TT mode vs LSA mode-difference between TT mode and LSA mode
This page on Gunn diode TT mode vs LSA mode mentions basic difference between TT mode and LSA modes of Gunn diode.
These are two modes which brings Gunn diode into oscillation region. TT mode stands for Transit-Time mode and LSA mode stands for Limited-Space Charge mode.
Gunn diode TT mode
When voltage across n+ n n+ GaAs crystal exceeds threshold voltage, electrons are transferred from (low energy,high mobility band) to (high energy,low mobility band). Here heavier electrons bunch together to form electric field near the cathode.
The TT mode of oscillation has low efficiency of power generation. In this mode, frequency can not be controlled by external circuit.
Gunn diode LSA mode
LSA mode in Gunn diode produce several watts of power at minimum efficiency of about 20%. The output power decreases with increase in frequency. For example, it generates 1Watt @ 10GHz and several mWatts @ 100GHz.
As shown it the figure, in LSA mode of operation, Gunn diode works as part of resonant circuit.