770 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 

 2. PROPERTIES OF FORMED POINT CONTACTS 



2.1 Effects of Electrical Forming on Point Contacts 



The simplest form of point-contact transistor collector is a metal to 

 semiconductor contact which has not been subjected to excessive power 

 dissipation either in short high energy pulses, or in the form of more 

 prolonged aging at lower power levels. Such contacts will be referred to 

 as vuiformed contacts, and their properties will be discussed in detail in 

 Section 3. Unformed point-contact transistors sometimes exhibit power 

 gain, but in general they are not suitable for use as active devices be- 

 cause the gain, although it may be highly variable from unit to unit, is 

 usually low. The electrical characteristics of such contacts depend on a 

 metal-semiconductor contact at the semiconductor surface, and control 

 of these properties requires exacting control of surface preparation, sur- 

 rounding ambient, and mechanical stability of the point. 



In early experiments, Brattain used electrical forming to improve 

 both the power gain and stability of the transistor. For present purposes, 

 the process of electrical forming will be defined as the passage of a short 

 pulse of reverse current through a point contact which produces perma- 

 nent changes in the electrical properties of the contact. This is usually 

 accomplished by charging a condenser to several hundred volts, and] 

 subsequently discharging it through a resistor in series with the transis- 

 tor collector. Bardeen and Pfann," investigating electrical forming of 

 phosphor bronze points on etched germanium surfaces, indicate, as a 

 possible explanation of their data, that the forming pulse changes the 

 height of the potential barrier at the germanium surface. This would, in 

 absence of large surface conductivity, increase the reverse current 

 through the point and increase the efficiency of hole collection by the 

 point.^ Thus, the formed point may, according to theory, act as a col- 

 lector with a current multiplication (a) greater than unity. Thermal 

 and potential probing of an ?i-germanium surface under a formed phos- 

 phor bronze point indicates, according to Valdes, that an appreciable 

 volume of germanium is converted to p-type conduction. Thus, the 

 reverse current through a formed point probably depends on the char- 

 acteristics of a p-n junction a small distance from the point, rather than 

 on a potential barrier at the germanium surface. 



A characteristic of the point-contact transistor is that the current 

 gain can be substantially greater than unity. The current gain, a, i^^ 

 usually defined as the current multiplication at constant voltage, that 

 is: 



dl 



a = 



die 



i 



(1)1 



