A HIGH-FREQUENCY DIFFUSED BASE GERMANIUM TRANSISTOR 25 



dilutant, and provided a convenient means of determining the degree of 

 dilution by a measurement of the conductivity. The arsenic concentra- 

 tions used in the source crystal were typically of the order of 10 '-10^^/cc. 

 These concentrations were rather high compared to the concentrations 

 desired in the diffused surface layers since compensation had to be made 

 for losses of arsenic due to the imperfect fit of the cover on the capsule 

 and due to some chemical reaction and adsorption which occurred on the 

 internal surfaces of the capsule. 



The layers obtained after diffusion were then evaluated for sheet con- 

 ductivity and thickness. To measure the sheet conductivity a four-point 

 probe method^ was used. An island of the surface layer was formed by 

 masking and etching to reveal the junction between the surface layer 

 and the p-type body. The island was then biased in the reverse direction 

 with respect to the body thus effectively isolating it electrically during 

 the measurement of its sheet conductivity. The thickness of the surface 

 layer was obtained by first lapping at a small angle to the original surface 

 (3^-2°~l°) and locating the junction on the beveled surface with a thermal 

 probe; then multiplying the tangent of the angle between the two sur- 

 faces by the distance from the edge of the bevel to the junction gives the 

 desired thickness. Another particularly convenient method of measuring 

 the thickness' is to place a half silvered mirror parallel to the original sur- 

 face and count fringes, of the sodium D-Yme for example, from the edge 

 of the bevel to the junction. Typically the transistors described here 

 were prepared from diffused layers with a sheet conductivity of about 

 200 ohms/square, and a layer thickness of (1.5 ± 0.3) X 10~ cm. 



When the surface layer had been evaluated, the emitter and base con- 

 tacts were made using techniques of vacuum evaporation and alloying. 



o 



For the emitter, a film of aluminum approximately 1,000 A thick was 

 evaporated onto the surface through a mask which defined an emitter 

 area of 1 X 2 mils. The bar with the evaporated aluminum was then 

 placed on a strip heater in a hydrogen atmosphere and momentarily 

 brought up to a temperature sufficient to alloy the alimiinum. The 

 emitter having been thus formed, the bar was again placed in the masking 

 jig and a film of gold-antimony alloy from 3,000 to 4,000 A thick was 

 evaporated onto the surface. This film was identical in area to the 

 emitter, and was placed parallel to and 0.5 to 1 mil away from the 

 emitter. The bar was again placed on the heater strip and heated to the 

 gold-germanium eutectic temperature, thus forming the ohmic base 

 contact. The masking jig was constructed to permit the simultaneous 

 evaporation of eight pairs of contacts on each bar. Thus, using a 3-mil 

 diamond saw, a bar could be cut into eight units. 



