5.1.4 Poly Silicon Structures

  A very promising technique is the patterning of a poly-silicon layer which is contacted with source, drain, and gate electrode. Fig. 5.4 shows the structure produced by K. Yano et al. [108] [109].

  

Figure 5.4: Poly-silicon batch contacted with a source, drain, and gate electrode. In the upper part the electrodes and oxide are left out. This structure behaves like a flash memory, where one electron is stored/trapped on a grain that lies close enough to a conducting path which connects source with drain, to modulate the current flow.

They successfully built with it a 64-bit memory cell. Grain boundaries and varying grain size produce a `Grand Canyon' like potential landscape. If a high enough bias voltage is applied to source and drain a narrow current path will form. An adjacent grain may act like a floating gate and modulate the current. By applying a high gate voltage, one electron is trapped in or ejected from this storage dot. It is a device which works close at the point, where one electron stores one bit of information. A detailed description of this and other memory cells is given in Section 5.2.

The advantage of semiconductor structures is that many production process are well understood, controllable and for many years in permanent use. Additionally, semiconductor quantum dots show a discrete energy spectrum which enhances the Coulomb blockade, and which is favorable to reach a room temperature operation (see Section 2.1.2). However, this same advantage can turn into an disadvantage, whenever the absolute size of the Coulomb blockade is important. Small changes in grain size change unpredictably the Coulomb blockade in semiconductor structures. Metallic structures are much more uniform in this respect.