Pre-Program Detection Of Threshold Voltages Of Select Gate Transistors In A Memory Device

摘要

A memory device includes memory cells arranged in NAND strings between select gate transistors. A threshold voltage (Vth) distribution of the select gate transistors is evaluated, such as in response to a program, erase or read command involving a block or sub-block of memory cells. For example, a lower tail and an upper tail of the Vth distribution can be evaluated using read voltages. If the Vth is out-of-range, such as due to read disturb, data retention loss or defects in the memory device, the block or sub-block is marked as being bad and previously-programmed data in the block or sub-block can be copied to another location. If the Vth is in range, the command can be executed. Also, a control gate voltage for the select gate transistors can be set based on a Vth metric which is obtained from the evaluation.

主权项

1. A method for operating a memory device, comprising:
receiving first instructions from a host device at a memory device, the first instructions comprise a program command, write data and a first address, the first address identifies memory cells in one plurality of NAND strings; and in response to the first instructions, evaluating a threshold voltage distribution of select gate transistors of the one plurality of NAND strings to determine whether a number of the select gate transistors which have a threshold voltage outside a specified range is greater than a maximum allowable number comprising at least 1-5% of the select gate transistors; if the number of the select gate transistors which have a threshold voltage outside the specified range is not greater than the maximum allowable number, executing program command by programming the memory cells corresponding to the first address with the write data and if the number of the select gate transistors which have a threshold voltage outside the specified range is greater than the maximum allowable number, returning fail status to the host device without programming the memory cells corresponding to the first address with the write data.