342 BKLL SYSTEM Jj-XJJMCAL JOl R.WIL 



between lattice atoms. In samples of germanium of such purity that the 

 amount of added donor or accei)tor was too small to determine by conven- 

 tional chemical methods, the conductivity was still controlled by the proc- 

 esses outlined above. And it is interesting to note that a portion of this 

 investigation was carried out with the aid of radioactive antimony alloyed 

 with the germanium, the radioactive property making possible an accurate 

 count of antimony atoms, though present only in extremely attenuated 

 amounts. 



The semiconductor papers in this issue of the Journal will explain how 

 these simple facts of electron exchange give rise to rectifying and amplify- 

 ing properties. 



Semiconductor Rectifiers and Amplifiers 



A contact between a metal and semiconductor may act as a rectitier, the 

 contact resistance being high for one direction of current flow and low for 

 the opposite. Rectification results from the presence in the semiconductor 

 adjacent to the interface of a potential barrier or hill which the current 

 carriers, electrons or holes, must surmount in order to flow across the junc- 

 tion. The direction of easy flow is that in which the carriers flow from the 

 semiconductor to the metal. An applied voltage which produces a current 

 flow in this direction reduces the height of the potential hill and allows the 

 carriers to flow more easily to the metal. When the voltage is applied in the 

 opposite direction the height of the barrier which the carriers must surmount 

 in going from the metal into the semiconductor is unchanged, to a first ap- 

 proximation, and the resistance of the contact remains high. A p-type semi- 

 conductor is positive, an »-type negative, relative to the metal, in the direc- 

 tion of easy flow. 



Rectifying contacts can also be made between two semiconductors of op- 

 posite conductivity types. The direction of easy flow is again that for which 

 the /'-type is positive, the «-type negative. The rectifying boundary may 

 separate two regions with different conductivity characteristics within the 

 same crystal. 



In some contact rectifiers it is necessary to consider the flow of both types 

 of carriers, electrons and holes, even though one type is overwhelmingly in 

 excess under equilibrium conditions. .\n example is the germanium point 

 contact rectifier such as the 400 tyi)e varistor. The germanium used is n- 

 type and the normal concentration of holes is small compared to the con- 

 centration of conduction electrons. Nevertheless, a large part of the current 

 in the forward direction consists of holes flowing away from the contact 

 rather than electrons flowing in. The flow increases the concentration of 

 holes in the vicinity of the contact and there is a corresponding increase in 

 the concentration of electrons to compensate for the space charge of the 

 holes. This increase in concentration of carriers increases the conductivity 



