By adopting atomic layer deposition (ALD) sidewall passivation, the electroluminescence (EL) and communication performances of mini-light emitting diodes (mini-LEDs) of different sizes were improved. In particular, the most significant improvement in the electroluminescence properties of the external quantum efficiency (EQE) (a 7.2% increase), leakage current, and the communication properties of the modulation bandwidth (a 20% increase) transmission rate and bit error rate (BER) was found in the smallest mini-LEDs (80 μm× 120 μm ). According to the results of time-resolved photoluminescence (TRPL) measurements, the carrier lifetime of the samples can be affected by both the size and ALD sidewall passivation. In addition, the effects of ALD sidewall passivation for visible-light communication (VLC) were demonstrated.