r-N-I-P AND N-P-I-N JUNCTION TRANSISTOR TRIODES 531 



EXPLORATORY MODELS 



Objectives 



While the p-n-i-p transistor will be useful for high voltiige and high 

 power operation, our exploratory development work has been directed 

 toward good performance at very high frequencies. The initial electrical 

 objectives set were those of p-n-i-p No. 1 of Table I: /„ = 100,mcps, 

 Cc < 1.0 mmf, and r^' = 34 ohms. The base thickness of 0.13 mil and 

 base resistivity of 0.14 ohm-cm are the critical structural parameters. 



Fabiication 



Although p-n-i-p's might conceivably be built in a single operation, 

 one procedure used has two major parts. The first is the production and 

 evaluation of 2-mil thick wafers of intrinsic germanium with a skin or 

 surface layer of 0.1-1.0 ohm-cm n-germanium 0.3-0.5 mils thick. The 

 second step is the alloying of collector, emitter, and base electrodes to 

 these wafers. 



Wafers mth n-type skins have been made by three methods. Intrinsic 

 crystals growing from a melt by the Teal-Little technique have been 

 doped with arsenic, grown for a few seconds longer (another 0.5-1.0 

 mils), and snatched mechanically from the melt. The resulting crystal 

 surface has a mirror finish and is relatively flat. N-type skin layers have 

 also been produced by alloying the wafer surfaces with lead-arsenic 

 and lead-antimony mixtures and by the diffusion of arsenic into wafer 

 surfaces. 



Collector and emitter electrodes are alloyed by the indium germanium 

 process ^vith times, temperatures, and quantities of indium selected to 

 give desired alloying depths. Ring-base connections of antimony and 

 gold plated kovar have been used. 



Measurements 



Progress toward the initial design objectives mentioned previously 

 has been encouraging. The predicted behavior has been verified semi- 

 quantitatively. The capacitance of a 15-mil diameter collector is usually 

 less than 1.0 mmf at Vc = —25 volts as predicted in design No. 1 of 

 Table 1. Ohmic base resistances generally less than 50 and as low as 5 

 ohms have been measured. However, the highest alpha cutoff freciuency 

 obtained as yet is 25 mcps. This has been limited primarily by the thick- 

 ness of the base layer. At present this is of the order of 0.30 mils so that 

 an alpha cutoff frequency of 25 mcps is about what would be predicted. 

 Further development of the technology of fabrication seems reasonably 



