5.2.10 Multi Island Memory

   The most recent and very promising design and experimental realization of a memory cell was done by K. Yano et al. [109]. This design fulfills three major design rules: it works at room temperature, is random background charge independent, and is manufacturable with todays process technology. The idea is somehow similar to the previous one, in that a flash memory like device is considered. With a gate electrode charges are trapped, which modulate the current through the structure. The   independence of random background charge is achieved in a different way, namely the use of many interconnected similar islands formed by a granular film, for instance, poly-silicon (see Fig. 5.13).

  

Figure 5.13: Circuit diagram of a multi-island memory cell. Arrows starting at islands indicate a connection to the gate capacitor.

A simplified qualitative explanation for Q₀-independence is as follows. Consider a piece of granular film across which we apply a small bias voltage V_b (see Fig. 5.14).

  

Figure 5.14: A granular film exhibits a Coulomb blockade regardless of the background charge Q₀. The threshold voltage V_th changes slightly if the background charge is changed.

In the case of zero background charge (Q₀ = 0) no current will flow, due to the Coulomb blockade (see Fig. 5.14a). If one increases the bias voltage further, then at one point a conducting path will form and current will flow (Fig. 5.14b). If random background charge is present, $Q_0 \neq 0$, single regions will exhibit no Coulomb blockade, due to the background charge, and others will show an increased Coulomb blockade (Fig. 5.14c; parts conduct others do not). But overall no conducting path has formed, thus the Coulomb blockade is still present. Increasing V_b will finally lead to a conducting path. The threshold voltage $V_{\text {th}}$ will be different, but not necessarily smaller.