10 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



by approximately 2 per cent. This amounts to only 150 parts per million 

 change in alpha per degree centigrade change in ambient temperature. 

 The decrease in alpha at low emitter currents shown in Fig. 7 has been 

 observed in every double diffused silicon transistor which has been made 

 to date. Although this effect is not completely understood at present it 

 could be caused by recombination centers in the base layer that can 

 be saturated at high injection levels. Such saturation would result in an 

 increase in effective lifetime and a corresponding increase in alpha. The 

 large increase in alpha with temperature at low emitter currents is con- 

 sistent with this proposal. It has also been observed that shining a strong 

 light on the transistor will produce an appreciable increase in alpha at 

 low emitter currents but has little effect at high emitter currents. A 

 strong light would also be expected to saturate recombination centers 

 which are active at low emitter currents and this behavior is also con- 

 sistent with the above proposal. 



3.0 DISCUSSION OF THE TRANSISTOR STRUCTURE 



Although the low frequency electrical characteristics of the double 

 diffused silicon transistor which are presented in Section 2 are quite 

 similar to those usually obtained in junction transistors, the structure 

 of the double diffused transistor is sufficiently different from that of the 

 grown junction or alloy transistor that a discussion of some design 

 principles is warranted. This section is devoted to a general discussion 

 of the factors which determine the electrical characteristics of the tran- 

 sistors. In Section 4 the general ideas of Section 3 are applied in a more 

 specialized fashion to the double diffused structure and a detailed cal- 

 culation of electrical parameters is presented. 



One essential difference between the double diffused transistor and 

 grown junction or alloy transistors arises from the manner in which the 

 impurities are distributed in the three active regions. In the ideal case 

 of a double-doped grown junction transistor or an alloy transistor the 

 concentration of impurities in a given region is essentially uniform and 

 the transition from one conductivity type to another at the emitter and 

 collector junctions is abrupt giving rise to step junctions. On the other 

 hand in the double diffused structure the distribution of impurities is 

 more closely described by the error function complement and the emitter 

 and collector junctions are graded. Tlu\se differences can have an appre- 

 ciable influence on the electrical beha\'ior of the transistors. 



Fig. 8(a) shows the probable distribution of donor impurities, No , 

 and acceptor impurities, A''^ , in a double diffused n-p-n. Fig. 8(b) is a 



