DIFFUSED p-n JUNCTION SILICON RECTIFIERS 



675 



5.4 The reverse breakdown voltage, Vb , of these devices is controlled 

 by the choice of resistivity of the starting material and the depth of 

 diffusion of the junction. By keeping the resistivity of the initial p-type 

 silicon above 20 ohm-cm., it is possible to keep Vb above 200 volts. 

 Units have been made with Vb greater than 1,000 volts. The deeper 

 diffusion causes the junction to be more "graded"^ and therefore re- 

 quire a greater voltage for the breakdown characteristic. This is in line 

 with the capacitance measurements where the exponent indicates that 

 the junction is neither a purely abrupt junction which would result in 

 an exponent of two nor a constant gradient junction which would result 

 in an exponent of three. 



5.5 Another interesting measurement, which is related to the life- 

 time of minority carriers in the high-resistivity region and the frequency 

 response, is the recovery time of these devices. During a forward bias on 

 a p-n junction, excess minoritj^ carriers are injected into either region. 

 When the applied voltage polarity is reversed, these excess minority 

 carriers flow out of these regions, giving rise initially to a large reverse 

 current until the excess carriers are removed. The magnitude and time 

 variation of this current will depend to some extent upon the level of 

 the forward current but mostly upon the circuit resistance. If one ad- 

 justs the circuit resistance such that the maximum initial current in 



CURRENT IN AMPERES 



Fig. 11 — Rectifier characteristics at 80°C. 



