NEGATIVE RESISTANCE IN SEMICONDUCTOR DIODES 805 



(c) Throughout A' the coucentratiou of holes is less by a factor of 10 

 than the electron concentration. 



(d) The thickness of X is large compared to the (lei)th of space charge 

 penetration into it. 



(e) The ^•oltage di-op across the space charge region *S2 is large com- 

 pared to the other \'oltage drops. 



The conditions lead at the operating frequency to the folhnving conse- 

 ([uences: 



(1) The current across the first junction is carried preponderantly by 

 holes. 



(2) The hole drift in .V is substantially unaffected Ijy the ac field and 

 thus represents the delayed diffusing and drifting current injected across 

 the first junction. 



(3) The ac voltage drop occurs chiefly across So . 



We shall show below (Section 3.2) how (a) to (e) lead to (1) to (3), 

 but we shall first bring out the importance of (1) to (3) by using them to 

 give a simple treatment of the impedance of the diode. 



3.1. Calculation of Impedance. 

 If the total current is 



J{t) = J ^je'"', "(3.1) 



then the ac hole current across *Si is also in the notation discussed in 

 Section 2 with the addition of the sj^mbol p to indicate holes 



j(p,S„t) =.7V"'. (3.2) 



This current flows through the n-layer unaffected by the ac field and 

 arrives at S2 delayed and attenuated by a complex factor 



/3 = \0\exp(-id). (3.3) 



Because of the high field in So , the transit time there is negligible so that 

 the hole current arriving at Po is 



j(p, S.;)e''" = ^je'"'. (3.4) 



In addition to this current, there is a dielectric displacement current in 

 So which is converted to hole conduction cm-rent in Po . If the voltage 

 drop across »S2 is 



V(S2 , t) = V{So) + v{S,)e'''\ (3.5) 



