TT Mode vs. LSA Mode in Gunn Diodes: Key Differences

This article explains the fundamental differences between TT (Transit-Time) mode and LSA (Limited-Space Charge) mode in Gunn diodes. These two modes are crucial for enabling oscillation in Gunn diodes.

TT mode stands for Transit-Time mode, while LSA mode stands for Limited-Space Charge mode. Let’s delve into the specifics of each mode:

Gunn Diode TT Mode

When the voltage across an n+ n n+ GaAs crystal surpasses the threshold voltage, electrons undergo a transition from a low-energy, high-mobility band to a high-energy, low-mobility band.

During this process, the heavier electrons cluster together, forming a concentrated electric field near the cathode. The Transit-Time mode (TT mode) is characterized by a relatively low efficiency in terms of power generation. Furthermore, in TT mode, the oscillation frequency cannot be easily controlled by an external circuit.

Gunn Diode LSA Mode

The LSA mode, on the other hand, enables the Gunn diode to produce several watts of power with a minimum efficiency of approximately 20%.

A key characteristic of LSA mode is the inverse relationship between output power and frequency. As the frequency increases, the output power decreases.

For instance, a Gunn diode operating in LSA mode might generate 1 Watt at 10 GHz, but only several milliwatts at 100 GHz.

Image: Gunn diode TT mode vs LSA mode

As illustrated in the figure, in the LSA mode of operation, the Gunn diode operates as an integral part of a resonant circuit. This allows for more controlled and efficient oscillation.