GUNN diode operates under different modes of oscillation:

1. Transit Time domain mode
2. Delayed mode
3. quenched mode
4. LSA mode

i) Transit Time mode: External circuit is tuned to a frequency such that the time period of the oscillation of tuned circuit τ0 is equal to the transit time τt of the electron cloud falling in high-filed domain. This mode of operation is also called as GUNN mode.

ii) Delayed domain mode: As shown in the figure transit time of high-filed domain is such that it is smaller than the oscillation period of the tuned circuit. Due to this the domain is collected at anode at time τt and while the oscillating signal from the tuned circuit is at negative peak and the voltage across the GUNN is less that the Eth(threshold voltage). New domain in GUNN is formed only after a delay when the voltage across the GUNN is larger than the Eth, this mode of operation is called delayed domain mode. Efficiency of the mode is 20 %.