664 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 | 



Rs is extremely small (<0.05 ohms). Although the measured reverse 

 currents are greater than those predicted by theory for temperatures 

 up to 100°C, the reverse losses are low and do not affect the efficiency 

 appreciably. ,1 



1.4 The diodes made by the diffusion of sihcon are very attractive' 

 from an engineering standpoint for several reasons. First of all, their ; 

 behavior is predictable from the theory of semiconductor devices, as 

 are junction transistors. This makes it possible to design rectifiers of 

 given electrical, thermal, and mechanical characteristics. Secondly, : 

 rectifier elements of many sizes are available from the same diffused < 

 wafers making it possible to use the same diffusion process, material, 

 and equipment for a range of devices. Thirdly, large enough elements 

 can be processed so that the power dissipation in the unit is limited 

 only by the thermal impedance of mount and package. 



2.0 DIFFUSION PROCESS 



t 



2.1 It will be shown in 3.2 that the forward characteristic of these 

 devices is practically independent of the type (n or p) and resistivity 

 of the starting material. The reverse breakdown voltage of a silicon p-n 

 junction depends primarily on the resistivity of the lightly doped region. 

 With these two considerations in mind; that is, to fabricate rectifiers 

 having the desirable excellent forward characteristic and at the same 

 time high reverse breakdown voltage, high resistivity siUcon is used as ; 

 the starting material for the diffused barrier silicon rectifiers. Single 

 crystal material has been found to give a better reverse characteristic 

 than multicrystalline material. Also, it has been found that p-type ma- 

 terial has yielded units with a better reverse characteristic than n-type 

 material. Therefore, in the remainder of this paper, we will limit dis-';' 

 cussion to rectifiers made from high resistivity, single crystalline, p-type /"i 

 silicon. We will designate this material as ir type silicon. 



2.2 In addition to the fine control one has in the diffusion process 

 (see 2.4), the process lends itself admirably to the semiconductor recti-' 

 fier field in as much as the distribution of impurities in this process re- ; 

 suits in a gradual transition from a degenerate semiconductor at the' 

 surface of the material to a non-degenerate semiconductor a short dis- 

 tance below the surface. This condition permits low resistance ohmic 

 metallic; contacts to be made to the surfaces of the diffused silicon. 



In order to create a p-n junction in the x silicon, it is necessary to 

 diffuse donor imjiurities into one side of the slice. Although several donor 

 type imi)urities have been diffused into siUcon, all the devices discussed 



I 



