NEGATIVE RESISTANCE IN SEMICONDUCTOR DIODES 813 



this \\as due to the bipolar nature of the mode of operation considered, 

 the majority cari-icrs in the region N acting to yhi(>l(l th(^ minority car- 

 riers from their own space charge. 



In this section we shall deal with unipohir diodes in which only one 

 type of carrier is present in sufficient number to have a major effect. In 

 these the influence of the space charge of the carriers upon their motion 

 plays an important role. 



Fig. 4.1 illustrates one example of the type of structure covered by 

 the theory of this section. It is again a p-n-p structure like that considered 

 in Section 3. However, in this case the dimensions, the donor density 

 and the applied potential are such that the space charge "punches 

 through" the device. ' Under these circumstances a condition of space 

 charge limited emission is set up so that holes are injected from the 

 positive region Pi to just such an extent that their flow is limited by 

 their own space charge. This limitation is associated with the maximum 

 of potential just inside A^". 



The potential maximum is evidently a "hook" for electrons generated 

 thermally in Po and in A^. Under some circumstances electrons may 

 accumulate and form a layer in which there is no electron flow and hole 

 flow is carried equally by diffusion and drift. Such stagnant regions will 

 tend to be suppressed if Pi is made of short lifetime material, so that 

 electrons are siphoned out of A^, or if p at the maximum is larger than p 

 for intrinsic material and the lifetime is locally low. 



We shall treat the transient response of this structure of Fig. 4.1 from 

 the point of view of the impulsive impedance discussed in Section 2. 

 Accordingly we suppose that a steady current J flows per unit area. At 

 ^ = an added pulse of current occurs carrying a total charge of 8Qi per 

 unit area, the subscript "i" signifying initial condition. Our problem is 

 to determine how this added charge is carried by a transient disturbance 

 in the hole flow and what is the resultant dependence of voltage upon 

 time ; by definition the added voltage across the device is 



v(t) = dQMt)- (4.1) 



Since we are dealing with a planar model, we shall suppose that the 

 initial condition at f = corresponds to added charges 8Qi and — SQ,- on 

 the metal plates on the P-regions. These charges set up an added field 



8Ei = 8Qi/K, (4.2) 



where 



K = Keo (4.3) 



