In the Optics and Photonics division of CEA-LETI, The Emissive Components Laboratory designs and fabricates micro-displays based on GaN micro-LEDs. One of the major issue of GaN µ-LEDs is the decrease of their efficiency with the size reduction. In literature, this degradation is attributed to SRH (Shockley-Read-Hall) non-radiative recombinations on pixel sidewalls, in defects induced during the pixel etching step. In the laboratory, we also observed that field effect on pixel sidewalls could have a major impact on µ-LEDs performance. Three phenomena may be in competition. Sidewalls defects can lead to SRH recombinations, but also to a presence of charge (field effect) depending on their ionization level (and so on their energy position in the semiconductor gap). Fixed or mobile charges may also exists on pixel sidewall or in the passivation dielectric. More over, a correlation between the carriers injection (P and N ohmic or Schottky contacts) and the impact of sidewalls on µ-LEDs performance was observed and could help to decorrelate the influence of the different factors. The goal of this thesis will be to study the impact of these different phenomena on µ-LEDs performance through TCAD simulations and electro-optical measurements on different test structures (different sizes LEDs, capacitances, TLM) and put in evidence the dominant phenomenon leading to the efficiency decrease of µ-LEDs.