Analog sensing of memory cells with a source follower driver in a semiconductor memory device
Memory devices, methods, and sample and hold circuits are disclosed, including a memory device that includes a sample and hold circuit coupled to a bit line. One such sample and hold circuit includes a read circuit, a verify circuit, and a reference circuit. The read circuit stores a read threshold voltage that was read from a selected memory cell. The verify circuit stores a target threshold voltage that is compared to the read threshold voltage to generate an inhibit signal when the target and read threshold voltages are substantially equal. The reference circuit stores a reference threshold voltage that can be used to translate the read threshold voltage to compensate for a transistor voltage drop and/or temperature variations.
1. A method for programming a selected memory cell, the method comprising:
storing a target threshold voltage into a verify circuit;
biasing the selected memory cell with a programming pulse;
verifying whether the selected memory cell is programmed to the target threshold voltage by:
reading a current threshold voltage of the selected memory cell into a read circuit having a source follower driver;
comparing, to the target threshold voltage, the current threshold voltage output from the read circuit through the source follower driver; and
generating an indication of a response to the comparison.
2. The method of claim 1 wherein reading the current threshold voltage comprises:
biasing the selected memory cell with a ramp voltage; and
storing, in a capacitor of the read circuit, the current threshold voltage from the ramp voltage.
3. The method of claim 2 wherein storing comprises closing an input switch to the read circuit.
4. The method of claim 1 wherein the indication of the response is an inhibit signal when the current threshold voltage is substantially equal to or greater than the target voltage.
5. The method of claim 1 and further including storing the current threshold voltage in a reference circuit for a subsequent read operation.
6. The method of claim 1 wherein biasing the selected memory cell comprises biasing a control gate of the selected memory cell.
7. The method of claim 1 wherein generating the indication comprises generating a logical high signal when the current threshold voltage output from the read circuit is equal to or greater than the target threshold voltage.
8. The method of claim 1 wherein the indication of the response to the comparison is used to inhibit further programming of the selected memory cell.
9. The method of claim 2 wherein the ramp voltage is a conditioned ramp voltage.
10. The method of claim 9 wherein the conditioned ramp voltage is one of: buffered, range reduced, level shifted, or any combination of buffered, range reduced, and level shifted.
11. The method of claim 2 and further comprising generating the ramp voltage through an attenuator and level shifter.
12. The method of claim 1 and further comprising generating the programming pulse by a word line generator receiving a command to generate one or more programming pulses.
13. The method of claim 2 wherein the ramp voltage starts at 0V and increases to 5V.
14. The method of claim 3 and further including opening the input switch to the read circuit in response to:
detecting a current in a bit line coupled to the selected memory cell;
generating a control signal in response to detecting the current; and
the control signal controlling operation of the input switch.
15. The method of claim 2 wherein a voltage storage device in the read circuit stores an indication of the ramp voltage as the ramp voltage increases.
16. The method of claim 15 wherein the voltage storage device is a capacitance.