Srabanti Chowdhury

Enhancement and Depletion Mode AlGaN/GaN CAVET With Mg-Ion-Implanted GaN as Current Blocking Layer

Current aperture vertical electron transistor (CAVET) was successfully demonstrated by using Mg-implanted GaN as a current blocking layer for nonalloyed source contacts. A maximum source-drain current of (corresponding to 0.22 A/mm of source) and an extrinsic transconductance of 54 mS/mm of source were achieved. Threshold voltage as high as 0.6 V was realized by plasma exposure for 10 min, increasing the transconductance of the device to 140 mS/mm of source. Thus, a normally off CAVET was demonstrated for the first time. The increase in by plasma exposure for a given bias was due to etching of the AlGaN barrier. The shift of threshold voltage and the varied directly with the time of exposure. There was no significant dispersion in these devices.

CAVET on Bulk GaN Substrates Achieved With MBE-Regrown AlGaN/GaN Layers to Suppress Dispersion

A current aperture vertical electron transistor (CAVET) with a Mg-ion-implanted current blocking layer (CBL) and a channel regrown by plasma assisted molecular beam epitaxy (MBE), is successfully demonstrated on bulk GaN to work as a high voltage device. The fabrication of the device combined a drift region grown by metalorganic chemical vapor deposition (MOCVD), to hold the blocking voltage, with AlGaN/GaN layers regrown by plasma-MBE to conduct current. The device registered a maximum current of 4 kA· cm^-2 under direct-current operation offering a specific on-state resistance <i>R</i>_on - <i>A</i> of 2.2 mΩ·cm². With 80 μs pulses applied to the gate, the devices showed no dispersion. The increased aperture length <i>L</i>_ap resulted in the decrease in specific <i>R</i>_on, as expected. The impact of the gate overlap to aperture <i>L</i>_go on the leakage current was studied, where the leakage current was found to increase with a smaller overlap.

Lateral and Vertical Transistors Using the AlGaN/GaN Heterostructure

Power conversion losses are endemic in all areas of electricity consumption, including motion control, lighting, air conditioning, and information technology. Si, the workhorse of the industry, has reached its material limits. Increasingly, the lateral AlGaN/GaN HEMT based on gallium nitride (GaN-on-Si) is becoming the device of choice for medium power electronics as it enables high-power conversion efficiency and reduced form factor at attractive pricing for wide market penetration. The reduced form factor enabled by high-efficiency operation at high frequency further enables significant system price reduction because of savings in bulky extensive passive elements and heat sink costs. The high-power market, however, still remains unaddressed by lateral GaN devices. The current and voltage demand for high power conversion application makes the chip area in a lateral topology so large that it becomes more difficult to manufacture. Vertical GaN devices would play a big role alongside of silicon carbide (SiC) to address the high power conversion needs. In this paper, the development, performance, and status of lateral and vertical GaN devices are discussed.

Effects of oxidation on surface chemical states and barrier height of AlGaN/GaN heterostructures

The effects of oxidation on the surface structure and chemical bonding states of AlGaN/GaN heterostructures were investigated using x-ray photoelectron spectroscopy (XPS). In comparing Al 2p core-level XPS spectra among as-grown and annealed samples, we found that Al atoms on the surface were highly oxidized after rapid thermal annealing (RTA) at high temperature; not only in an O2 but also in an N2 gas atmosphere. The Al oxidation level was almost identical for the samples annealed at 800 °C, irrespective of the annealing atmosphere and time; yet there was a strong dependence on the annealing temperature. The dependence of surface barrier height on the annealing condition is associated with Al oxidation behavior. Before the RTA, the barrier height increased together with the AlGaN thickness, indicating an unpinned Fermi level and the existence of low-density and distributed surface donor states. After the high-temperature RTA, however, the height is maintained at a certain value, regardless of the thickn...

Current status and scope of gallium nitride-based vertical transistors for high-power electronics application*

Gallium nitride (GaN) is becoming the material of choice for power electronics to enable the roadmap of increasing power density by simultaneously enabling high-power conversion efficiency and reduced form factor. This is because the low switching losses of GaN enable high-frequency operation which reduces bulky passive components with negligible change in efficiency. Commercialization of GaN-on-Si materials for power electronics has led to the entry of GaN devices into the medium-power market since the performance-over-cost of even first-generation products looks very attractive compared to todays mature Si-based solutions. On the other hand, the high-power market still remains unaddressed by lateral GaN devices. The current and voltage demand for high-power conversion application makes the chip area in a lateral topology so large that it becomes difficult to manufacture. Vertical GaN devices would play a big role alongside silicon carbide (SiC) to address the high-power conversion needs. In this paper vertical GaN devices are discussed with emphasis on current aperture vertical electron transistors (CAVETs) which have shown promising performance. The fabrication-related challenges and the future possibilities enabled by the availability of good-quality, cost-competitive bulk GaN material are also evaluated for CAVETs.

Distribution of donor states on etched surface of AlGaN/GaN heterostructures

The dependence of electron density (ns) on AlGaN barrier thickness (dAlGaN) was studied for AlGaN/GaN single heterostructures whose dAlGaN was controlled by low-power Cl-based reactive ion etching (RIE) instead of growth. The samples showed a constant increase not only in ns but also in AlGaN surface barrier height (eϕB) with dAlGaN, indicating the existence of low-density and distributed donor states on the AlGaN surface. Such a distribution of donor states differs from the commonly accepted model based on high-density and single-level surface donor states as the source of electrons in the two-dimensional electron gas (2DEG). The presence of a distribution of donor states is confirmed by first-principles calculations for a variety of surface structures for oxidized AlGaN surfaces. Donor states arise from areas of the surface that deviate from the electron-counting rule, leading to occupied surface states in the upper half of the band gap. The oxide formed on the surface after RIE results in a low-density...

Distributed surface donor states and the two-dimensional electron gas at AlGaN/GaN heterojunctions

Surface donor states with distributed and finite density are implemented in Schrodinger–Poisson simulations of AlGaN/GaN high electron mobility transistors, with the goal of studying their effects on the two-dimensional electron gas. Our recent experimental observations of an increasing surface barrier height with increasing AlGaN thickness are fitted very well by simulations including surface donor levels represented by a constant density of states (DOS) with a density on the order of 1013 cm−2 eV−1. The highest occupied surface states are found to be around 1 eV below the conduction-band minimum, considerably higher in energy than previously reported single surface donor levels. These trends can be explained by the features of oxidized AlGaN surfaces. Furthermore, the surface DOS that fit the experimental results are found to be larger for samples with higher Al concentration.

Design, fabrication, and performance analysis of GaN vertical electron transistors with a buried p/n junction

The Current Aperture Vertical Electron Transistor (CAVET) combines the high conductivity of the two dimensional electron gas channel at the AlGaN/GaN heterojunction with better field distribution offered by a vertical design. In this work, CAVETs with buried, conductive p-GaN layers as the current blocking layer are reported. The p-GaN layer was regrown by metalorganic chemical vapor deposition and the subsequent channel regrowth was done by ammonia molecular beam epitaxy to maintain the p-GaN conductivity. Transistors with high ON current (10.9 kA/cm2) and low ON-resistance (0.4 mΩ cm2) are demonstrated. Non-planar selective area regrowth is identified as the limiting factor to transistor breakdown, using planar and non-planar n/p/n structures. Planar n/p/n structures recorded an estimated electric field of 3.1 MV/cm, while non-planar structures showed a much lower breakdown voltage. Lowering the p-GaN regrowth temperature improved breakdown in the non-planar n/p/n structure. Combining high breakdown vol...

Normally OFF Trench CAVET With Active Mg-Doped GaN as Current Blocking Layer

A normally OFF trench current aperture vertical electron transistor (CAVET) was designed and successfully fabricated with Mg-doped p-GaN current blocking layers. The buried Mg-doped GaN was activated using a postregrowth annealing process. The source-to-drain body diode showed an excellent p-n junction characteristics, blocking over 1 kV, sustaining a maximum blocking electric field of 3.8 MV/cm. Three-terminal breakdown voltages of trench-CAVETs, measured up to 225 V, were limited by dielectric breakdown. This paper highlights the achievement of the well-behaved buried p-n junction that has been a formidable challenge in the success of vertical GaN devices.

Design of 1.2 kV Power Switches With Low

Two novel gallium nitride-based vertical junction FETs (VJFETs), one with a vertical channel and the other with a lateral channel, are proposed, designed, and modeled to achieve a 1.2 kV normally OFF power switch with very low ON resistance (R_ON). The 2-D drift diffusion model of the proposed devices was implemented using Silvaco ATLAS. A comprehensive design space was generated for the vertical channel VJFET (VC-VJFET). For a well-designed VC-VJFET, the breakdown voltage (V_BR) obtained was 1260 V, which is defined in this study as the drain-to-source voltage at an OFF-state current of 1 μA · cm^-2 and a peak electric field not exceeding 2.4 MV/cm. The corresponding R_ON was 5.2 mΩ · cm². To further improve the switching device figure of merit, a merged lateral-vertical geometry was proposed and modeled in the form of a lateral channel VJFET (LC-VJFET). For the LC-VJFET, a breakdown voltage of 1310 V with a corresponding R_ON of 1.7 mQ · cm² was achieved for similar thicknesses of the drift region. This paper studies the design space in detail and discusses the associated tradeoffs in the R_ON and V_BR in conjunction with the threshold voltage (V_T) desired for the normally OFF operation.