In Chapter 11 (also see Fig.  11.1 ), we saw that the sizes of the electronic devicesNanoelectronics like transistors are shrinking accompanied by the increase of their density per unit area. With increased device density, in general, the cost of the product not only goes down but the device performance also improves. The performance is limited by the increased heat generation which in turn would restrict the size of the device that should be made. Further, as we go on reducing the size the physics of low dimensional materials does not remain the same as for the bulk according to the discussions in the previous chapters. In fact this gives rise to new nanodevices which would be discussed in this chapter just to understand the basic principle behind their peculiar behaviour which is not seen in the microdevices. Although very interesting, we shall not discuss the nanodevices in which carbon nanotubes or grapheneGraphene are being used to obtain more efficient devices. Here we are restricting only to certain phenomena rather than special nanomaterials in the devices.