800 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



It is instructive, however, to speculate on some of these ultimate po- 

 tentialities. For example a grain boundary formed of edge type disloca- 

 tions is in a sense an analogue of a grid. Possibly it can be made into a 

 grid by acting as a locus for an atmosphere of donors or acceptors. 

 Evidently such a grid ^^'ill approach the smallest spacing that can be 

 achieved with any kno^vn form of matter. If the spacings perpendicular 

 to the grid are made comparable to a mean free path of the carriers used, 

 the device ^^ill operate like a vacuum tube with carrier velocities con- 

 trolled by inertia rather than by mobility. It is not easy to conceive of a 

 structure having the potentialitj^ of operating at higher frequencies. 



It is evident that the difficulty of making small structures increases 

 with the number of electrodes. For example, it is now possible to make 

 diodes which give usable rectification at frequencies above 10^° cps. In 

 these the "working volume" is a very thin layer under the metal point. 

 The thickness of this layer is controlled by surface treatments and the 

 applied voltages. The diameter of the point, which is the minimum di- 

 mension mechanically controlled, is much larger than this thickness of 

 the layer. In order to make a transistor of comparable frequency, it 

 would be necessary to make structural elements having dimensions 

 comparable to the thickness of the layer and this would be a much more 

 exacting task than making the diode. 



These considerations point out the importance of giving serious con- 

 sideration to two-terminal structures as amplifying elements. It is pos- 

 sible, in principle at least, to have structures which are much smaller 

 in one dimension than the other two and which exhibit negative re- 

 sistance and thus give ac power at frequencies comparable to the 

 reciprocal of the transit time across the small dimension. 



The attractiveness of such negative resistance diodes for amplification 

 is enhanced by the possibility of using them in dissected amplifiers ' in 

 combination with nonreciprocal elements such as gyrators or Hall effect 

 plates. Combinations of negative resistance elements and nonreciprocal 

 elements can lead to structures having gain and unsymmetrical trans- 

 mission that simulate conventional amphfiers. The adjective dissected 

 has been suggested for them since elements giving power gain are 

 physically separated from those giving one-way transmission. 



In this article we shall not consider the possible forms of dissected 

 amplifiers, of which there are a wide variety. Instead we shall give an 

 introductory treatment of some forms of negative resistance that may 

 arise from transit time effects. In some cases the most instructive way 

 of treating the structure is by way of the "impulsive impedance" and 

 we devote most of the next section to considering this method. 



