NEGATIVP: resistance in SEMirONDUCTOIt DIODES 



821 



then relax with a relaxation constant of about iJ.p(rciax)/K where p(max) 

 is the hole charge density. Actually the relaxation may be somewhat 

 (juicker because the concentration gradient of the added lioles also 

 contributes to the flow over the maximum. Since the charge density at 

 k'T/q below the maximum is comparable to that at the maximum the 

 entire relaxation process will proceed at about this rate. Thus a criterion 

 for the applicability of the theory is that /C/^ip(max) be much less than 

 S, the transit time or total decaj' time for D(t). 



5. MOBILITY AND GEOMETRY EFFECTS 



5.1. The Effect of a Region of Negative n* 



In very high electric fields holes may be expected on the basis of 

 theory to exhibit a negative value of /x*. This theory^ ^ is founded on the 

 idea that a hole can lose energy to phonons at a certain maximum aver- 



100 200 300 400 500 600 700 800 

 e IN DEGREES 



Fig. 4.5 — Impedance of a p-n-p structure, (h) Imaginary part of impedance. 



