GaN-based MIS-HEMTs with Al2O3 dielectric deposited by low-cost and environmental-friendly mist-CVD technique

We report on the fabrication and characterization of AlGaN/GaN metal-insulator-semiconductor (MIS) capacitors and high-electron-mobility transistors (MIS-HEMTs) using a 5 nm thick Al2O3 dielectric deposited by cost-effective and environmental-friendly mist chemical vapor deposition (mist-CVD) technique. Practically hysteresis-free capacitance–voltage profiles were obtained from the fabricated two-terminal MIS-capacitors indicating high quality of the mist-Al2O3/AlGaN interface. Compared with reference Schottky-gate HEMTs, mist MIS-HEMTs exhibited much improved performance including higher drain current on-to-off ratio, much lower gate leakage current in both forward and reverse directions and lower subthreshold swing. These results demonstrate the potential and viability of non-vacuum mist-CVD Al2O3 in the development of high-performance GaN-based MIS-HEMTs

Insulated gate and surface passivation structures for GaN-based power transistors

Recent years have witnessed GaN-based devices delivering their promise of unprecedented power and frequency levels and demonstrating their capability as an able replacement for Si-based devices. High-electron-mobility transistors (HEMTs), a key representative architecture of GaN-based devices, are well-suited for high-power and high frequency device applications, owing to highly desirable III-nitride physical properties. However, these devices are still hounded by issues not previously encountered in their more established Si- and GaAs-based devices counterparts. Metal–insulator–semiconductor (MIS) structures are usually employed with varying degrees of success in sidestepping the major problematic issues such as excessive leakage current and current instability. While different insulator materials have been applied to GaN-based transistors, the properties of insulator/III-N interfaces are still not fully understood. This is mainly due to the difficulty of characterizing insulator/AlGaN interfaces in a MIS HEMT because of the two resulting interfaces: insulator/AlGaN and AlGaN/GaN, making the potential modulation rather complicated. Although there have been many reports of low interface-trap densities in HEMT MIS capacitors, several papers have incorrectly evaluated their capacitance–voltage (C–V) characteristics. A HEMT MIS structure typically shows a 2-step C–V behavior. However, several groups reported C–V curves without the characteristic step at the forward bias regime, which is likely to the high-density states at the insulator/AlGaN interface impeding the potential control of the AlGaN surface by the gate bias. In this review paper, first we describe critical issues and problems including leakage current, current collapse and threshold voltage instability in AlGaN/GaN HEMTs. Then we present interface properties, focusing on interface states, of GaN MIS systems using oxides, nitrides and high-κ dielectrics. Next, the properties of a variety of AlGaN/GaN MIS structures as well as different characterization methods, including our own photo-assisted C–V technique, essential for understanding and developing successful surface passivation and interface control schemes, are given in the subsequent section. Finally we highlight the important progress in GaN MIS interfaces that have recently pushed the frontier of nitride-based device technology

Interface trap states in Al2O3/AlGaN/GaN structure induced by inductively coupled plasma etching of AlGaN surfaces

We have investigated the effects of the inductively coupled plasma (ICP) etching of AlGaN surface on the resulting interface properties of the Al2O3/AlGaN/GaN structures. The experimentally measured capacitance-voltage (C-V) characteristics were compared with those calculated taking into account the interface states density at the Al2O3/AlGaN interface. As a complementary method, photoassisted C-V method utilizing photons with energies less than the bandgap of GaN was also used to probe the interface state density located near AlGaN midgap. It was found that the ICP etching of the AlGaN surface significantly increased the interface state density at the Al2O3/AlGaN interface. It is likely that ICP etching induced the interface roughness, disorder of chemical bonds and formation of various type of defect complexes including nitrogen-vacancy-related defects at the AlGaN surface, leading to poor C-V curve due to higher interface state density at the Al2O3/AlGaN interface

Calculating relaxation time distribution function from power spectrum based on inverse integral transformation method

A novel method is presented for obtaining the distribution function of relaxation times G(tau) from power spectrum 1/f(alpha) (1 <= alpha <= 2). It is derived using McWhorter model and its inverse Stieltjes transform. Unlike the pre-assumed conventional g(tau) distribution, the extracted G(tau) has a peak whose width increases as the slope of the power spectrum alpha decreases. The peak position determines the dominant time constant of the system. Our method is unique because the distribution function is directly extracted from the measured power spectrum. We then demonstrate the validity of this method in the analysis of noise in transistor

Reduced thermal resistance in AlGaN/GaN multi-mesa-channel high electron mobility transistors

Dramatic reduction of thermal resistance was achieved in AlGaN/GaN Multi-Mesa-Channel (MMC) high electron mobility transistors (HEMTs) on sapphire substrates. Compared with the conventional planar device, the MMC HEMT exhibits much less negative slope of the I-D-V-DS curves at high V-DS regime, indicating less self-heating. Using a method proposed by Menozzi and co-workers, we obtained a thermal resistance of 4.8 K-mm/W at ambient temperature of similar to 350K and power dissipation of similar to 9W/mm. This value compares well to 4.1 K-mm/W, which is the thermal resistance of AlGaN/GaN HEMTs on expensive single crystal diamond substrates and the lowest reported value in literature

Highly-stable and low-state-density Al2O3/GaN interfaces using epitaxial n-GaN layers grown on free-standing GaN substrates

Interface characterization was carried out on Al2O3/GaN structures using epitaxial n-GaN layers grown on free-standing GaN substrates with relatively low dislocation density (<3 x 10(6) cm(-2)). The Al2O3 layer was prepared by atomic layer deposition. The as-deposited metal-oxide-semiconductor (MOS) sample showed a significant frequency dispersion and a bump-like feature in capacitance-voltage (C-V) curves at reverse bias, showing high-density interface states in the range of 10(12) cm(-1) eV(-1). On the other hand, excellent C-V characteristics with negligible frequency dispersion were observed from the MOS sample after annealing under a reverse bias at 300 degrees C in air for 3 h. The reverse-bias-annealed sample showed state densities less than 1 x 10(11) cm(-1) eV(-1) and small shifts of flat-band voltage. In addition, the C-V curve measured at 200 degrees C remained essentially similar compared with the room-temperature C-V curves. These results indicate that the present process realizes a stable Al2O3/GaN interface with low interface state densities

Controlling surface/interface states in GaN-based transistors: Surface model, insulated gate, and surface passivation

Gallium nitride (GaN) is one of the front-runner materials among the so-called wide bandgap semiconductors that can provide devices having high breakdown voltages and are capable of performing efficiently even at high temperatures. The wide bandgap, however, naturally leads to a high density of surface states on bare GaN-based devices or interface states along insulator/semiconductor interfaces distributed over a wide energy range. These electronic states can lead to instabilities and other problems when not appropriately managed. In this Tutorial, we intend to provide a pedagogical presentation of the models of electronic states, their effects on device performance, and the presently accepted approaches to minimize their effects such as surface passivation and insulated gate technologies. We also re-evaluate standard characterization methods and discuss their possible pitfalls and current limitations in probing electronic states located deep within the bandgap. We then introduce our own photo-assisted capacitance-voltage (C-V) technique, which is capable of identifying and examining near mid-gap interface states. Finally, we attempt to propose some directions to which some audience can venture for future development