Ceiling test mode to characterize the threshold voltage distribution of over programmed memory cells

A method of determining a threshold voltage associated with a programmed dual bit flash memory cell, comprising the steps of:

• (a) applying a characterization voltage signal on a gate terminal of the programmed dual bit flash memory cell;
• (b) varying the applied characterization voltage signal over a predetermined range;
• (c) evaluating a conduction current associated with the programmed dual bit flash memory cell for each of the applied characterization voltage signals over the predetermined range;
• and
• (d) determining the threshold voltage based on the evaluation.

A system for determining a threshold voltage of a flash memory cell, comprising:

• a program control circuit configured to conduct therethrough a varying core cell characterization signal to the gate portion of a core cell;
• a flash memory array of core cells operably coupled to core address control circuitry, bit/column control circuitry and word/row control circuitry, wherein the flash memory array is operable to generate a core cell characterization current for a selected core cell having the core cell characterization signal applied thereto;
• and
• a program verify control circuit configured to generate a reference cell reference voltage, which is coupled to the gate of a reference cell operable to generate a reference cell current using the reference cell reference voltage, and compare the core cell characterization current to the reference cell current and provide a comparison indication associated therewith, wherein a change in the comparison indication provides information related to the threshold voltage of the selected core cell.