NEGATIVi: ItESISTANCE IN SEMICONDUCTOR DIODES 801 



2. THE IMPULSIVE IMPEDANCE AND NEGATIVE RESISTANCE 



The impulsive impedance D{t) for a two terminal (knice is defined in 

 terms of its transient response to an impulse of current. Thus if the 

 current through the de\ice is 



J{t) = J + jit), 

 where J is the dc current and 



m = 



except very near t = and 



j j{t)dt = bQ, 



then the voltage is 



F(0 = F + v{t), 

 where V is the dc voltage and 



v{t) = 8Q D{t). 



In other words, if in addition to the dc biasing current, a charge 8Q is 

 instantaneously forced through the circuit at time t = 0, the added 

 voltage is D(t) per unit charge. These equations also serve to introduce 

 the notation used in this article: 



Notation. In general, cjuantities that are functions of time or position 

 will haxe the functional dependence explicitly indicated. In Sections 4 

 and 5, however, the symbol 5 will be used to distinguish the transient 

 parts bE and 6p from the dc parts of the electric field and charge density. 



Capital V{t) and J{t) stand for total voltages and current. Without 

 functional dependence upon {t) they are the dc parts. Similarly v{t) and 

 j{t) are the ac or transient parts. A sinusoidal disturbance is represented 

 by 



v{t) = V exp iwt, 



j(t) = j exp iut. 



where v and j are not functions of time. Where it is necessary to distin- 

 guish the displacement current at a particular location from the con- 

 duction current, as in the next section, we shall write 



J(D, S2,t), 



