Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories.
Both EEPROM and flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage of the transistor is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
When programming an EEPROM or flash memory device, typically a program voltage is applied to the control gate and the bit line is grounded. Electrons from the channel are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the memory cell is raised so that the memory cell is in the programmed state. More information about programming can be found in U.S. Pat. No. 6,859,397, titled “Source Side Self Boosting Technique For Non-Volatile Memory;” and U.S. Pat. No. 6,917,542, titled “Detecting Over Programmed Memory,” both patents are incorporated herein by reference in their entirety.
Some EEPROM and flash memory devices have a floating gate that is used to store two ranges of charges and, therefore, the memory cell can be programmed/erased between two states, an erased state and a programmed state that correspond to data “1” and data “0.” Such a device is referred to as a binary or two-state device.
A multi-state flash memory cell is implemented by identifying multiple, distinct allowed threshold voltage ranges. Each distinct threshold voltage range corresponds to a predetermined value for the set of data bits. The specific relationship between the data programmed into the memory cell and the threshold voltage ranges of the memory cell depends upon the data encoding scheme adopted for the memory cells. For example, U.S. Pat. No. 6,222,762 and U.S. Patent Application Publication No. 2004/0255090, both of which are incorporated herein by reference in their entirety, describe various data encoding schemes for multi-state flash memory cells.
Typically, the program voltage (Vpgm) is applied to the control gates of the memory cells as a series of pulses. The magnitude of the pulses is increased with each successive pulse by a predetermined step size (e.g. 0.2 v, 0.3 v, 0.4 v, or others). In the periods between the pulses, verify operations are carried out. That is, the programming level of each memory cell of a group of memory cells being programmed in parallel is sensed between each programming pulse to determine whether it is equal to or greater than a verify level to which it is being programmed. One means of verifying the programming is to test conduction at a specific compare point. The memory cells that are verified to be sufficiently programmed are locked out, for example, by raising their respective bit line voltage to stop the programming process for those memory cells. The above described techniques, and others described herein, can be used in combination with various boosting techniques to prevent program disturb and with various efficient verify techniques known in the art.
As mentioned above, verifying programming can be done by testing that the memory cell conducts when a certain compare voltage is applied to the control gate of the memory cell. The same holds true for read operations. In many systems, a memory cell is considered conducting if the current conducted through the bit line connected to the memory cell exceeds a certain threshold amount. Earlier programmed word lines often see low bit line resistances during verify operations due to the fact that other word lines have not yet been programmed. The same is also sometimes true during read operations. As a result, when a compare voltage in excess of the threshold voltages of the memory cells connected thereto is applied to lower word lines, the memory cells often conduct current in excess of the required amount of current necessary for detection. While this does not create a problem from a logic point of view, it can represent an excessive use of power.