A simulation study of the impact of traps in the GaN substrate on the electrical characteristics of an AlGaN/GaN HEMT with a thin channel layer

Abstract

Gallium nitride (GaN) substrates are promising candidates for GaN high-electron-mobility transistors (HEMTs) because of their epitaxial layer growth with low defect density. We perform device simulations to study the influence of substrate acceptor traps in GaN HEMTs on semiinsulating GaN substrates with a thin (0.02 μm) channel layer. When the trap concentration in the GaN substrate increases at a constant channel trap concentration of 1.0 × 1015 cm−3, the drain leakage current decreases but the transient response worsens. These phenomena result from an increase of the conduction-band energy due to ionized acceptor traps with negative charge. For GaN buffer structures, we obtain moderate suppression of the drain leakage current (1.7 × 10−9 A/mm) and a good transient response (with a normalized drain current of 0.86 at 1 ms for the transient response from the off- to on-state condition) at a substrate trap concentration of 5.0 × 1015 cm−3. The aluminum gallium nitride (AlGaN) back-barrier structure is highly effective for suppressing the drain leakage current at a low trap concentration of 1.0 × 1015 cm−3 in GaN substrates. Although its maximum drain current is decreased, this structure exhibits a low drain leakage current (4.7 × 10−11 A/mm) and a high normalized drain current (0.95) in the transient response. Increasing the Al content in the barriers of the GaN HEMT structure increases its maximum drain current to 1.2 A/mm, whereas the drain leakage current and transient response are well maintained. Moreover, the traps in the GaN substrate affect the low-frequency S21, which is important for the linearity of power amplifiers, and the characteristics of S21 are similar to those of the transient responses.