Comparison between trap and self-heating induced mobility degradation in AlGaN/GaN HEMTs
Introduction
Owing to very high breakdown voltages and sheet carrier densities, GaN HEMTs (high electron mobility transistors) are useful in high power and high frequency applications. Self-heating effects are a serious concern in GaN HEMTs because of their large power densities [1], [2], [3], [4]. The maximum power densities in GaN HEMTs are ten times those that can be obtained in silicon and GaAs devices. This is attributed to a larger current density for the same device area and higher breakdown voltage [2]. The elevated temperatures near the two-dimensional electron gas (2DEG) locally decrease the mobility and thus the drain current [5]. Bulk defects also degrade mobility in AlGaN/GaN HEMTs [6], [7], [8]. In a previous paper we quantified how charged defects degrade mobility [9]. The results presented here suggest that mobility in the 2DEG is limited by charged bulk defects near the 2DEG and by Joule heating near the gate–drain access region.
Section snippets
Device structure
The structure of the AlGaN/GaN HEMTs examined in this work is shown in Fig. 1. The sapphire substrate is not shown in the figure. The gate length is 0.7 μm and the gate width is 150 μm. A semi-insulating GaN layer was grown over the sapphire substrate and a 1 nm interfacial AlN layer was grown over the semi-insulating GaN buffer layer. On top of the AlN layer, there is a 27.5 nm layer of n-type Al0.22Ga0.78N. Over this layer is a 0.25 nm unintentionally doped (UID) layer of Al0.22Ga0.78N. The n-type
Impurity scattering
Fig. 4 shows the experimentally observed impact of proton irradiation on the drain current (ID) vs. drain voltage (VD) characteristics. The figure shows results at VG = −1 and −2 V. These results are similar to those reported in previous studies on proton-irradiated AlGaN/AlN/GaN HEMTs [14], [15], [16]. The figure shows the pre-irradiation I–V characteristics, as well as those at proton fluences of 6 × 1013 and 5 × 1014 cm−2. The energy of the protons was 1.8 MeV. As the particle fluence increases, the
Mobility and saturation velocity degradation models
Mobility and saturation velocity are functions of temperature. Joule heating due to large current densities is the main driver for the change in mobility and saturation velocity in these devices [5]. The temperature dependence of these two quantities is described in Eqs. (1), (2). Eq. (3) relates the mobility to the proton fluence and lattice temperature. This is consistent with previous mobility degradation models that take into account the effects of traps. Here μ0(Φ) is the mobility at a
Results
Fig. 5 shows the simulated lattice temperature cross section of the HEMT at VG = −1 V and VD = 9 V. There is a hot spot near the drain side gate edge with a maximum temperature of approximately 370 K. The experimentally measured 2DEG electron mobility at 298 K in these devices is 1200 cm2/V s, obtained from Hall measurements [2]. Fig. 6, Fig. 7, Fig. 8, Fig. 9 show the relationship between lattice temperature and mobility near the gate–drain access region. The hot spot is created due to the presence of
Conclusions
Experimental data and simulations are used to understand the interplay between bulk defects, mobility degradation, vsat degradation, and self-heating in AlGaN/AlN/GaN HEMTs. The drain current in the device degrades due to the cumulative effect of both charged defects and temperature. At higher current densities temperature-induced degradation in mobility is greater than charged defect-induced degradation near the hot spot. Away from the hot spot, charged defect-induced degradation dominates.
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