DIFFUSED EMITTER AND BASE SILICON TRANSISTORS 21 



space charge associated with this gradient will extend approximately 

 2 X 10 ■' cm into the base region. The base thickness over which re- 

 tarding fields extend is AR times the base width or 7.6 X 10~^ cm. Thus 

 the first quarter of region R will be space charge and can be neglected. 

 The frequency cutoff from pure diffusion transit is given by 



2A3D ,. , 



where W is the measured base layer thickness. Assuming D — 25 cmVsec 

 for electrons in the base region, ,/'„ = (w mc/sec. Since the measured 

 cutoff was 120 mc/sec, the predicted aiding effect of the built-in field 

 is evidently present. 



These computations illustrate how the measured electrical parameters 

 can be used to check the values of the surface concentrations and dif- 

 fusion coefficients. Conversely knowledge of the concentrations and 

 diffusion coefficients aid in the design of devices which will have pre- 

 scribed electrical parameters. The agreement in the case of the transistor 

 described above is not perfect and indicates errors in the proposed values 

 of the concentrations and diffusion coefficients. However, it is sufficiently 

 close to be encouraging and indicate the value of the calculations. 



The discussion of design has been limited to a very few of the important 

 parameters. Junction capacitances, emitter and collector resistances are 

 among the other important characteristics which have been omitted 

 here. Presumably all of these quantities can be calculated if the detailed 

 structure of the device is known and the structure should be susceptible 

 to the type of analysis used above. Another fact, which has been ignored, 

 is that these transistors were operated at high injection levels and a low 

 level analysis of electrical parameters was used. All of these other factors 

 must be considered for a detailed understanding of the device. The object 

 of this last section has been to indicate one path which the more detailed 

 analysis might take. 



5.0 CONCLUSIONS 



By means of multiple diffusion, it has been possible to produce silicon 

 transistors with alpha-cutoff above 100 mc/sec. Refinements of the 

 described technicjues offer the possibility of even higher frequency per- 

 formance. These transistors show the other advantages expected from 

 silicon such as low saturation currents and satisfactory operation at 

 high temperatures. 



The structure of the double diffused transistor is susceptible to design 



