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THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



to as the type (1) anomaly, is of interest here since it represents a col- 

 lector contact whose physical properties are between the extremes listed 

 in Section 2.2. Miller has shown that the source of this kind of outpvit 

 characteristic can be identified as the formed area under the collector 

 point. 



Essentially this anomaly consists of an abrupt rise in the current gain 

 as the collector voltage Vc is increased at constant emitter current. Be- 

 yond the critical value of Vc , the characteristic of the unit resembles 

 that of a well formed transistor. One is led to consider that such a con- 

 tact is under-formed, in the sense that at low Vc , collection of holes is 

 inadequate. Further support is lent to such a definition by the data of 

 Miller, which shows a definite increase in the occurrence of anomalous 

 units with a decrease in the Ico of the contact. Such an increase occurs 

 regardless of whether the Ico decrease is obtained by decreasing the donor 

 concentration of the point wire, or by increasing the time constant of 

 the forming pulse. In Table II are compared collector capacity and Ico 

 measurements made in units with and without output characteristic 

 anomalies. The capacity of these anomalous collectors also appears to 

 range between the two extremes listed in Table I. Thus there is evidence 

 that these collectors are intermediate between the extremes cited in 

 Table I in the sense that at low reverse biases the drift field is low, and 

 the properties of the formed barrier resemble those of a formed donor- 

 free point. 



The results of detailed investigation of the properties of such anoma- 

 lous characteristics now being conducted will be published at a later 

 date. The present experimental results indicate that the instability oc- 

 curs when the extra current to the collector. Ale , reaches a critical value. 

 In this respect, increasing the transport factor (3, by increasing Vc , or 

 increasing the emitter current are equivalent. At a roughly critical Ale , 

 the transition between a low a and a higher value of a occurs. After the 

 transition, the unit behaves like a conventional point contact transistor, 

 with a current multiplication on the order of (1 + &) at higher values of 

 le . Thus the origin of this kind of anomaly may lie in the lowering of 

 the formed barrier by the space charge of the holes, a mechanism sug- 

 gested by Bardeen. 



Table II 



Idle = 0, Fc = -10 volts) 



Typical Transistor 



Typical Anomalous Transistor . 



] .0 Ilia 

 0.2 ma 



Cede = 0, ^0 = -10 volts) 



(I. 1 nix( 



0.5 fi/jif 



