5.2.7 Multi Tunnel Junction Memory

       K. Nakazato and H. Ahmed [88] proposed a dynamic memory cell pushed to its extreme limit. A small number of electrons is stored on a single quantum dot. Their presence on the quantum dot QD corresponds to logical '1' and their absence to '0' (see Fig. 5.8).

  

Figure 5.8: Circuit diagram of the multi-tunnel-junction memory cell.

To write in this cell a voltage pulse is applied ( $V_{\text{in}}$). A positive pulse $V_{\text{in}}$ forces electrons to tunnel through junctions J₆, J₅,..., J₁ off of quantum dot QD. A negative voltage pulse $V_{\text{in}}$ forces electrons to tunnel onto QD. The state of the memory dot QD is sensed at $V_{\text{out}}$. The central island of a SET transistor which is extremely charge sensitive, may be used as sensor for the logic states. However exactly this charge sensitivity makes these devices prone to random background charge. The more tunnel junctions are used the less likely it is that electrons are co-tunneling to ground. That is why this memory is a dynamic memory, since the charges can not be trapped infinitely. Nevertheless, the refresh rate can easily extend to the second or minute range. At least two tunnel junctions are needed to achieve a memory effect. We have chosen the multi-tunnel-junction (MTJ) memory to consist of six tunnel junctions, so that flip-flop, MTJ memory and ring memory have the same number of junctions and are better comparable.

Recently S. Amakawa et al. [2] proposed a MTJ memory with improved persistence to co-tunneling (see Fig. 5.9).

  

Figure 5.9: Circuit diagram of the dual-array multi-tunnel-junction memory cell.

Instead of trapping only electrons, they suggested to trap electrons and holes on separate quantum dots which are coupled by a capacitor C_c. This could be called an exciton trap. Increasing C_c increases the coupling between electrons and holes, but also increases the island capacitances and thus reduces the Coulomb blockade and operation temperature. The additional binding of the exciton makes it more resistant to co-tunneling with the same number of tunnel junctions.