Ultraviolet light sources based on III-nitride semiconductors are attracting much attention because of their promising applications in medicine, electronic devices and military systems. Because of wide band gap, AlGaN/GaN structures are a very promising system to create high power electronic devices operating at high temperatures. However, the realization of high performance devices requires many material investigations by using various experimental techniques including electromodulation (EM) spectroscopy [i.e. photoreflectance (PR) and/or contactless electroreflectance (CER)]. This technique, due to its absorption and differential character is an excellent tool to study the energies of optical transitions (including the excited state ones) in quantum wells (QWs) and quantum dots as well as energies of bulk-like QW barriers or intermediate layers. Because of its high sensitivity to the Franz Keldysh effect, this technique can be used to determine the value of built-in electric fields in semiconductor structures. This issue is especially important for III-nitrides since they are polar structures with high internal piezoelectric fields. However, the application of PR spectroscopy in the UV region can be limited due to the wavelength of modulation beam (deep UV laser sources are still less available and very expensive). This problem can be avoided in CER spectroscopy when a wirelike electrode is used in the capacitor system. In this work we will present our recent progress in the application of CER spectroscopy to study GaN-based structures including AlGaN/GaN transistor and AlGaN/GaN QW structures. We will discuss electromodulation mechanism in the CER technique as well as the efficiency of electromodulation in CER technique depending on the capacitor construction.