P. M. Bridger

High voltage (450 V) GaN Schottky rectifiers

We fabricated high standoff voltage (450 V) Schottky rectifiers on hydride vapor phase epitaxy grown GaN on sapphire substrate. Several Schottky device geometries were investigated, including lateral geometry with rectangular and circular contacts, mesa devices, and Schottky metal field plate overlapping a SiO2 layer. The best devices were characterized by an ON-state voltage of 4.2 V at a current density of 100 A/cm2 and a saturation current density of 10^–5 A/cm2 at a reverse bias of 100 V. From the measured breakdown voltage we estimated the critical field for electric breakdown in GaN to be (2.2 ± 0.7) × 10^6 V/cm. This value for the critical field is a lower limit since most of the devices exhibited abrupt and premature breakdown associated with corner and edge effects.

The values of minority carrier diffusion lengths and lifetimes in GaN and their implications for bipolar devices

Abstract The wide bandgap semiconductors GaN and AlGaN show promise as the high voltage standoff layers in high power heterostructure bipolar transistors and thyristors due to their electric breakdown characteristics. Material properties which significantly influence the design and performance of these devices are electron and hole diffusion lengths and recombination lifetimes. We report direct measurements of minority carrier diffusion lengths for both holes and electrons by electron beam induced current. For planar Schottky diodes on unintentionally doped n-type and p-type GaN grown by metal organic vapor phase deposition (MOCVD), the diffusion lengths were found to be (0.28±0.03) μm for holes and (0.2±0.05) μm for electrons. Minority carrier lifetimes of approximately 7 ns for holes and 0.1 ns for electrons were estimated from these measured diffusion lengths and mobilities. In the case of GaN grown by halide vapor phase epitaxy (HVPE) diffusion lengths in the 1–2 μm range were found. We attempt to correlate the measured diffusion lengths and lifetimes with the structural properties of GaN and to explain why linear dislocations might act as a recombination centers. We calculate the performance of nitride based bipolar devices, in particular thyristor switches. The forward voltage drop across standoff layer of the nitride based thyristor switch is shown to significantly depend on the minority carrier (hole) lifetime.

Charge storage in CeO2/Si/CeO2/Si(111) structures by electrostatic force microscopy

An electrostatic force microscope was used to write and image localized dots of charge in a double barrier CeO2/Si/CeO2/Si(111) structure. By applying a relatively large tip voltage and reducing the tip to sample separation to 3–5 nm, charge dots 60–200 nm full width at half maximum of both positive and negative charge have been written. The total stored charge is found to be Q = ±(20–200)e per charge dot. These dots of charge are shown to be stable over periods of time greater than 24 h, with an initial charge decay time constant of tau ~ 9.5 h followed by a period of much slower decay with tau > 24 h. The dependence of dot size and total stored charge on various writing parameters such as tip writing bias, tip to sample separation, and write time is examined.

Nitride Based High Power Devices: Transport Properties, Linear Defects And Goals

The wide bandgap semiconductors GaN and AlGaN show promise for high voltage standoff layers in high power devices such as GaN Schottky rectifiers and GaN/AlGaN thyristorlike switches. The material properties which significantly influence the device design and performance are electron and hole diffusion lengths, recombination lifetimes and the critical field for electric breakdown. We have fabricated high standoff voltage (> 450 V) GaN Schot-tky rectifiers, and measured a lower limit for the critical field for electric breakdown to be (2 ± 0.5) · 10 6 V/cm. Diffusion lengths and recombination lifetimes were measured by electron beam induced current on unintentionally doped, n and p-type GaN samples grown by various epitaxial techniques. To establish the possible effects of linear dislocations and other defects on the transport and breakdown properties, the same sample surfaces were analyzed by AFM. On some of the samples, our measurements indicate that the dislocations appear to be electrically active and that recombination at dislocations occupying grain boundaries limit the minority carrier lifetime to the nanosecond range. Based on the measurements of transport properties, critical fields and the modeling of the devices proposed, our estimates indicate that DARPA/EPRI goals for megawatt electronics set at 5 kV standoff voltage and 200 A on-state current might be achieved with 15 – 20 μm thick layers grown by HVPE, at approximately 1. 10 16 cm −3 doping levels, and 1 – 2cm 2 device active area.

Silicon nanoelectronics: prospects and promises

It is widely recognized that the holy grail for nanoelectronics is a technology that is compatible with standard silicon. We review the current prospects for the development of such a technology. We will discuss the current prospects for Si based heterojunctions including SiGeC, CaF/sub 2/, CeO/sub 2/, SiO/sub 2/ and ZnS to name just a few. Further, we review the status of one device structures, the tunnel switched diode, which can currently be deployed in a number of applications.