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Ultrawide bandgap Al0.7Ga0.3N MESFETs with refractory Tungsten Schottky and Ohmic contacts are studied in 300–675 K environments. Variable-temperature dc electrical transport reveals large ON-state drain current densities for an AlGaN device: 209 mA/mm at 300 K and 156 mA/mm at 675 K in the ON-state (25% reduction). Drain and gate currents are only weakly temperature-dependent, suggesting potential for engineering temperature invariant operation. The ON-/ OFF-ratio is limited by OFF-state leakage through the gate, which is attributed to damage from sputter deposition. Future work using refractory metals with larger work functions that are deposited by electron beam deposition is proposed.

Advanced GaN power devices are promising for many applications in high power electronics but performance limitations due to material quality in etched-and-regrown junctions prevent their widespread use. Carrier diffusion length is a critical parameter that not only determines device performance but is also a diagnostic of material quality. Here we present the use of electron-beam induced current to measure carrier diffusion lengths in continuously grown and etched-and-regrown GaN pin diodes as models for interfaces in more complex devices. Variations in the quality of the etched-and-regrown junctions are observed and shown to be due to the degradation of the n-type material. We observe an etched-and-regrown junction with properties comparable to a continuously grown junction.

Etched-and-regrown GaN pn-diodes capable of high breakdown voltage (1610 V), low reverse current leakage (1 nA = 6 $\mu \text{A}$ /cm2 at 1250 V), excellent forward characteristics (ideality factor ~1.6), and low specific on-resistance (1.1 $\text{m}\Omega $ .cm2) were realized by mitigating plasma etch-related defects at the regrown interface. Epitaxial ${n}$ -GaN layers grown by metal-organic chemical vapor deposition on free-standing GaN substrates were etched using inductively coupled plasma etching (ICP), and we demonstrate that a slow reactive ion etch (RIE) prior to ${p}$ -GaN regrowth dramatically increases diode electrical performance compared to wet chemical surface treatments. Etched-and-regrown diodes without a junction termination extension (JTE) were characterized to compare diode performance using the post-ICP RIE method with prior studies of other post-ICP treatments. Then, etched-and-regrown diodes using the post-ICP RIE etch steps prior to regrowth were fabricated with a multi-step JTE to demonstrate kV-class operation.

Aluminum gallium nitride (AlGaN) high electron mobility transistors (HEMTs) are candidates for next-generation power conversion and radio frequency (RF) applications. AlxGa1-xN channel HEMT devices (x = 0.3, x = 0.7) were investigated using multiple in-situ thermal characterization methods and electro-thermal simulation. The thermal conductivity, contact resistivity, and channel mobility were characterized as a function of temperature to understand and compare the heat generation profile and electro-thermal transport within these devices. In contrast to GaN-based HEMTs, the electrical output characteristics of Al0.70Ga0.30 N channel HEMTs exhibit remarkably lower sensitivity to the ambient temperature rise. Also, during 10kHz pulsed operation, the difference in peak temperature between the AlGaN channel HEMTs and GaN HEMTs reduced significantly.

We report high-voltage regrown nonpolar ${m}$ -plane p-n diodes on freestanding GaN substrates. A high blocking voltage of 540 V at ~1 mA/cm $^{\textsf {2}}$ (corresponding to an electric field of E ~ 3.35 MV/cm), turn- ON voltages between 2.9 and 3.1 V, specific on-resistance of 1.7 $\text{m}\Omega \cdot \textsf {cm}^{\textsf {2}}$ at 300 A/cm $^{\textsf {2}}$ , and a minimum ideality factor of 1.7 were obtained for the regrown diodes. Our results suggest that Si, O, and C interfacial impurity levels up to $\textsf {2}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , $\textsf {8}\times \textsf {10}^{\textsf {17}}$ cm $^{-\textsf {3}}$ , and $\textsf {1}\times \textsf {10}^{\textsf {19}}$ cm $^{-\textsf {3}}$ , respectively, at the metallurgical junction of ${m}$ -plane, p-n diodes do not result in very early breakdown in the reverse bias although the off-state leakage current in the forward bias is affected. The impact of the growth interruption/regrowth on diode performance is also investigated.

We present heavy ion and proton data on AlGaN high-voltage HEMTs showing single event burnout (SEB), total ionizing dose, and displacement damage responses. These are the first such data for materials of this type. Two different designs of the epitaxial structure were tested for SEB. The default layout design showed burnout voltages that decreased rapidly with increasing LET, falling to about 25% of nominal breakdown voltage for ions with LET of about 34 MeV $\cdot $ cm2/mg for both structures. Samples of the device structure with lower AlN content were tested with varying gate–drain spacing and revealed an improved robustness to heavy ions, resulting in burnout voltages that did not decrease up to at least 33.9 MeV $\cdot $ cm2/mg. Failure analysis showed that there was consistently a point, location random, where gate and drain had been shorted. Oscilloscope traces of terminal voltages and currents during burnout events lend support to the hypothesis that burnout events begin with a heavy ion strike in the vulnerable region between gate and drain. This subsequently initiates a cascade of events resulting in damage that is largely manifested elsewhere in the device. This hypothesis also suggests a path for greatly improving the susceptibility to SEB as development of this technology goes forward. Testing with 2.5-MeV protons showed only minor changes in device characteristics.

We report on ultra-wide bandgap (UWBG) Al0.7Ga0.3N channel metal–oxide–semiconductor field-effect transistors (MOSFETs) grown by metal-organic chemical vapor deposition. Employing reverse Al composition graded ohmic contact layers and 20 nm Al2O3 gate-dielectric, 250 nm thick Al0.7Ga0.3N:Si channel MOSFETs resulted in the maximum current density of 0.5 A/mm, which is the highest value reported for AlGaN channels with Al composition >0.25. Transistors with a gate-drain spacing ( $L_{\mathrm{ GD}})$ of $1.7~\mu \text{m}$ demonstrated a breakdown voltage ( $V_{\mathrm{ DG}})$ of ~620 V, translating in an average lateral breakdown field of ~3.6 MV/cm. This work establishes UWBG AlGaN as a promising candidate for advanced RF applications.

Devices based on GaN have shown great promise for high power electronics, including their potential use as radiation tolerant components. An important step to realizing high power diodes is the design and implementation of an edge termination to mitigate field crowding, which can lead to premature breakdown. However, little is known about the effects of radiation on edge termination functionality. We experimentally examine the effects of proton irradiation on multiple field ring edge terminations in high power vertical GaN PIN diodes using in operando imaging with electron beam induced current (EBIC). We find that exposure to proton irradiation influences field spreading in the edge termination as well as carrier transport near the anode. By using depth-dependent EBIC measurements of hole diffusion length in homoepitaxial n-GaN we demonstrate that the carrier transport effect is due to a reduction in hole diffusion length following proton irradiation.

Simulations of reverse breakdown behavior of GaN power diodes with junction termination extensions (JTEs) are presented. The p-type JTE is located at the edge of the main p-n-junction, and under reverse bias, the charge in the JTE causes spreading and reduction of the peak electric fields to avoid premature avalanche breakdown. To determine the available charge in the JTE, it is shown that the electric field under reverse bias causes severe band bending within the JTE and full ionization of the Mg acceptor. Therefore, all the Mg dopants contribute charge and determine the performance of the JTE. The dependence of the breakdown voltage on the JTE’s acceptor concentration and thickness is shown. When the JTE is properly designed, the simulations show improved reverse breakdown behavior and breakdown efficiencies approaching 98% of the ideal limit for planar geometry. Finally, the challenges of creating JTEs within GaN power diodes are discussed.

Vertical GaN power diodes with a bilayer edge termination (ET) are demonstrated. The GaN p-n junction is formed on a low threading dislocation defect density ( $10^{4}-10^{5}$ cm $^{-2}$ ) GaN substrate, and has a 15- $\mu \text{m}$ -thick n-type drift layer with a free carrier concentration of $5\times 10^{15}$ cm $^{-3}$ . The ET structure is formed by $N$ implantation into the p+-GaN epilayer just outside the p-type contact to create compensating defects. The implant defect profile may be approximated by a bilayer structure consisting of a fully compensated layer near the surface, followed by a 90% compensated (p) layer near the n-type drift region. These devices exhibit avalanche breakdown as high as 2.6 kV at room temperature. Simulations show that the ET created by implantation is an effective way to laterally distribute the electric field over a large area. This increases the voltage at which impact ionization occurs and leads to the observed higher breakdown voltages.

Electrical performance and defect characterization of vertical GaN P-i-N diodes before and after irradiation with 2.5 MeV protons and neutrons is investigated. Devices exhibit increase in specific on-resistance following irradiation with protons and neutrons, indicating displacement damage introduces defects into the p-GaN and n- drift regions of the device that impact on-state device performance. The breakdown voltage of these devices, initially above 1700 V, is observed to decrease only slightly for particle fluence

Al-rich AlGaN-channel high electron mobility transistors with 80-nm long gates and 85% (70%) Al in the barrier (channel) were evaluated for RF performance. The dc characteristics include a maximum current of 160 mA/mm with a transconductance of 24 mS/mm, limited by source and drain contacts, and an on/off current ratio of $10^{{9}}$ . ${f}_{T}$ of 28.4 GHz and ${f}_{\text {MAX}}$ of 18.5 GHz were determined from small-signal S-parameter measurements. Output power density of 0.38 W/mm was realized at 3 GHz in a power sweep using on-wafer load pull techniques.

The temperature dependence of threshold current Ith in vertical-cavity surface-emitting lasers (VCSELs) can be approximated by the equation Ith(T)=alpha+beta(T-Tmin)2 , where Tmin is the temperature of lowest Ith,alpha and beta are parameters, and temperature is T. We compare the temperature dependence of threshold current in VCSELs with GaAs, InGaAs, and strain compensated InGaAs-GaAsP quantum wells. From our analysis we find the coefficient beta is related to the gain properties of the quantum well, and is shown to serve as a benchmark for the VCSEL temperature sensitivity. The incorporation of strain-compensated high-barrier GaAsP layers in the active region of 980-nm VCSELs is demonstrated to reduce the threshold dependence on temperature

We have measured on picosecond time scales the polarization behavior of vertical-cavity surface-emitting lasers (VCSELs) pumped with short optical pulses. Our data describe the temporal properties of the intensity and intensity noise for orthogonally polarized modes as well as noise correlations between the modes in both the circular and the linear detection bases. A variety of behavior is observed that depends on the polarization of the pump, the strength of the pump, the VCSEL operating temperature which determines the carrier spin-flip rates, and the degree of birefringence and dichroism in the laser. We develop an extension of the "split density" model which includes carrier relaxation processes that are important on picosecond time scales. Our numerical simulations illustrate how spin-flip processes lead to anti-correlated fluctuations in the circularly polarized modes and how anti-correlated noise can arise between the linearly polarized modes without switching between two "preferred" modes, as is commonly seen on longer time scales.

The wavelength splitting between the LP/sub 01/ and LP/sub 11/ modes of selectively oxidized, ion implanted, and hybrid ion implanted/selectively oxidized vertical-cavity surface-emitting lasers is studied by experiment and theory. Measured splittings at threshold show marked differences between the different laser structures due to the effects of index guiding and thermal lensing. Theoretical results were obtained using a vector optical mode solver and show good agreement with experimental results. The hybrid lasers exhibited behavior intermediate between the ion implanted and selectively oxidized lasers and could be optimized for high power single transverse mode emission.

Massively parallel interconnects and scannerless imaging are applications that would benefit from high-density two-dimensional arrays of lasers. Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited for these applications due to their small size and high efficiency. We have successfully fabricated 64 /spl times/ 64 element arrays containing alternating rows of selectively-oxidized 850 nm VCSELs and resonant-cavity photodetectors (RCPDs) monolithically integrated on semi-insulating GaAs substrates. In order to reduce the input and output connections to the array, we employ a matrix addressable architecture, where all the VCSELs (or RCPDs) in each row are connected by a common metal trace at the base of their mesas. The columns are connected by metal traces that bridge from mesa top to mesa top, connecting every other row (i.e., only VCSELs or only RCPDs). The pitch of devices in the array is 55 /spl mu/m, and the total resistance contributed by the long (up to 3.5 mm) row and column traces is below 50 /spl Omega/. The design, fabrication, and performance of these arrays are discussed.

Two custom GaAs integrated circuits (ICs) have been developed for enabling vertical cavity surface emitting laser (VCSEL) arrays to be used for high throughput spatial division multiplexed (SDM) optical data links. A 16-channel driver IC was developed to drive the VCSEL array and an 8/spl times/8 monolithic photoreceiver, which spatially matches the VCSEL array, was developed for receive. Both of these circuits were fabricated in a standard commercial GaAs MESFET process with parasitic photodetectors used for the photoreceivers. Power dissipation and circuit size were primary design challenges for both circuits. The present 8/spl times/8 array size along with an estimated usable channel speed of 1 Gb/s allows for an aggregate throughput of 64 Gb/s.

We report the nondestructive, optical characterization of a heterojunction bipolar transistor (HBT) structure using photoreflectance spectroscopy (PR) and spectral ellipsometry (SE). The PR results show good agreement with capacitance-voltage (C-V) measurements for the dopant concentration in the n-GaAs collector layer. We find that PR provides a reasonable N/sub d/ for the emitter layer; whereas C-V analysis of the thin emitter layer provides only an upper-limit to the dopant concentration, PR provides a useful means of obtaining emitter and collector dopant concentrations quickly and nondestructively. We have developed a detailed SE model of the HBT structure and applied this model to obtain alloy compositions and layer thicknesses for the upper five of the nine HBT layers. Combining the structure and dopant concentration data obtained from critical layers of the HBT, we can screen the epitaxial material for potentially harmful structural deviations.

Control of electric fields with edge terminations is critical to maximize the performance of high-power electronic devices. While a variety of edge termination designs have been proposed, the optimization of such designs is challenging due to many parameters that impact their effectiveness. While modeling has recently allowed new insight into the detailed workings of edge terminations, the experimental verification of the design effectiveness is usually done through indirect means, such as the impact on breakdown voltages. In this letter, we use scanning photocurrent microscopy to spatially map the electric fields in vertical GaN p-n junction diodes in operando. We reveal the complex behavior of seemingly simple edge termination designs, and show how the device breakdown voltage correlates with the electric field behavior. Modeling suggests that an incomplete compensation of the p-type layer in the edge termination creates a bilayer structure that leads to these effects, with variations that significantly impact the breakdown voltage.

Using a hybrid ion implanted/selectively oxidized device structure, we report high-power single-mode operation of an 850-nm vertical-cavity laser. Under continuous-wave operation, >4 mW of single-mode power with 45 dB of side-mode suppression is achieved. The spectral behavior under pulsed modulation is determined to be influenced by thermal lensing. When biased to threshold, single-mode operation with >35-dB side-mode suppression is obtained for large signal modulation.

A wide-aperture vertical-cavity surface-emitting laser has been fabricated so as to enhance the transverse mode selection. It was found that the far field emission can be made to exhibit two distinct steerable beams. The laser beams may be steered from 30/spl deg/ away from the normal right up to normal emission by varying the driving current. The steering effect is a thermal effect as confirmed by transient waveform analysis. A two-channel switch was built with switching time between channels of 10 /spl mu/s and crosstalk of greater than 18 dB.

Efficient continuous wave operation of oxide-confined double intracavity-contacted InGaAs-GaAs vertical-cavity surface-emitting lasers (VCSEL's) with low-threshold voltage, low-threshold current and low-power dissipation has been achieved over a wide range of cryogenic temperatures (77 K-250 K). Low operating voltages were obtained by routing current through two intracavity contacts to bypass both distributed Bragg reflector (DBR) mirrors, while lower optical losses were achieved by using undoped DBR mirrors with abrupt heterointerfaces. This resulted in low operating voltages (

We demonstrate the high-speed (gigabit-per-second) operation of a wavelength-division-multiplexed optical interconnect, which is implemented by multiplexing the optical data from a multiple-wavelength vertical-cavity surface-emitting laser (VCSEL) array into a single optical fiber, and demultiplexing the composite data stream using an array of resonance-enhanced photodetectors (REPDs) with matching resonance wavelengths. By using VCSELs and REPDs with a new quasi-planar oxide-confinement design for improved high-speed performance, and using strained InGaAs-GaAs quantum wells to achieve a better tradeoff between optical responsivity and wavelength selectivity, wavelength-division-multiplexing (WDM) operation has been demonstrated under 1-Gb/s data modulation, with an optical crosstalk rejection ratio of better than -10 dB for wavelength channels that are spaced 4 nm apart.

By employing a reactive low-temperature wafer bonding technique, we have demonstrated oxide-defined 850 nm vertical-cavity surface-emitting lasers (VCSEL's) on Si substrates. Devices reach a differential quantum efficiency of 53% and a light output power of 7.1 mW under room temperature and continuous-wave operation without a heat sink.