CONDUCTIVITY-MODULATED SILICON RECTIFIER 



993 



to permit the high resistivity region to be at least as wide as 0.025 cen- 

 timeters. Thus even in high voltage rectifiers it is still possible to design 

 the forward and reverse current voltage characteristics independently. 



V DEVICE PROCESSING 



5.1 Silicon Material 



Fig. 5 shows that step junctions which break down at over a thou- 

 sand volts must have a background impurity concentration ^ 10^^ 

 atoms/cm^ The highest grade commercial semiconductor silicon has 

 5 X 10^* impurities/cm^ (20-50 Q cm P type). This material must be 

 processed to reduce the impurity level. To date, high voltage devices 

 have been processed from four types of high resistivity material: float- 

 ing zone refined, compensated, gold diffused, and horizontal zone refined 

 silicon. 



Some silicon was prepared by adding N-type impurities to reduce 

 I Nd — N A I < lO'*. Maintaining this delicate balance in material where 

 Nd ^^ Na is difficult. The boron is relatively uniformly distributed since 

 the distribution constant is close to unity. N-type impurities are less 

 uniformly distributed in the crystal since the distribution constants are 

 considerably less than unity. High resistivity compensated silicon is full 

 of N- and P-region striations. The units processed from this material 

 generally had poor electrical characteristics. 



Table II is a typical contour of a compensated crystal. The resistivity 

 varies around the crystal and changes along the length of the crystal. 

 At the bottom of the crystal the resistivity goes through a maximum. 

 The tail end is converted from P to N type. 



A number of devices have been fabricated from silicon processed with 



Table II — ^ A Typical Contour of a High 



Resistivity Compensated Crystal 

 Crystal A-161, Oriented 111, Rotated ^ RPM 



