A bulk acoustic wave resonator, BAWR, comprising: a substrate and an air cavity disposed above the substrate; a bulk acoustic wave resonant unit (540, 550, 560) comprising a first electrode, a second electrode, and a piezoelectric layer disposed between the first electrode and the second electrode; reflective layers (520, 530, 570, 580) to reflect bulk acoustic waves at a resonant frequency that are generated from the piezoelectric layer due to a signal applied to the first electrode and the second electrode, wherein the reflective layers (520, 530, 570, 580) comprise: a first reflective layer disposed below the bulk acoustic wave resonant unit, a second reflective layer disposed below the first reflective layer and on the air cavity, and having a higher acoustic impedance than the first reflective layer, a third reflective layer disposed on the bulk acoustic wave resonant unit, and a fourth reflective layer disposed on the third reflective layer, and having a higher acoustic impedance than the third reflective layer, wherein the first and third reflective layers (530, 570) comprise a material having a temperature coefficient of frequency, TCF, that has a sign that is opposite to the sign of the TCF of the bulk acoustic wave resonant unit (540, 550, 560), such that the sum of the TCF of the material of the reflective layers and the TCF of the bulk acoustic wave resonant unit is approximately zero. a substrate and an air cavity disposed above the substrate; a bulk acoustic wave resonant unit (540, 550, 560) comprising a first electrode, a second electrode, and a piezoelectric layer disposed between the first electrode and the second electrode; reflective layers (520, 530, 570, 580) to reflect bulk acoustic waves at a resonant frequency that are generated from the piezoelectric layer due to a signal applied to the first electrode and the second electrode, wherein the reflective layers (520, 530, 570, 580) comprise: a first reflective layer disposed below the bulk acoustic wave resonant unit, a second reflective layer disposed below the first reflective layer and on the air cavity, and having a higher acoustic impedance than the first reflective layer, a third reflective layer disposed on the bulk acoustic wave resonant unit, and a fourth reflective layer disposed on the third reflective layer, and having a higher acoustic impedance than the third reflective layer, wherein the first and third reflective layers (530, 570) comprise a material having a temperature coefficient of frequency, TCF, that has a sign that is opposite to the sign of the TCF of the bulk acoustic wave resonant unit (540, 550, 560), such that the sum of the TCF of the material of the reflective layers and the TCF of the bulk acoustic wave resonant unit is approximately zero. a first reflective layer disposed below the bulk acoustic wave resonant unit, a second reflective layer disposed below the first reflective layer and on the air cavity, and having a higher acoustic impedance than the first reflective layer, a third reflective layer disposed on the bulk acoustic wave resonant unit, and a fourth reflective layer disposed on the third reflective layer, and having a higher acoustic impedance than the third reflective layer, wherein the first and third reflective layers (530, 570) comprise a material having a temperature coefficient of frequency, TCF, that has a sign that is opposite to the sign of the TCF of the bulk acoustic wave resonant unit (540, 550, 560), such that the sum of the TCF of the material of the reflective layers and the TCF of the bulk acoustic wave resonant unit is approximately zero.