1240 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



is considerably less well understood than the simple NP junction. Most 

 of the assumptions that make the NP case relatively simple to deal with 

 do not apply to junctions where one side is intrinsic. Specifically, the 

 space charge is that of the mobile carriers; thus the flow and electrostatic 

 problems cannot be separated as they can in PN junction under reverse 

 bias. The following sections analyze the iV-intrinsic - P structure under 

 reverse bias. 



For a given material with fairly highly doped extrinsic regions, the 

 problem is defined by the length of the intrinsic region and the applied 

 voltage. Taking the intrinsic region infinitely long gives the solution for 

 a simple A^-intrinsic or P-intrinsic structure. The results are given and 

 plotted in terms of the electric field distribution. From this the potential, 

 space charge and carrier concentrations can be found; so also can the 

 current-voltage curve. The final section considers the case where the 

 middle layer contains some fixed charge but where the carrier charge 

 cannot be neglected. 



Qualitative Discussion of an N-intrinsic-P Structure 



Consider an A^-intrinsic-P structure where the intrinsic, or /, region is 

 considerably wider than the zero bias, or built-in, space charge regions 

 at the junctions, so that there is normal intrinsic material between the 

 junctions. The field distribution at zero bias can be found exactly from 

 the zero-current analysis of Prim.' Throughout the intrinsic region, hole 

 and electron pairs are always being thermally generated and recombining 

 at a rate determined by the density and properties of the traps, or recom- 

 bination centers. Under zero bias the rates of generation and recombina- 

 tion are everywhere equal. Suppose now a reverse bias is applied causing 

 holes to flow to the right and electrons to the left. Some of the carriers 

 generated in the intrinsic region will be swept out before recombining. | 

 This depletes the carrier concentration in the intrinsic region and hence 

 raises the resistivity. It also produces a space charge extending into the 

 intrinsic layer. The electrons are displaced to the left and the holes, to ■ 

 the right. Thus the space charge opposes the penetration of the field 

 into the intrinsic region; that is, the negative charge of the electrons on i 

 the left and positive charge of the holes on the right gives a field 

 distribution with a minimum somewhere in the interior of the intrinsic 

 region and maxima at the NI and IP junctions. If holes and electrons 

 had equal mobilities, the field distribution would be symmetrical with a 

 minimum in the center of the intrinsic region. Likewise, the total carrier 



1 R. C. Prim, B. S. T. J., 32, p. 665, May, 1953. 



