High-frequency oscillations of p++-n+-n-n++avalanche diodes below the transit-time cutoff

According to Read, n
⁺⁺
-P
⁺
-i-P
⁺⁺
diodes should oscillate at special high frequencies determined by carrier transit time in the space-charge layer. Oscillations not affected by transit time were observed with p
⁺⁺
-n
⁺
- n-n
⁺⁺
silicon diodes. The corresponding current-voltage characteristic revealed a negative resistance setting in at a critical current. Theoretical considerations show that one-sided avalanche injection in n
⁺⁺
-p
⁺
-p
⁺⁺
structures may lead to a slight negative resistance for carrier concentrations smaller than the impurity concentration and for certain widths of the depletion layer. This type of negative resistance disappears in n
⁺⁺
-p
⁺
-i-P
⁺⁺
structures, but with increasing injection multiplication is induced in the intrinsic layer. Therefore the carrier space-charge is reduced and a negative resistance appears at a critical current density. The onset of this second injection is an upper current limit of the Read transit-time mode. The frequency range of oscillations due to avalanche space-charge feedback generally will not be separated from the range of transit-time oscillations. Thus, it must be judged carefully which mechanism is responsible for observed high-frequency oscillations. On the other hand, space-charge feedback may give additional stability to the transit-time mode.