POINT-CONTACT TEANSISTOR SURFACE EFFECTS 809 



5.1 Point-Contact Transistors with High Current Gain 



In most switching applications the combination of high current gain 

 and low reverse current is desirable. The measurements of current gain, 

 taken together with the potential probe measurements in Section 2.2.1, 

 indicate that, for the structures used here, the reverse collector current 

 at operating voltage must be large enough to set up a substantial drift 

 field before efficient collection of holes can occur. If this condition is not 

 met, either the unit has low gain at all values of emitter current (un- 

 formed), or develops a bistability of the kind described in Section 2.3 

 (partially formed). For a given structure, the drift field can be increased 

 by increasing resistivity of the germanium at the expense of increased 

 base resistance. Here thermal stability of the contact also provides a 

 limit. A more likely expedient, in the case of germanium, is to decrease 

 the area of the formed collector junction by using sharper points and 

 modified forming technique. The limits here are produced by reliability 

 requirements for mechanical stability of the point structure. 



5.2 Current Multiplication in Unformed Transistors 



Many experiments have reported on junction transistors with high 

 current gains which are attributable to the p-n hook mechanism. The 

 high values of current gain observed with conventionally formed point 

 contact transistors have been attributed to various mechanisms, among 

 , which is the hypothesis of a p-n hook structure, primarily in the bulk of 

 the germanium, introduced by the pulsing of the donor-doped point. In 

 particular, at small emitter currents small signal a-values in conven- 

 tionally formed collectors may reach values as high as ten, and values of 

 a as large as 100 are encountered in formed collectors exhibiting anoma- 

 lous output characteristics. However, the average a over a 6-ma emitter 

 current range is usually near the value of 3.1 which would be expected 

 from the mobility ratio of holes and electrons with the Type-A transis- 

 tor geometry. The increase in reverse current of a formed collector by 

 t addition of donor to the point wire may result from the production of a 

 hook structure. However, information is needed concerning the impor- 

 tance of the hook structure in accounting for the high values of a en- 

 countered at low emitter currents, or in connection with collector char- 

 acteristic anomalies in conventionally formed point-contact transistors. 



The unformed transistors discussed in this article differ from electri- 

 cally formed units in that the collector barrier is the one at the metal- 

 semiconductor surface. It has been found that certain chemical treat- 

 ments can produce a collector barrier which allows an increased reverse 



