Normally-OFF AlGaN/GaN-based HEMTs with decreasingly graded AlGaN cap layer

Abstract

In this work, an E-mode AlGaN/GaN-based HEMTs with a graded AlGaN cap layer (GACL) is proposed and numerically studied by Silvaco TCAD. The GACL is designed with a decreasingly graded Al composition x along [0001] direction and the initial x is smaller than the Al composition of the Al0.2Ga0.8N barrier layer (BL). This GACL scheme can simultaneously produce high-concentration polarization-induced holes and negative net polarization charges at the GACL/BL interface. This can facilitate the separation of the conduction band and Fermi level at the 2DEG channel and therefore benefit the normally-OFF operation of the device. The optimized graded-AlGaN-gated (GAG) metal-semiconductor (MES) HEMT can achieve a large threshold voltage of 4 V. Furthermore, we demonstrated that shortening the gate length on the GACL and inserting an oxide layer between the gate and GACL can be both effective to suppress gate leakage current, enhance gate voltage swing, and improve on-state drain current of the device. These numerical investigations can provide insights into the physical mechanisms and structure innovations of the E-mode GaN-based HEMTs of the future.

Enhanced performance of normally-OFF GaN HEMTs with stair-shaped p-GaN cap layer

“…The numerical investigations in this study were carried out by technology computer-aided design simulation provided by Silvaco International [20]. The key models employed in the device simulation include the parallel electric field model for velocity saturation, Fermi-Dirac statistics model (FERMI), Shockley-Read-Hall model (SRH), Auger-Recombination model (AUGER), the Albrecht model (ALBRCT.N) and the polarization models (POLARIZATION and CALC.STRAIN for spontaneous polarization and piezoelectric polarization, respectively) [14].…”

“…However, the performance trade-off among output current (I DS ), threshold voltage (V TH ), and breakdown voltage (V BD ) of the P-HEMTs still needs to be tackled at the moment. To be specific, the high Mg doping and larger hole concentration (N A ) in the p-GaN cap is required to achieve a large V TH of the P-HEMT [14,15]. The higher N A , on the other hand, would lead to a lower I DS , degraded gate reliability, and early device breakdown [16,17].…”

GaN-based E-mode p-FETs with polarization-doped p-type graded AlGaN channels

“…The key models employed in the simulation include Shockley-Read-Hall model, Fermi-Dirac model, polarization models (POLARIZATION and CALC. STRAIN for spontaneous polarization and piezoelectric polarization, respectively), Auger recombination (AUGER), and constant low-field mobility model [32,33]. In order to well match the actual device characteristics, based on previous reports, the simulation parameters are set as follows: the gate metal work function of 4.8 eV [19]; the Al 2 O 3 relative dielectric constant of 9.3; the specific Ohmic contact resistance (ρ C ) of 1.4 × 10 −4 Ω•cm 2 for all devices, same as the value reported in [34].…”

Special issue on wide-bandgap semiconductors and applications

“…They are essentially suitable for various applications targeting low losses and high operating frequencies and temperatures. This special issue evaluates a series of GaN-and AlGaN-based electronic devices proposed by Chinese researchers, such as Schottky barrier diodes (SBDs) [19], metal-oxide-semiconductor field effect transistors (MOSFETs) [20], high electron mobility transistors (HEMTs) [21][22][23][24][25], and so forth.…”

“…They found that, at low temperatures, the device's threshold voltage shifts positively, and both maximum transconductance and ON-resistance significantly improve. Furthermore, Xing et al [23] demonstrated an E-mode AlGaN/GaN HEMT with a graded AlGaN cap layer (GACL). In their work, the threshold voltage of gradient AlGaN gate metal semiconductor HEMT rose to 4 V, proving that 80 nm GACL metal semiconductor HEMT was the best structure.…”