24 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



a diffused surface layer, several problems become immediately apparent : 



(1) Control of body resistivity and lifetime during the diffusion heat- 

 ing cycle. 



(2) Control of the surface concentration of the diffusant. 



(3) INIaking an emitter on the surface of a thin diffused layer and 

 controlling the depth of penetration. 



(4) Making an ohmic base contact to the diffused surface layer. 

 One approach to the solution of these problems in germanium which has 

 enabled us to make transistors with alpha-cutoff frequencies in excess 

 of 500 mc/sec is described in the main body of the paper. 



An important characteristic feature of the diffusion technique is that 

 it produces an impurity gradient in the base region of the transistor. 

 This impurity gradiant produces a "built-in" electric field in such a 

 direction as to aid the transport of minority carriers from emitter to 

 collector. Such a drift field may considerably enhance the frequency 

 response of a transistor for given physical dimensions. 



The capabilities of these new techniques are only partially realized 

 by their application to the making of high frequency transistors, and 

 even in this field their potential has not been completely explored. For 

 example, with these techniques applied to making a p-n-i-p structure 

 the possibility of constructing transistor amplifiers with usable gain at 

 frequencies in excess of 1,000 mc/sec now seems feasible. 



DESCRIPTION OF TRANSISTOR FABRICATION AND PHYSICAL CHARACTERIS- 

 TICS 



As starting material for a p-n-p structure, p-type germanium of 0.8 

 ohm-cm resistivity was used. From the single crystal ingot rectangular 

 bars were cut and then lapped and polished to the approximate dimen- 

 sions: 200 X 60 X 15 mils. After a slight etch, the bars were washed in 

 deionized water and placed in a vacuum oven for the diffusion of an 

 n-type impurity into the surface. The vacuum oven consisted of a small 

 molybdenum capsule heated by radiation from a tungsten coil and sur- 

 rounded by suitable radiation shields made also of molybdenum. The 

 capsule could be baked out at about 1,900°C in order that impurities 

 detrimental to the electrical characteristics of the germaniinn be evapo- 

 rated to sufficiently low levels. 



As a source of n-type impurity to be placed with the p-type bars in 

 the molybdenum oven, arsenic doped germanium was used. The rela- 

 tively high vapor pressure of the arsenic was reduced to a desirable range 

 (about lO"* nun of Ilg) by diluting it in germanium. The use of ger- 

 manium eliminated any additional problems of contamination by the 



