NEGATIVE RESISTANCE 1\ SEMK '<)\Dl( TOH DIODES 



823 



into account, it is found that a device in wliicli most of the curreiit flow- 

 occurs in a ne<j;ative m* region does not necessarily sliow a dc negative 

 resistance characteristic. On the other hand, such a structure may liave 

 a veiy faxorable D(l) characteristic. 



We shaU illustrate these conclusions by considering a (p^)p{p^) 

 structure having heavily doped ends, so that ohmic non-injecting con- 

 tacts may be made to the ends. Fig. 5.2 shows the potential distribution 

 and hole distribution for two cases of applied potential. The first case, 

 represented in (a) and (b), corresponds to moderate fields such that the 

 peak velocity Um. is not reached. 



In the second case the \'oltage is so high that the average value of 

 E = V/L e.xceeds the critical field E^ ■ Under these conditions u is 

 approximately equal to ;/„, over a large part of the F-region and a sub- 

 stantial portion of the voltage drop occurs near one end. An increase of 

 the applied \-oltage occurs chiefly at this end with a small increase in 

 current. Thus no negative dc resistance occurs. 



The abo^•e conclusions are reached by considering the differential 

 equation for the space charge again as in Section 4 neglecting diffusion 

 and starting with E = at the left edge of the P-region. This leads to 



dx = KdE/[(J/u) - Pa], 

 where we ha^'e introduced 



(5.3) 



Pa = -pf = -qXa (5.4) 



for the charge densit}' of the acceptors. Evidently if J exceeds ./,„ where 



(5.5) 



(b) 



(d) 



Fig. 5.2 — Hole density and potontial distribution including influence of 

 'staueffekt." 



