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Electroluminescence (EL) spectroscopy in combination with drift-diffusion simulations was used to prove the presence of impact ionization in AlGaN/GaN HEMTs illustrated on InGaN back-barrier devices. Regardless of the level of gate leakage current, which is dominated by contributions such as surface leakage current and others, EL enabled to reveal hole generation due to impact ionization. Hole currents as low as 10pA were detectable by the optical technique used.Introduction: AlGaN/GaN HEMTs represent a promising technology for high power RF applications. Besides charge carrier trapping, impact ionization due to high electric fields in the device channel [1-3] can limit device reliability and consequently the use of GaNbased HEMTs. The presence of impact ionization has been debated for GaN HEMTs with controversial evidence due to high leakage in many devices [3], illustrating the limitations of the commonly applied method of gate current analysis for GaN HEMTs.Therefore, a new methodology is needed to probe impact ionization in AlGaN/GaN HEMTs independently of the gate leakage current level. A bell-shaped dependence of the gate current versus gate-source voltage has been used in AlGaAs/GaAs HEMTs [4] to prove impact ionization, supported by studies of interband electroluminescence (EL).So far neither has been unambiguously observed in GaN-based HEMTs. This letter Ref. [6]. Initially, the commonly used method of gate current analysis for detecting impact ionization [2] was applied to attempt to probe hole currents. The hole current generated by, e.g., impact ionization is expected to be in the range of nA [1], which requires a very low gate leakage current level in order to distinguish between hole current by impact ionization and other gate leakage current contributions. The inset of Fig. 1 shows, however, that the leakage current level of the devices used is too high in order to draw conclusions on the presence of a hole current. EL spectroscopy was used to probe the hole current independently from the level of gate leakage current. EL spectra emitted from the source-drain gap were recorded using a Renishaw RM system, while the device was operated at a drain-source and gate-source bias of 20 V and 0 V, respectively. Results:The EL spectrum shown in Fig. 1 reveals a tail in the red-infrared spectral range typically observed in GaN-based HEMTs, which is related to hot carrier relaxation in the active device region [7]. Furthermore, a peak of the EL signal is apparent at the bandgap energy of InGaN, evidencing the recombination of electrons and holes in the InGaN layer. This clearly proves the presence of holes in the InGaN layer of the device under operation, in addition to electrons from normal device operation. The InGaN device layer 3 acts as charge carrier collecting layer in the device (inset of Fig. 2), and as optical hole probe due to its high optical quantum efficiency. We note that EL around the GaN bandgap energy tends to be optically less efficient, possibly explaining why this has not been observed to...

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