There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

By doping an organic compound functioning as an electron donor (hereinafter referred to as donor molecules) into an organic compound layer contacting a cathode, donor levels can be formed between respective LUMO (lowest unoccupied molecular orbital) levels between the cathode and the organic compound layer, and therefore electrons can be injected from the cathode, and transmission of the injected electrons can be performed with good efficiency. Further, there are no problems such as excessive energy loss, deterioration of the organic compound layer itself, and the like accompanying electron movement, and therefore an increase in the electron injecting characteristics and a decrease in the driver voltage can both be achieved without depending on the work function of the cathode material.

Light Emitting Device

Vertical structured GaN power devices have recently been attracting a great interest because of their potential on extremely high-power conversion efficiency. This letter describes increased breakdown voltages in the vertical GaN p-n diodes fabricated on the free-standing GaN substrates. By applying multiple lightly Si doped n-GaN drift layers to the p-n diode, the record breakdown voltages ( $V_{B}$ ) of 4.7 kV combined with low specific differential ON-resistance ( $R_{\mathrm{\scriptscriptstyle ON}}$ ) of 1.7 $\text{m}\Omega $ cm2 were achieved. With reducing the Si-doping concentration of the top n-GaN drift layer adjacent to the p-n junction using well-controlled metal-organic vapor phase epitaxy systems, the peak electric field at the p-n junction could be suppressed under high negatively biased conditions. The second drift layer with a moderate doping concentration contributed to the low $R_{\mathrm{\scriptscriptstyle ON}}$ . A Baliga’s figure of merit ( $V_{B}^{2}/R_{\mathrm{\scriptscriptstyle ON}}$ ) was 13 GW/cm2. These are the best values ever reported among those achieved by GaN p-n junction diodes on the free-standing GaN substrates.

Vertical GaN p-n Junction Diodes With High Breakdown Voltages Over 4 kV

Highly scaled indium phosphide (InP) heterojunction bipolar transistor (HBT) technologies have been demonstrated with maximum frequencies of oscillation ( $f_{\max}$ ) of >1 THz and circuit operation has been extended into the lower end of the terahertz (THz) frequency band. InP HBTs offer high radio-frequency (RF) output power density, millivolt (mV) threshold uniformity, and high levels of integration. Integration with multilevel thin-film wiring permits the realization of compact and complex THz monolithic integrated circuits (TMICs). Circuit results reported from InP HBT technologies include: 200-mW power amplifiers at 210 GHz, 670-GHz amplifiers and fundamental oscillators, and fully integrated 600-GHz transmitter circuits. We review the state of the art in THz-capable InP HBT devices and integrated circuit (IC) technologies. Challenges in extending transistor bandwidth and in circuit design at THz frequencies will also be addressed.

InP HBT Technologies for THz Integrated Circuits

This paper reports the characteristics of vertical GaN-based trench metal–oxide–semiconductor field-effect transistors on a free-standing GaN substrate with a blocking voltage of 16 kV The high blocking voltage was obtained by using field plate edge termination around the isolation mesa of the transistor To our knowledge, the blocking voltage is the highest ever reported for vertical GaN-based transistors on free-standing GaN substrates Normally off operation with a threshold voltage of 7 V is also demonstrated

https://iopscience.iop.org/article/10.7567/APEX.7.021002/pdf

Vertical GaN-based trench metal oxide semiconductor field-effect transistors on a free-standing GaN substrate with blocking voltage of 1.6 kV

In this letter, we describe the characteristics of Gallium Nitride (GaN) p–n junction diodes fabricated on free-standing GaN substrates with low specific on-resistance Ron and high breakdown voltage VB. The breakdown voltage of the diodes with the field-plate (FP) structure was over 3 kV, and the leakage current was low, i.e., in the range of 10-4 A/cm2. The specific on-resistance of the diodes of 60 µm diameter with the FP structure was 0.9 mΩcm2. Baliga's figure of merit (VB2/Ron) of 10 GW/cm2 is obtained. Although a certain number of dislocations were included in the device, these excellent results indicated a definite availability of this material system for power-device applications.

https://iopscience.iop.org/article/10.7567/JJAP.52.028007/pdf

High-Breakdown-Voltage and Low-Specific-on-Resistance GaN p-n Junction Diodes on Free-Standing GaN Substrates Fabricated Through Low-Damage Field-Plate Process

A gas detection method capable of solving the problem with respect to the operation at normal temperature that was impossible so far in the existent catalyst type sensor and detection with high sensitivity that was impossible by the light absorption type sensor. A multi-layered film formed of a first layer adsorbing a specified gas and a second layer having less adsorption are utilized as a detection film, and the detection film is disposed in the direction perpendicular to the optical channel and optically detects the change of stress caused in the detection film by gas adsorption as coupling loss. Alternatively, the stress generated in the detection film caused by gas adsorption is electrically detected by a piezoelectric element or capacitance element.

Gas detecting method and gas sensors

The MEMS switch comprises a first anchor formed over a substrate, a first spring connected to the first anchor, an upper electrode which is connected to the first spring and makes a motion above the substrate, elastically deforming the first spring, a lower electrode formed over the substrate, positioned under the upper electrode, a second spring connected to the upper electrode, and a second anchor connected to the second spring. When voltage is applied between the upper and lower electrodes and the upper electrode makes a downward motion, the second anchor is brought into contact with the substrate. As a result, the second spring is elastically deformed. When the upper electrode is subsequently brought into contact with the lower electrode, thereby the upper and lower electrodes are electrically connected. The first and second anchors, first and second springs, and upper electrode are formed of identical metal in integral structure.

RF-MEMS switch and its fabrication method

The forward current (IF) of GaN p–n diodes forward biased at 5 V was found to be proportional to anode perimeter and independent of temperature (273–373 K). Although this IF characteristic contributes to reduction in specific on-resistance (RonA) of small GaN p–n diodes, the mechanism involved is yet to be understood. Accordingly, in this study, an optical-thermo-transition model for the reduced RonA in small GaN p–n diodes is proposed. The energy of photons that are created through radiative recombination around the edge of anode electrode is considered to be used for optical-thermo-transition; that is, electrons are emitted from ionized acceptors to conduction bands, and the resultant neutralized acceptors are ionized through electron capture from valence bands. Since known self-heating and intrinsic-photon-recycling effects are considered to be negligible, optical-thermo transition and enhanced radiative recombination (due to large perimeter-to-area ratio) are concluded to play a dominant role in reducing RonA.

Optical-Thermo-Transition Model of Reduction in On-Resistance of Small GaN p-n Diodes

Forward-current-density JF/forward-voltage VF characteristics of vertical gallium-nitride (GaN) Schottky-barrier diodes (SBDs) and p-n diodes on free-standing GaN substrates were computationally, as well as experimentally, investigated. Based on the thermionic emission model, electron-drift mobility μn was used as a parameter to fit the JF-VF characteristics of both reported and fabricated GaN SBDs. At 300 K, μn was fitted to be 600 cm2/V ·s when electron concentration n was 1 × 1016 cm-3 and 750 cm2/V ·s when n was 5 ×1015 cm-3. Accordingly, the theoretical μn-n curve for a carrier compensation ratio of 0.90 was applied in the case of n-GaN layers on GaN substrates. With respect to GaN p-n diodes, the reported JF -VF characteristics were successfully fitted with dislocation-mediated carrier lifetimes in the high-injection region and with Shockley-Read-Hall lifetimes in the generation-recombination current region.

Akihisa Terano

Papers

High-Breakdown-Voltage and Low-Specific-on-Resistance GaN p-n Junction Diodes on Free-Standing GaN Substrates Fabricated Through Low-Damage Field-Plate Process

Gas detecting method and gas sensors

RF-MEMS switch and its fabrication method

Optical-Thermo-Transition Model of Reduction in On-Resistance of Small GaN p-n Diodes

Numerical Analysis of Forward-Current/Voltage Characteristics of Vertical GaN Schottky-Barrier Diodes and p-n Diodes on Free-Standing GaN Substrates