20 



THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



Each unit, with an alloyed emitter and base contact, was then soldered 

 to a platinum tab with indium, a sufficient quantity of indium being- 

 used to alloy through the n-type surface layer on the back of the unit. 

 One of the last steps was to mask the emitter and base contacts with a 

 6- to 8-mil diameter dot of wax and form a small area collector junction 

 by etching the unit attached to the platinum tab, in CP4. After washing 

 in solvents to remove the wax, the unit was mounted in a header designed 

 to allow electrolytically pointed wire contacts to be made to the base and 

 emitter areas of the transistor. These spring contacts were made of 1-mil 

 phosphor bronze wire. 



ELECTRICAL CHARACTERIZATION 



Of the parameters that characterize the performance of a transistor, 

 one of the most important is the short circuit current gain (alpha) ver- 

 sus frequency. The measured variation of a and q:/(1 — a) (short-circuit 

 current gain in the grounded emitter circuit) as a function of frequency 

 for a typical unit is shown in Fig. 1 . For comparison the same parameters 

 for an exceptionally good unit are shown in Fig. 2. 



In order that the alpha-cutoff frequency be a measure of the transit 

 time of minority carriers through the active regions of the transistor, any 

 resistance-capacity cutoffs, of the emitter and collector circuits, must lie 

 considerably higher than the measured /„ . In the emitter circuit, an 

 external contact resistance to the aluminum emitter of the order of 10 



U1 



_J 



LU 



eg 



o 



lij 



Q 



0.1 0.2 0.4 0,6 1 2 4 6 8 10 20 40 60 100 200 400 1000 



FREQUENCY IN MEGACYCLES PER SECOND 



Fig. 1 — The grounded emitter and grounded base response versus frequency 

 for a typical unit. 



