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The relatively low breakdown voltage (BV) seriously restricts the high power application of GaN based high electron mobility transistors (HEMTs). In this work, a novel AlGaN/GaN HEMT with buried p-n junctions is investigated to improve the breakdown characteristics by introducing six equidistant p-GaN islands buried buffer layer (PIBL) into the n-GaN epitaxial layer. The p-GaN islands act as reversed p-n junctions, which produces new electric field peaks at the edges of p-GaN islands, then realizing a much high breakdown voltage, and the reversed p-n junctions can help to suppress punch-through effect in buffer layer. Furthermore, the characteristics of proposed device are analyzed in detail from the aspects of off-state I-V characteristics, equipotential contour distribution, off-state electric field distribution, offstate carrier distribution and output characteristics. Simulated equipotential contour distribution shows that under the condition of high-voltage blocking state, multiple reverse p-n junctions introduced by the buried p-GaN islands produce five new electric field peaks, realizing a more uniform equipotential contour distribution especially at the edges of the buried p-islands. Then off-state electric field distribution demonstrates that p-GaN islands modulate the surface and bulk electric fields, which makes the voltage distributed in a larger area, therefore presenting a much higher breakdown voltage. It can be seen from off-state carrier distribution that the electrons in the buffer layer fully depleted in PIBL HEMT effectively suppress the buffer leakage current, thus alleviating the buffer-leakage-induced impact ionization leading to a high breakdown BV of over 1700 V with gate-to-drain length of 10μm, which is nearly 3 times larger than BV of 580 V in conventional AlGaN/GaN HEMT. Although, the introduction of p-type buried layer narrows the current path and causes an improved on-resistance, simulation shows that the specific on-resistance (Ron,sp) of PIBL HEMT is only about 1.47 mΩ·cm2, while the BV of the PIBL device is over 1700 V, and the obtained figure of merit (FOM=BV2/Ron,sp) reaches as high as 1966 MW·cm-2. The optimization of device structure reveals that when the distance between p-GaN layer and AlGaN layer (t) is 0.2μm, a thinner buried p-GaN island (tp) should help to realize a more significant electric field modulation, and PIBL HEMT can achieve a maximum BV of 1789 V with a tp=0.1μm. Compared with the traditional AlGaN/GaN HEMT, the PIBL HEMT reveals a higher breakdown voltage, meanwhile ensuring low Ron,sp, which makes this structure a promising candidate in the applications of high power electronic devices.

Background: Premature rupture of membranes (PROM) may cause intra-uterine infection and fetal death. A diagnostic tool that is non-invasive, specific and quick is needed to predict PROM. Human chorionic gonadotrophin (hCG), α-fetoprotein (AFP) and interleukin 6 (IL-6), which are present in vaginal fluid, were reported to be potential markers for PROM but have not yet been used in the clinic. This study was designed in a daily routine semi-emergency setting to evaluate the clinical practicability of using these markers in diagnosing PROM. Methods: Using a random-access automated luminescence immunoassay system, 81 vaginal washing samples collected from third trimester pregnant women (43 from PROM patients, 38 from patients with intact membranes) were analysed for β-hCG, AFP and IL-6 in a semi-emergency setting. The Mann-Whitney U-test was used to test the difference between the two groups. Receiver operator curve (ROC) analysis was used to evaluate the performance of the three markers and to determine the cut-off value for a positive diagnosis. Results: Vaginal fluid concentrations of the three markers were significantly different ( P < 0.001) between the two groups. ROC analysis indicated that AFP had a 97.7% diagnostic sensitivity and 100% specificity; the other two markers had lower diagnostic sensitivity and specificity (95.3% and 89.5% for β-hCG and 83.7% and 78.9% for IL-6, respectively). Discussion: This work demonstrates that, of the three markers investigated, AFP has the highest diagnostic sensitivity and specificity. Using the 'stat' function provided by the automated luminescence immunoassay system, the reporting time of the results was less than 1h. We conclude that vaginal fluid AFP measured by random-access automated luminescence immunoassay is an ideal marker for the diagnosis of PROM. The technique could be introduced into the laboratory as a semi-emergency service to meet clinical needs.

First-order Raman scatterings of hexagonal GaN layers deposited by the hydride vapour phase epitaxy and by metal-organic chemical vapour deposition on SiC and sapphire substrates are studied in a temperature range between 303 K and 503 K. The temperature dependences of two GaN Raman modes (A1 (LO) and E2 (high)) are obtained. We focus our attention on the temperature dependence of E2 (high) mode and find that for different types of GaN epilayers their temperature dependences are somewhat different. We compare their differences and give them an explanation. The simplified formulas we obtained are in good accordance with experiment data. The results can be used to determine the temperature of a GaN sample.

Nonpolar (112?0) a-plane GaN films are grown by metal?organic chemical vapour deposition (MOCVD) on r-plane (11?02) sapphire. The samples are irradiated with neutrons under a dose of 1 ? 1015 cm?2. The surface morphology, the crystal defects and the optical properties of the samples before and after irradiation are analysed using atomic force microscopy (AFM), high resolution X-ray diffraction (HRXRD) and photoluminescence (PL). The AFM result shows deteriorated sample surface after the irradiation. Careful fitting of the XRD rocking curve is carried out to obtain the Lorentzian weight fraction. Broadening due to Lorentzian type is more obvious in the as-grown sample compared with that of the irradiated sample, indicating that more point defects appear in the irradiated sample. The variations of line width and intensity of the PL band edge emission peak are consistent with the XRD results. The activation energy decreases from 82.5 meV to 29.9 meV after irradiation by neutron.

Quality properties and internal defects of unintentionally doped GaN films grown on 0.3 vicinal sapphire (0001) substrates by MOCVD are investigated by TEM. The results show that plenty of dislocations in the GaN films prepared on vicinal sapphire substrates are annihilated in the areas with a distance of 0.8 m away from substrates, and that dislocations gather in the GaN films. Based on these phenomena, a mechanism for dislocation annihilation in the GaN film prepared on vicinal substrate is proposed, which is capable of explaining the fact that vicinal substrates are able to improve the qualities of GaN films.

We studied the performance of AlGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with an AlGaN buffer layer, which leads to a higher potential barrier at the backside of the two-dimensional electron gas channel and better carrier confinement. This, remarkably, reduces the drain leakage current and improves the device breakdown voltage. The breakdown voltage of AlGaN/GaN double heterojunction HEMTs (~100 V) was significantly improved compared to that of conventional AlGaN/GaN HEMTs (~50 V) for the device with gate dimensions of 0.5 × 100 μm and a gate—drain distance of 1 μm. The DH-HEMTs also demonstrated a maximum output power of 7.78 W/mm, a maximum power-added efficiency of 62.3% and a linear gain of 23 dB at the drain supply voltage of 35 V at 4 GHz.

The anisotropic strain of a nonpolar (110) a-plane GaN epilayer on an r-plane (102) sapphire substrate, grown by low-pressure metal-organic vapour deposition is investigated by Raman spectroscopy. The room-temperature Raman scattering spectra of nonpolar a-plane GaN are measured in surface and edge backscattering geometries. The lattice is contracted in both the c- and the m-axis directions, and the stress in the m-axis direction is larger than that in the c-axis direction. On the surface of this sample, a number of cracks appear only along the m-axis, which is confirmed by the scanning electron micrograph. Atomic force microscopy images reveal a significant decrease in the root-mean-square roughness and the density of submicron pits after the stress relief.