To be short, there are lots of times when someone needs an oscillator of very high frequencies. Usual approaches don't really fit because frequencies are frequently to high. For example, transistors stop working because of all the parasitic capacitances that you don't care at low frequencies. That's where all these diodes become useful : Gunn, IMPATT, TRAPATT and BARRIT.
I don't know if you care about their working principles at a very low level so I will explain just the basics. IMPATT and TRAPATT are usually made of silicon and their voltamperic characteristic usually look like a usual diode. What is different then? Well, they're usually connected in reverse so that effects at the reverse bias could be exploited such as avalanche breakdown. A very strong electric field and diode's structure makes it generate high frequency sinusoidal waves. Generating is possible because of negative resistance. They both have fairly high efficiency but are noisy.
To add further, a good analogue would be using Gunn diodes. They usually have worse efficiency which could be made better using a resonator (around 20 percent) and are not noisy. Gunn diode is quite unique as it doesn't have a pn junction. It is usually a n+nn+ diode. In materials like GaAs electron velocity increases as electric field strength increases only to some point and later decreases. This brings diode into negative resistance region where generating of very high frequencies can happen.
You could check the main technical differences here.