Propagating avalanche-zone mode for high-efficiency GaAs lo--hi--lo IMPATT structures

The superiority of high-efficiency hi--lo and lo--hi--lo impatt structures over conventional structures and the distinctive tuning characteristics of these structures have been largely explained in terms of a variety of drift-region mechanisms common to both structures. There are, however, sometimes distinct differences between the large-signal behaviour of the two structures, and these have not been explained so far. In the letter, a reason for the differences is suggested: it is shown that large-signal impatt oscillations may trigger a `propagating avalanche-zone mode´ in lo--hi--lo structures, but that this will not happen in hi--lo structures. The existence of such a mode is thought to be capable of explaining observed differences between the large-signal properties of the two structures.