IMPATT diode operation.

? what is the "Junction formation"?

An impatt diode is reverse biased enough so that it breaks down...like 20 volts. An electron pops out, gets accelerated by the big electric field inside the diode, smashes into a molecule, which releases multiple electrons, which get accelerated by the big electric field, which smash into molecules, generating many more electrons which....hence the impact avalanche effect

And then on the outside, it looks like a negative resistance is formed at the terminals. The total length of the interaction region, the exact doping, and the diode capacitance, make it only useable over a small frequency range.

Thank you,
junction refers to depletion region as in normal diode.