On the Practical Limitations for the Generation of Gunn Oscillations in Highly Doped GaN Diodes

Abstract

Planar Gunn diodes based on doped GaN active layers with different geometries have been fabricated and characterized. Gunn oscillations have not been observed due to the catastrophic breakdown of the diodes for applied voltages around 20-25 V, much below the bias theoretically needed for the onset of Gunn oscillations. The breakdown of the diodes has been analyzed by pulsed I-V measurements at low temperature, and it has been observed to be almost independent of the geometry of the channels, thus allowing to discard self-heating effects as the origin of the device burning. The other possible mechanism for the device failure is impact-ionization avalanche due to the high electric fields present at the anode corner of the isolating trenches. However, Monte Carlo simulations using the typical value of the intervalley energy separation of GaN, 𝜺 𝟏−𝟐 =2.2 eV, show that impact ionization mechanisms are not significant for the voltages for which the experimental failure is observed. But recent experiments showed that 𝜺 𝟏−𝟐 is lower, around 0.9 eV. This lower intervalley separation leads to a much lower threshold voltage for the Gunn oscillations, not far from the experimental breakdown. Therefore, we attribute the devices failure to an avalanche process just when Gunn domains start to form, since they increase the population of electrons at the high electric field region, thus strongly enhancing impact ionization mechanisms which lead to the diode failure.

A Deep Learning-Monte Carlo Combined Prediction of Side-Effect Impact Ionization in Highly Doped GaN Diodes

“…This would be unimportant if there is no transport through the buffer, but at the high biases needed for the generation of Gunn oscillations, high vertical fields force the electrons to penetrate into the undoped region. Consequently, some hot carriers, susceptible of provoking impact ionizations, can appear, thus leading to avalanche and the potential failure of the device [22]. Besides, defects and impurities could play a role in the appearance of tunneling leakage current in other types of GaN-based diodes [23], [24], but in the case of the planar diode architecture studied here, they are mainly effective at low electron energies, and do not have a significant influence.…”