GaN based power high electron mobility transistors (HEMTs) have emerged as a promising candidate for next generation power switching applications as these devices can deliver high switching speed and high power conversion efficiency. Although the GaN HEMTs have been commercialized these years, there are still challenges when the GaN HEMTs are adopted in power applications. In particular, the enhancement-mode (E-mode) GaN HEMTs feature a weak reverse-conduction capability and are lack of reverse-blocking capability. Employing GaN SBDs monolithically integrated with the GaN HEMTs is a compact approach to provide these functions. However, the GaN SBDs at present features both high reverse leakage current and high turn-on voltage. Thus, the reverse-conducting and reverse-blocking GaN HEMTs...[
Read more ]
GaN based power high electron mobility transistors (HEMTs) have emerged as a promising candidate for next generation power switching applications as these devices can deliver high switching speed and high power conversion efficiency. Although the GaN HEMTs have been commercialized these years, there are still challenges when the GaN HEMTs are adopted in power applications. In particular, the enhancement-mode (E-mode) GaN HEMTs feature a weak reverse-conduction capability and are lack of reverse-blocking capability. Employing GaN SBDs monolithically integrated with the GaN HEMTs is a compact approach to provide these functions. However, the GaN SBDs at present features both high reverse leakage current and high turn-on voltage. Thus, the reverse-conducting and reverse-blocking GaN HEMTs exhibit inferior performance at present.
In this work, high performance multifunctional GaN power devices, which include SBDs, reverse-condcuting MOS-HEMT and reverse-blocking MOS-HEMT, are demostrated on an AlGaN/GaN double-channel platform. The drain current model for double-channel HEMT is also derived, since the HEMT developed on this heterojunction features a unique “double-peak” behavior in transconductance-gate voltage curves. This work is divided into the following four parts:
Firstly, a 650-V AlGaN/GaN double-channel SBD with double-recessed gated anode is demonstrated. The SBD features two recess steps. The deep one cuts through two channels and the anode metal contacts 2DEG directly from the sidewall of the heterostructure to form a metal-2DEG Schottky junction, delivering a low turn-on voltage. The shallow one terminates at the upper channel layer to form a leakage suppression MOS field plate. At reverse state, the MOS field plate can pinch-off the channel underneath and shield the Schottky contact from strong electric field, which leads to a low reverse leakage current. At forward state, the channel under the MOS field plate connects the Schottky junction to the access region with a low channel resistance, as the lower channel in MOS region is separated from the etched upper channel surface and a high mobility is maintained. Thus, the SBD exhibits an enhanced forward and reverse performance. In fabricating the double-channel SBDs with gated anode, the Schottky barrier height is affected by the carrier density in the adjacent MOS-channel. It is experimentally revealed that a higher carrier density in MOS-channel would lead to a lower Schottky barrier height, as the carriers in SBDs with higher MOS-channel charge density can tunnel through the barrier at a lower energy level.
Second, a low-loss reverse-conducting normally-off AlGaN/GaN power transistor with built-in SBD was demonstrated. This device features a MOS-gate section and SBD-anode section paralleled in an interdigitated layout along the gate width direction. A common access region that conducts current at both forward and reverse ON-state is employed, which is beneficial to reducing the conduction loss. Owing to the high performance SBD, the power switch shows low reverse turn-on voltage and low OFF-state leakage current, as well as a high breakdown voltage.
Thirdly, a gated Schottky-drain was employed to replace the ohmic drain of the AlGaN/GaN MOS-HEMT, thus a reverse-blocking AlGaN/GaN MOS-HEMT is obtained. Under the reverse bias, the Schottky-drain is reverse biased and the depletion region expansion from the Schottky-drain towards the gate. The leakage suppression MOS field plate pinches off the channel underneath at reverse state, shielding the metal-2DEG Schottky contact from the strong electric field; the metal-2DEG Schottky-drain can effectively lowering the turn-on voltage at the forward ON-state. The device also shows a high forward and reverse breakdown voltage.
Finally, an analytical model of surface potential (SP) and drain current for double-channel AlGaN/GaN HEMTs is presented. By modeling the two channels as two triangular potential well and applying the Poisson’s equation and Fermi-Dirac statistics, a transcendental charge-control equation set is established for the two dimensional electron gas (2DEG) and Fermi levels of the two channels. The modeled capacitance-voltage (C-V) characteristics exhibit a unique “double-plateau” behavior and agree well with the experiment. Based on this calculation, a SP-based drain current model is established. The “double-peak” behavior in transconductance-gate voltage (g
m-V
GS) characteristics is well described.
Post a Comment