Study of the leakage current suppression for hybrid-Schottky/ohmic drain AlGaN/GaN HEMT

The leakage current suppression mechanism in AlGaN/GaN High Electron Mobility Transistors (HEMTs) is investigated. It is known that leakage current can cause severe reliability problems for HEMT devices and conventional AlGaN/GaN HEMT devices suffer from detrimental off-state drain leakage current issues, especially under high off-state drain bias. Therefore, a leakage current suppression technique featuring hybrid-Schottky/ohmic-drain contact is discussed. Through the 2-zones leakage current suppression mechanism by the hybrid-Schottky/drain metal including the shielding effect of the rough ohmic-drain metal morphology and the drain side electric field modulation, AlGaN/GaN HEMT featuring this novel technique can significantly enhance the leakage current suppression capability and improve the breakdown voltage. An analytical method using loop-voltage-scanning is proposed to illustrate the optimization procedure of the hybrid-Schottky/ohmic drain metallization on leakage current suppression. Through the comparison of the loop leakage current hysteresis of conventional ohmic drain HEMT and hybrid-Schottky/ohmic drain, the leakage current suppression mechanism is verified through the leakage current considering surface acceptor-like trap charging/discharging model. Device featuring the hybrid-Schottky/ohmic drain technique shows an improvement in breakdown voltage from 450 V (with no Schottky drain metal) to 855 V with a total drift region length of 9 μm, indicating enhanced off-state reliability characteristics for the AlGaN/GaN HEMT devices.     

Blue/green luminescence based on Zn(S)Se/GaAs heterostructures

Substantial advances have been realized in the aim to achieve blue–green light emitting devices based on Zn(S)Se wide band gap II–VI semi-conductor materials. Two light emitting diodes p on n and n on p heterostructures were grown on GaAs substrate by molecular beam epitaxy. The active layer was a single ZnCdSe quantum well, with ZnSSe guiding layers and ZnSe cladding layers. p-GaInP, p-AlGaAs and p-CdZnSe buffer layers were deposited at the p-ZnSe/GaAs interface to reduce the valence band offset in the case of n on p heterostructures. Electrical and optical properties were investigated using current voltage, capacitance voltage, electroluminescence, photoluminescence and photocurrent measurements at room temperature. Blue–green luminescence centered at 516.7 nm is observed. The highest luminescence intensity is observed under 7 V forward bias. Photoluminescence spectrum shows two wide peaks at 2.2 and 1.9 eV energies. These energies are attributed to the transitions between ZnSe and GaAs conduction bands and the deep level at E_v−0.6 eV. Absorption process from ZnSe and ZnSSe conduction bands to the shallow nitrogen acceptor level (2.6 and 2.8 eV, respectively) have been observed using photocurrent measurements. From these results we present a band alignment diagram which confirms the presence of the two levels at 0.1 and 0.6 eV from the valence band of ZnSe.     

Influence of AlGaN deep level defects on AlGaN/GaN 2-DEG carrierconfinement

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures and to correlate their effect on two-dimensional electron gas (2-DEG) confinement. We investigated AlGaN/GaN heterostructures with different electrical properties using incident electron beam energies of 0.5 to 15 keV to probe electronic state transitions within each of the heterostructure layers. AlGaN heterostructures of 25 nm thickness and nominal 30% Al concentration grown on GaN buffer layers on sapphire substrates by plasma-assisted molecular beam epitaxy exhibited a range of polarization-induced electron densities and room temperature mobilities. In general, the spectra exhibit AlGaN band edge emission at ~3.8 eV or ~4.0 eV, GaN band edge emission at ~3.4 eV, yellow luminescence (YL) features at 2.18 eV and 2.34 eV, and a large emission in the infrared (<1.6 eV) from the GaN cap layer used to passivate the AlGaN outer surface. These heterostructures also show high strain in the 2 nm-thick GaN layer with evidence for a Franz-Keldysh red shift due to piezoelectric charging. The LEEN depth profiles reveal differences between the structures with and without 2-DEG confinement and highlight the importance of AlGaN defects in the near 2-DEG region     

Defect characterization of Tl4GaIn3Se2S6 layered single crystals by photoluminescence

Photoluminescence (PL) spectra of Tl₄GaIn₃Se₂S₆ layered crystals grown by the Bridgman method have been studied in the energy region of 2.02–2.35 eV and in the temperature range of 16–45 K. A broad PL band centered at 2.20 eV was observed at T=16 K. Variations of emission band has been studied as a function of excitation laser intensity in the 0.1 to 149.9 mW cm⁻² range. Radiative transitions from shallow donor level located at 10 meV below the bottom of conduction band to moderately deep acceptor level located at 180 meV above the top of the valence band were suggested to be responsible for the observed PL band. An energy level diagram showing transitions in the band gap of the crystal was plotted taking into account the results of present work and previously reported paper on thermally stimulated current measurements carried out below room temperature. Analysis of the transmission and reflection measurements performed in the wavelength range of 400–1030 nm at room temperature revealed the presence of indirect transitions with 2.22 eV band gap energy.     

2 MeV ion irradiation effects on AlGaN/GaN HFET devices

AlGaN/GaN heterostructure field effect transistors (HFETs) were irradiated with 2 MeV protons, carbon, oxygen, iron and krypton ions with fluences ranging from 1 × 10⁹ cm^?2 to 1 × 10¹³ cm^?2. DC, pulsed I–V characteristics, loadpull and S-parameters of the AlGaN HFET devices were measured before and after irradiation. In parallel, a thick GaN reference layer was also irradiated with the same ions and was characterized by X-ray diffraction, photoluminescence, Hall measurements before and after irradiation. Small changes in the device performance were observed after irradiation with carbon and oxygen at a fluence of 5 × 10¹⁰ cm^?2. Remarkable changes in device characteristics were seen at a fluence of 1 × 10¹² cm^?2 for carbon, oxygen, iron and krypton irradiation. Similarly, remarkable changes were also observed in the GaN layer for irradiations with fluence of 1 × 10¹² cm^?2. The results found on devices and on the GaN layer were compared and correlated.     

Critical design issues for high-power GaN/AlGaN anti-serial Schottky varactor frequency triplers

In this paper the importance of a new design variable for high power anti-serial Schottky varactors, the aluminum composition of the AlGaN barrier layer, is demonstrated. AlGaN/GaN varactors containing either (1) a high-doped/low-doped GaN region or (2) just a low doped GaN region have been compared demonstrating that the selection of the device structure also depends on the amplitude of the input signal being tripled in frequency. Stronger susceptance modulation is exhibited in AlGaN/GaN ASVs made from Ga-face polar material compared to N-face polar material. Results indicate choosing the proper aluminum composition results in 27% conversion efficiency with an input signal of 5 GHz and over 7% conversion efficiency with an input signal of 60 GHz along with optimization trends. With input voltage amplitudes over 10 V an AlGaN/GaN structure with 15% Al provides greater conversion efficiency than one with 5% Al. Power absorbed in the varactor also increases as aluminum percent increases affecting reliability and power transfer. Results of a GaN ASV performing as a frequency tripler for fundamental frequencies up to 110 GHz indicate an advantage to using an AlGaN/GaN epi-structure over only a GaN epi-structure.     

Study on mobility enhancement in MOVPE-grown AlGaN/AlN/GaN HEMT structures using a thin AlN interfacial layer

Al_0.26Ga_0.74N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AlN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (μ_Hall) of 1770 cm²/Vs with a low sheet resistance (ρ_s) of 365 Ω/sq. (2DEG density n_s = 1.0 × 10¹³/cm²) at room temperature compared with those of a sample without the AlN interfacial layer (μ_Hall = 1287 cm²/Vs, ρ_s = 539 Ω/sq., and n_s = 0.9 × 10¹³/cm²). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs.     

Ⅲ族氮化物外延层中的缺陷

采用光荧光和阴极荧光方法 ,对 Ga N外延层中的黄色和蓝色发光进行测量分析 ;同时 ,采用原子力显微镜、扫描电镜及其能谱测量外延层中的缺陷。结果表明 ,黄色和蓝色发光与残留杂质有关。采用第一原理计算结果显示 ,残留 C、O杂质、本征缺陷等是黄色和蓝色的可能物理起源。采用原子力显微镜、扫描电镜、透射电镜及其能谱对 Ga N/Al Ga N异质结中的纳米管进行观测 ,了解了纳米管的形貌。结果表明 ,构成纳米管的小面可能是外延过程中表面吸附引起的 ;计算结果显示 ,纳米管形貌变化与 Ga N/Al Ga N界面处晶格失配应力有关。采用透射电镜观察外延层中沉积物及其周围位错的结构表明 ,沉积物附近应力的存在是位错产生的主要原因     

A comparative study of electrical and optical properties of InN and In0.48Ga0.52N

We present results of our studies concerning electrical and optical properties of In_0.48Ga_0.52N and InN. Hall measurement were carried out at temperatures between T=77 and 300 K. Photoluminescence (PL) spectrum in InN and In_0.48Ga_0.52N. InN has a single peak at 0.77 eV at 300 K. However, the PL in In_0.48Ga_0.52N has two peaks; a prominent peak at 1.16 eV and a smaller peak at 1.55 eV. These two peaks are attributed to Indium segregation corresponding to a high Indium concentration of 48% and a low concentration of 36%. High electric field measurements indicate that drift velocity that tends to saturate at around V_d=1.0×10⁷ cm/s at 77 K in InN at an electric field of F=12 kV/cm. However, in In_0.48Ga_0.52N the I–V curve is almost linear up to an electric field of F=45 kV/cm, where the drift velocity is V_d=1.39×10⁶ cm/s. At applied electric fields above this value a S-type negative differential resistance (NDR) is observed leading to an instability in the current and to the irreversible destruction of the sample.     

Electron transport in passivated AlGaN/GaN/Si HEMTs

AlGaN/GaN/Si high electron mobility transistors (HEMTs) grown by molecular beam epitaxy are investigated using direct-current and radio-frequency measurements. As has been found, the maximum of drain current achieves 881 mA/mm with an extrinsic current gain cutoff frequency of 37 GHz for a 0.25 µm gate length. Pulsed characteristics also showed a reduction of trapping centers that improves the quality of the epilayers.     

Role of barrier and buffer layer defect states in AlGaN/GaN HEMT structures

We have used low energy electron-excited nanoscale luminescence spectroscopy (LEEN) to detect the defects in each layer of AlGaN/GaN HEMT device structures to correlate their effect on two-dimensional electron gas (2DEG) confinement. Also, we have used Auger electron spectroscopy (AES) to detect the chemical composition as a function of lateral position on a growth wafer and to correlate chemical effects with electronic properties. We investigated several high-quality AlGaN/GaN heterostructures of varying electrical properties using incident electron beam energies of 0.5–15 keV to probe electronic state transitions within each of the heterostructure layers. The LEEN depth profiles reveal differences between sucessful and failed structures and highlight the importance of acceptor deep defect levels in the near 2DEG region. Variations in the GaN and AlGaN band edge emissions, as well as the yellow defect emission across an AlGaN/GaN heterostructure growth wafer have been observed. AES and LEEN spectroscopy of the growth wafer suggest that variation in the cation concentration may play a role in the mechanism responsible for the deep aceceptor level emission in the AlGaN barrier layer.     

Giant piezoelectric effect in GaN self-assembled quantum dots

We observe in strained GaN self-assembled quantum dots grown on an AlN layer, a dramatic modification of the optical emission spectra as the dot size varies. In “large” quantum dots with an average height of 4.1 nm, the photoluminescence (PL) peak is centered at 2.95 eV, nearly 0.5 eV below the bulk GaN bandgap. We attribute this enormous redshift to a giant 5.5 MV cm⁻¹ piezoelectric field present in our dots. Temperature-dependent PL studies reveal the strongly zero-dimensional character of this QD system and are consistent with an intrinsic PL mechanism.     

The excellent spontaneous ultraviolet emission of GaN nanostructures grown on silicon substrates by thermal vapor deposition

In this study, GaN nanostructures were grown on p-Si (111) substrate by thermal chemical vapor deposition (TCVD). Ga vapor directly reacted with NH₃ solution in N₂ carrier gas flow of 2 L/min at different temperatures (950–1050 °C). The influence of NH₃ solution and growth temperature on the morphology, structure, optical and photoresponse properties of GaN nanostructures was investigated. Scanning electron microscopy images showed that the densities of the NWs varied with increasing temperature. The use of NH₃ solution and increased growth temperature improved the crystalline quality of GaN nanostructures. The photoluminescence (PL) spectra of nanostructures displayed a near band-edge (NBE) emission at around 363–367 nm. Higher growth temperature (1050 °C) resulted in a strong NBE emission with no yellow emission peak. With +5 V applied bias, the NWs metal–semiconductor–metal UV photodetector exhibited a high photocurrent of 1.6×10⁻³ A. The photocurrent to dark current contrast ratio was 120.     

Drain-to-gate field engineering for improved frequency response of GaN-based HEMTs

We report on a novel approach for designing high-frequency AlGaN/GaN HEMTs based on gate-drain field engineering. This approach uses a drain-connected field controlling electrode (FCE). The devices with gate-to-FCE separation of 0.5–0.7 μm exhibit much smaller frequency behavior degradation with drain bias at least up to 30 V and yield RF gain and output power improvement up to ～2 times compared to conventional devices. These results show that the FCE is a powerful technique of improving the high-frequency, high power performance of GaN HEMTs at high drain biases.     

Determination of the average channel temperature of GaN MOSHFETs under continuous wave and periodic-pulsed RF operational conditions

We present a method to determine the average device channel temperature of AlGaN/GaN metal–oxide–semiconductor heterostructure field effect transistors (MOSHFETs) in the time domain under continuous wave (CW) and periodic-pulsed RF (radiation frequency) operational conditions. The temporal profiles of microwave output power densities of GaN MOSHFETs were measured at 2 GHz under such conditions and used for determination of the average channel temperature. The measurement technique in this work is also being utilized to determine the thermal time constant of the devices. Analytical temporal solutions of temperature profile in MOSHFETs are provided to support the method. The analytical solutions can also apply to generic field effect transistors (FETs) with an arbitrary form of time-dependent heat input at the top surface of the wafer. It is found that the average channel temperature of GaN MOSHFETs on a 300 μm sapphire substrate with the output power of 10 W/mm can be over 400 °C in the CW mode while the average channel temperature of GaN MOSHFETs on a SiC substrate with the same thickness only reaches 50 °C under the same condition. The highest average channel temperature in a pulsed RF mode will vary with respect to the duty cycle of the pulse and type of the substrate.     

A 24 W Ku band GaN based power amplifier with 9.1 dB linear gain

A Ku-band power amplifier is successfully developed with a single chip 4.8 mm AlGaN/GaN high electron mobility transistors (HEMTs). The AlGaN/GaN HEMTs device, achieved by E-beam lithography г-gate process, exhibited a gate-drain reverse breakdown voltage of larger than 100 V, a cutoff frequency of f_T=30 GHz and a maximum available gain of 13 dB at 14 GHz. The pulsed condition (100 μs pulse period and 10% duty cycle) was used to test the power characteristic of the power amplifier. At the frequency of 13.9 GHz, the developed GaN HEMTs power amplifier delivers a 43.8 dBm (24 W) saturated output power with 9.1 dB linear gain and 34.6% maximum power-added efficiency (PAE) with a drain voltage of 30 V. To our best knowledge, it is the state-of-the-art result ever reported for internal-matched 4.8 mm single chip GaN HEMTs power amplifier at Ku-band.     

An analytical two-dimensional model for AlGaN/GaN HEMT with polarization effects for high power applications

An analytical two-dimensional model for AlGaN/GaN modulation-doped field effect transistor is developed. The spontaneous and piezoelectric polarization effects have been included. Two-dimensional analysis has been carried out in the high field region. The output characteristics, device transconductance and cut off frequency for 120 nm gate length device are obtained. Peak transconductance of 320 mS/mm and a cut off frequency of 120 GHz has been obtained. The results show excellent agreement when compared with experimental data thereby proving the validity of the model.     

Development and characterization of the thermal behavior of packaged cascode GaN HEMTs

This study investigates the heat generation behavior of packaged normally-on multi-finger AlGaN/GaN high electron mobility transistors (HEMTs) that are cascoded with a low-voltage MOSFET (LVMOS) and a SiC Schottky barrier diode (SBD). By foremost carrying out electro-thermal simulation and related thermal measurements with infrared thermography and Raman spectroscopy for basic 5 mm GaN HEMTs, the location of hot spot in operating device can be obtained. Based on the outcome, further packaged cascode GaN HEMT is analyzed. A hybrid integration of the GaN-HEMT, LVMOS, and SiC SBD are assembled on a directly bonded copper (DBC) substrate in the four-pin metal case TO-257 package. The metal plate is used as both the source terminal and heat sink. The analytical results of thermal investigation are confirmed by comparing them with the infrared thermographic measurements and numerical results obtained from a simulation using Ansys Icepak. For a power dissipation of less than 11.8 W, the peak temperature of the GaN HEMTs is 118.7 °C, obtained from thermal measurements.     

New degradation mechanism observed for AlGaN/GaN HEMTs with sub 100 nm scale unpassivated regions around the gate periphery

AlGaN/GaN HEMTs with low gate leakage current in the μA/mm range have been fabricated with a small-unpassivated region close to the gate foot. They showed considerably higher critical voltage values (average VCR = 60 V) if subjected to step stress testing at OFF-state conditions and room temperature as compared to standard devices with conventional gate technology. This is due to the fact that electrons injected from the gate can be accumulated at the unpassivated region and thus builds up negative charge. The lower gate leakage is due to virtual gate formation, which is reducing local electric field in the vicinity of the gate. In contrast to devices with standard gate technology, degradation during step stressing is not associated with a simultaneous gate leakage and drain leakage current increase but with a strong increase of drain current at OFF-state conditions while the gate leakage is practically not affected. Then a relatively higher critical voltage of around 60 V is achieved. An abrupt increase of subthreshold drain current implies the formation of a conductive channel bypassing the gate region without influencing gate leakage. It is believed that hopping conductivity via point defects formed during device stressing creates this channel. Once this degradation mode takes place, the drain current of affected devices significantly drops. This can be explained by negative trap formation in the channel region affecting the total charge balance in 2DEG region. Electroluminescence measurements on both fresh and degraded devices showed no hot spots at OFF-state conditions. However, there is additional emission at ON-state bias, which suggests additional energetic states that lead to radiative electron transition effects in the degraded devices, most possibly defect states in the buffer.     

Analysis of current collapse effect in AlGaN/GaN HEMT: Experiments and numerical simulations

In this work, current collapse effects in AlGaN/GaN HEMTs are investigated by means of measurements and two-dimensional physical simulations. According to pulsed measurements, the used devices exhibit a significant gate-lag and a less pronounced drain-lag ascribed to the presence of surface/barrier and buffer traps, respectively. As a matter of fact, two trap levels (0.45 eV and 0.78 eV) were extracted by trapping analysis based on isothermal current transient. On the other hand, 2D physical simulations suggest that the kink effect can be explained by electron trapping into barrier traps and a consequent electron emission after a certain electric-field is reached.