NAND flash also uses floating-gate transistors, but they are connected in a way that resembles a NAND gate.  Several transistors are connected in series, and only if all word lines are pulled high is the bit line pulled low.  These groups are then connected via some additional transistors to a bit line array. NAND flash uses tunnel injection for writing and tunnel release for erasing.  NAND flash memory forms the core of the removable USB storage devices known as USB flash drives, as well as most memory card formats and solid-state drives available today.