: 365/185.03
• CPC Class Codes: G11C 16/10 Programming or data input circuits
• CPC Class Codes: G11C 16/12 Programming voltage switching circuits
• CPC Class Codes: G11C 16/3481 Circuits or methods to verify correct programming of nonvolatile memory cells whilst programming is in progress, e.g. by detecting onset or cessation of current flow in cells and using the detector output to terminate programming
• CPC Class Codes: G11C 16/3486 Circuits or methods to prevent overprogramming of nonvolatile memory cells, e.g. by detecting onset or cessation of current flow in cells and using the detector output to terminate programming

Abstract

A method and circuit programs and automatically verifies the programming of selected EEPROM cells without alternating between programming and reading modes like prior art methods and circuitry. The circuitry includes a programming circuit and a bit line voltage regulation circuit. The programming circuit further includes a novel sense amplifier which unlike prior art sense amplifiers, is operable during both cell reading and programming modes. Included in the sense amplifier are two current providing circuits. A first circuit provides current to a selected EEPROM cell which is sufficient for reading the programmed state of the cell, and a second circuit which automatically provides additional current when required, for programming the cell. The sense amplifier detects when programming of a selected EEPROM cell has completed and causes programming of that cell to be terminated. The voltage regulation circuitry regulates the bit line voltage to the selected EEPROM cell'"'"'s drain electrode. The programming circuitry, including the sense amplifier, and voltage regulation circuitry are shown to be shared between a plurality of bit lines through a bit line selection circuit.

Claims

1. An EEPROM having a plurality of memory cells, each memory cell having a control gate, a drain electrode, and a source electrode, wherein a selected one of said plurality of memory cells has its control gate connected to a word line, its source electrode connected to a first bit line, and its drain electrode connected to a second bit line, said selected one memory cell being accessed by providing appropriate voltages to said word line, said first bit line, and said second bit line, and wherein said selected one memory cell draws through said second bit line, a first current, if said selected one memory cell is in an unprogrammed state, or a second current less than said first current, if said selected one memory cell is in a programmed state, said EEPROM including a sense amplifier, connected to a current source providing a reference current, said sense amplifier comprising:

a first circuit connected to a voltage supply and said current source, and selectably connected to said second bit line through a first node, said first circuit providing a first circuit current to said first node, not to exceed said reference current, in response to a cell current being drawn through said second bit line by said selected one memory cell when a programming voltage is applied to said second bit line; and

a second circuit connected to said voltage supply and said first node, said second circuit automatically providing an additional current to said first node when said cell current being drawn through said second bit line by said selected one memory cell is greater than said reference current;

wherein said reference current is greater than said second current and less than said first current, and said voltage supply provides a voltage greater than said programming voltage being applied to said second bit line when programming said selected one memory cell.

2. The EEPROM as recited in claim 1, wherein said first circuit of said sense amplifier comprises a first p-mos transistor having a control gate responsive to said current source, a source electrode connected to said voltage supply, and a drain electrode connected to said first node.

3. The EEPROM as recited in claim 1, wherein said second circuit of said sense amplifier comprises:

a first n-mos transistor having a control gate, a drain electrode connected to said voltage supply, and a source electrode connected to said first node; and

a detection circuit, connected to said first node and said control gate of said first n-mos transistor, wherein said detection circuit turns on said first n-mos transistor when said cell current drawn through said second bit line is greater than said reference current.

4. The EEPROM as recited in claim 3, said first node having a first node voltage which is pulled towards ground when said second bit line draws a cell current greater than said reference current, wherein said detection circuit of said sense amplifier comprises a circuit which detects said first node voltage and turns on said first n-mos transistor when said first node voltage is pulled towards ground.

5. The EEPROM as recited in claim 4, wherein said detection circuit of said sense amplifier comprises a second p-mos transistor having a source electrode connected to said voltage supply, a control gate connected to said first node, and a drain electrode connected to said control gate of said first n-mos transistor, so that when said first node is pulled down towards ground by said second bit line, said second p-mos transistor is turned on, thus pulling the drain electrode of said second p-mos transistor up towards the voltage supply, and turning said first n-mos transistor on as a result thereof.

6. The EEPROM as recited in claim 5, wherein said detection circuit further comprises a current sink connected, at a second node having a second node voltage, to said drain electrode of said second p-mos transistor, wherein said current sink normally pulls said second node voltage down towards ground, except when said second p-mos transistor is turned on, in which case said second p-mos transistor overpowers said current sink and pulls said second node voltage up towards said voltage supply.

7. The EEPROM as recited in claim 1, further comprising a voltage regulation circuit having an input connected to a reference voltage and an output connected to said second bit line, wherein said second bit line is maintained at said reference voltage as a result thereof.

8. The EEPROM as recited in claim 7 further including a second n-mos transistor having a drain electrode connected to said first node, a control gate, and a source electrode connected to said second bit line, thereby connecting said first node to said second bit line when said control gate of said second n-mos transistor is activated, wherein said voltage regulation circuit comprises:

a differential amplifier having a first input connected to said reference voltage, a second input connected to said second bit line, and an output connected, at a third node having a third node voltage, to said control gate of said second n-mos transistor.

9. The EEPROM as recited in claim 1, further including a bit line selection circuit comprising a plurality of bit-line-selection switches, each of said plurality of bit-line-selection switches having an input connected to said first node and an output connected to one of a plurality of bit lines, wherein said sense amplifier is shared among said plurality of bit lines by selectively turning on or off each of said plurality of bit-line-selection switches.

10. The EEPROM as recited in claim 7, further including a bit line selection circuit comprising a plurality of bit-line-selection switches, each of said plurality of bit-line-selection switches having an input connected to said first node and an output connected to one of a plurality of bit lines, wherein said sense amplifier is shared among said plurality of bit lines by selectively turning on or off each of said plurality of bit-line-selection switches.