Tunnel Diodes:

The other type of diode which can oscillate at microwave frequencies is the tunnel diode, also termed as the Esaki diode. It generates a very small quantity of rf power.

The tunnel diodes work well as amplifiers in microwave receivers. This is particularly true of GaAs devices that act to raise the amplitudes of weak signals without introducing any unwanted rf noise, or signals of their own which cover a large variety of frequencies. (An illustration of noise is the hiss which you hear in a stereo hi-fi amplifier with the gain turned up and no audio input. The less is the noise, the better it is.)