NEGATIVE RESISTANCE IN SKMirONDrCTOIt DIODES 811 



The ratio of the admittances is 





9~l 



Air coo-pi 4^17' 



(3.43) 

 47ro-pi qAV a pi 



For our example this expression is much greater than 1 as may be seen 

 as follows : The first fraction is the ratio of the dielectric decay constant 

 to oj. This is 10 or more larger than co need be. The next term is about 10 

 and the last term is the ratio of hole to electron density at Xi and is 

 about 10~ . Hence the ratio of impedances is about 10: 1. 



We shall next consider why the expression for A for holes must be 

 examined more closely. The admittance formula used above applies to 

 the case of zero field to the right of the junction. The aiding field will 

 increase the flow of holes into the n-layer and raise the admittance 

 somewhat. Correcting for this will increase A in respect to wC and \\dll 

 thus strengthen rather than weaken the argument. 



Also in the expression for A, no account was taken of the transit time 

 across the region W. If we assume a uniform field in this region for 

 purposes of making estimates, then the solution of equation (3.20) may 

 be applied. Since now m corresponds to drift velocity due to 9kT/q of 

 voltage drop across W which is much less than L in length, it is evident 

 that 7 will be less by a large factor in this region compared to its value 

 in .V. This leads to the conclusion that phase lags will be unimportant 

 in this region. 



We have neglected the effect of electron injection into Pi . By the 

 customary arguments for unsymmetrically doped junctions, it follows 

 that this current is very small compared to the hole current. 



This justifies consequence (1). 



Consequence (2) may be justified as follows: At Xi all the ac current 

 j exp (lot) is carried by holes. If a pure drift case occurred, the hole cur- 

 rent might be reversed at some point in the n-Iayer and be —j exp (iwt). 

 Under these conditions the electron current would have to be 2j exp 

 (io)t). Under no conditions, however, \xi\\ the electron current be larger 

 than this. This maximum possible electron current will require an electric 

 field and this field \x\\\ also affect the hole flow. Since the electron con- 

 ductivity is at least 10 times larger than the hole conductivity, the hole 

 current due to the ac field will only be about }/{q of 7 at most. Thus the 

 hole current is only slightly affected by the ac field. 



Consequence (3) follows from the fact that the reverse biased junction 



