Current collapse in AlGaN/GaN transistors studied using time-resolved Raman thermography

Abstract

Current collapse in AlGaN/GaN high electron mobility transistors was investigated using time-resolved Raman thermography. The virtual-gate mechanism was visualized by changes in the device temperature distribution, illustrating an effective gate lengthening up to 0.6 um. Two devices with different levels of current collapse are compared, demonstrating that the effective gate length increases for greater current collapse. A comparison of two-dimensional drift diffusion simulations with experimental data was used to estimate a lower limit for the charge trapping density in the virtual-gate region. This was found to be of the order of 2×1013 cm−2 for a device exhibiting relatively little current collapse.

A Review of Raman Thermography for Electronic and Opto-Electronic Device Measurement With Submicron Spatial and Nanosecond Temporal Resolution

“…Time resolved Raman thermography has also provided valuable information about dynamic temperatures in GaN and GaAsbased microwave devices during actual pulsed RF operation, closely matching the conditions of intended applications [67]. Investigating transistors that exhibit current collapse is another example of where time resolved Raman thermography measurements can provide additional information: Simms et al have used this technique to visualize the effect of the virtual gate mechanism on the temperature profile during pulsed operation [68].…”

Non-localized trapping effects in AlGaN/GaN heterojunction field-effect transistors subjected to on-state bias stress

“…Owing to the success of using surface passivation to suppress the current collapse, [6][7][8][9] the virtual gate mechanism has become widely accepted as the main mechanism for current collapse. 10 Following this understanding, regarding the reliability of AlGaN/ GaN HFETs, degradation at the gate edge has been recognized as the only degradation mechanism regardless of whether onstress or off-stress is applied. [11][12][13][14] In short, the reported trapping and degradation mechanisms of AlGaN/GaN HFETs have mainly focused on the gate edge.…”

Thermoreflectance CCD imaging of self heating in AlGaN/GaN high electron mobility power transistors at high drain voltage

“…GaN HEMTs have shown saturation current dependence on temperature [2]. Due to the high power levels reachable in GaN HEMTs and the high electric fields achievable that can force this power to be dissipated in a small volume, temperature gradients can be extreme [3]. Temperature stress affects many degradation phenomena observed in GaN power devices such as current collapse during fast switching [3] and pit formation [4,5].…”

“…Due to the high power levels reachable in GaN HEMTs and the high electric fields achievable that can force this power to be dissipated in a small volume, temperature gradients can be extreme [3]. Temperature stress affects many degradation phenomena observed in GaN power devices such as current collapse during fast switching [3] and pit formation [4,5]. For these reasons self heating in GaN devices remains an active area of study, and robust and versatile thermal characterization methods are needed.…”

“…For these reasons self heating in GaN devices remains an active area of study, and robust and versatile thermal characterization methods are needed. While Raman thermometry is a scanning laser method that has been used to measure both temperature and stress in GaN heterostructures at specific points near the surface and with some limited spatial resolution below the GaN surface [3,6], one method that is particularly well suited to measure heating across the entire surface of microelectronic devices is thermoreflectance CCD imaging [7][8][9]. Thermoreflectance imaging measures reflected visible wavelength illumination to provide two dimension maps of surface temperature distribution with submicron spatial resolution and 50 mK temperature resolution.…”