ABSTRACT
EL distribution was measured by combining an objective lens (*100) and a CCD camera. After EL signal image was obtained in the dark, top-view image of the device was also taken under illumination. We could precisely study the EL distribution on the device by superimposing these two images. For the spectroscopic measurements, the EL signal emitted from whole surface of the device was collected by an objective lens, dispersed through a 1 -m double monochromator, and detected with a cooled GaAs photomultiplier. The photon counting method was employed to detect a weak luminescence signal. The detectable spectrum range was 420-890 nm (2.8-1.4 eV), which was limited by the objective lens and GaAs photomultiplier. The measured spectra were calibrated by the sensitivity of the measurement system which was obtained by studying its response to the blackbody radiation, and underwent a 1/E2 correction required for the translation from the measured intensity per unit wavelength to intensity per unit energy. 3
3. EL distribution
Figure 1 shows (a) the drain current (ID) and (b) the gate current (Iq) of the AlGaN/GaN HEMT as a function of Vds at Vgs = -2 V. The ID slightly increased at VDs larger than 69 V, suggesting the occurrence of carrier multiplication due to impact ionization process. The |Iq| superlinearly increased with increase in Vds at voltages larger than 45 V due to holes generated by the impact ionization process at high-field region [3]. The V ds of 45 V is smaller than Vds (69 V) for Id increase. This suggests that Ig is a sensitive probe for studying the impact ionization phenomena. |Ig| drastically increased at VDs = 70 V. The Ioat low V ds below 45 V is reverse-bias gate leakage current (Iieak) due to electrons via surface states and/or defects in AlGaN barrier layer.
