6 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



The transistors whose characteristics are reported in this paper were 

 prepared from 3 ohm-cm n-type siHcon using antimony and ahmiinum 

 as the diffusants. The base contact was produced by evaporating alumi- 

 num through a mask so that a hne approximately 0.005 X 0.015 cm in 



o 



lateral dimensions and 100,000 A thick was formed on the surface. This 

 aluminum line was alloyed through the emitter layer in a subsequent 

 operation. The wafer was then alloyed onto the plated kovar tab. A 

 small area approximately 0.015 cm in diameter was masked around the 

 line and the wafer was etched to remove the unwanted layers. The unit 

 was then mounted in a header. Electrical contact to the collector was 

 made by soldering to the kovar tab. Contact to the base was made with 

 a tungsten point pressure contact to the alloyed aluminum. Contact 

 to the emitter was made by bringing a gold-antimony plated tungsten 

 point into pressure contact with the emitter layer. The gold-antimony 

 plate was then alloyed by passing a controlled electrical pulse between 

 the plated point and the transistor collector lead. Fig. 3 is a photograph 

 of a mounted unit. 



2.0 ELECTRICAL CHARACTERISTICS 



The frequency cutoffs of experimental double diffused silicon tran- 

 sistors fabricated as described above are an order of magnitude higher 

 than the known cutoff frequencies of earlier silicon transistors. This is 

 shown in Fig. 4 which gives the measured common base and common 

 emitter current gains for one of these units as a function of frequency. 

 The common base short-circuit current gain is seen to have a cutoff fre- 

 quency of about 120 mc/sec. The common emitter short-circuit current 

 gain is shown on the same figure. The low-freciuency current gain is 

 better than thirty decibels and the cutoff frequency which is indicated 

 by the freciuency at which the gain is 3 db below its low-frequency 

 value is 3 mc/sec. This is an exceptionally large common emitter band- 

 width for a thirty db common emitter current gain and is of the same 

 order of magnitude as that obtained with the highest frequency ger- 

 manium transistors (e.q., p-n-i-p or tetrode) which had been made 

 prior to the diffused base germanium transistor. 



^ Tlio iiicroasp in (•oiiiinon haso current gain ahovc unity (indicated by current 

 gain in decibels being positive) in the vicinity of 50 mc/sec is caused by a reactance 

 gain error in the common base measurement. This error is caused by a combination 

 of the emitter to ground parasitic capacitance and the i)ositive reactance com- 

 ponent of the transistor input impedance resulting from phase shift in the ali)ha 

 current gain. 



' C. A. Lee, A High-Frequency Diffused Base Germanium Transistor, see 

 page 23. 



