Three periods of Al0.1Ga0.9N/Al0.15Ga0.85 N multiple quantum wells (MQWs) were used as the active region of a p-i-n diode fabricated on 6H-SiC substrate. Electroluminescence (EL) of these MQWs has been investigated in both injection and avalanche modes. Band-to-band luminescence of the Al0.1Ga0.9N wells was found to peak at 364 nm in the injection mode and in the range of 364-372 nm in the avalanche mode. The most striking phenomenon is that band-to-band EL of the Al0.15Ga0.85N barriers has also been observed in the injection mode, while it is not seen in the avalanche mode. This is explained by considering different sources of carriers and different carrier transportation mechanisms in the two modes. The luminescence intensity IEL has a power-law dependence on the current I by IEL prop I2 in the injection mode and by IEL prop I4 in the avalanche mode.