5.2.9 Qo Independent Memory

        The first random background charge or Q₀-independent memory was proposed by K. Likharev and A. Korotkov [81]. The basic idea is the following (see Fig. 5.12).

  

Figure 5.12: Circuit diagram of random background charge independent memory cell.

Electrons are stored on an island or floating gate QD $_{\text{1}}$. They are moved through a tunnel junction J_g on or off the floating gate. A SET transistor (J₁, J₂, QD $_{\text{2}}$) which is very charge sensitive on its gate, is used to sense the changes of charge on the floating gate. The trick to achieve the Q₀-independence is not to sense any absolute charges, but to sense the current oscillations in the SET transistor, which are caused by changes in the charge on the gate, with a FET amplifier. In other words, the charge change on the floating gate induces current oscillations in J₁ and J₂. These oscillations occur at any background charge. Only the phase, not the amplitude is background charge dependent. The cell can only be read destructively by discharging the floating gate. If the FET sense amplifier picks up current oscillations, the floating gate was charged with electrons. If no oscillations are detected, no charge was on the floating gate.