GaN Heterostructure Field-effect Transistors Research Articles

Investigation of trap effects in AlGaN∕GaN field-effect transistors by temperature dependent threshold voltage analysis

We report on a temperature dependent threshold voltage analysis of the AlGaN∕GaN heterostructure field-effect transistors (HFETs) and Al2O3∕AlGaN∕GaN metal-oxide-semiconductor HFETs (MOSHFETs) in order to investigate the trap effects in these devices. The threshold voltage of both types of devices decreases with increased ambient temperature up to 450°C. This indicates on donor traps to be present. The temperature induced threshold voltage shift is −1.6 and −8.5mV∕°C for the HFETs and MOSHFETs, respectively. A thermally activated energy level of ∼0.2eV is evaluated and attributed to the nitrogen vacancy in the AlGaN near surface. The trap density for the MOSHFETs is about two times higher than that for the HFETs. This might be due to the high-temperature treatment (∼600°C) of the MOSHFET structure during the gate insulator deposition.

Influence of crystalline quality of low-temperature GaN cap layer on current collapse in AlGaN∕GaN heterostructure field-effect transistors

The correlation between the crystalline quality of low-temperature GaN (LT-GaN) films and current collapse in AlGaN∕GaN heterostructure field-effect transistors with a LT-GaN cap layer was investigated. LT-GaN cap layers were grown at various temperatures ranging from 450to700°C. No current collapse was observed for heterostructure field-effect transistors with LT-GaN cap layers grown at 500 and 550°C. The crystalline quality of the 1-μm-thick LT-GaN films grown at 450–700°C was then studied through x-ray diffraction and selected area electron diffraction measurements. The crystalline quality, considered by measuring the full width at half maximum through x-ray diffraction, deteriorated as the growth temperature decreased. The crystal structure of LT-GaN films grown at 500 and 550°C was confirmed as polycrystalline. Thus, the results demonstrated that in AlGaN∕GaN heterostructure field-effect transistors, a polycrystalline structure provides the most effective LT-GaN cap layer for suppressing current collapse.

Accurate thermal analysis of GaN HFETs

AlGaN–GaN heterostructure field effect transistors (HFETs), while capable of delivering large output power densities at high efficiency into the mm W range of frequencies, also dissipate high power densities in the form of heat. Elevated channel temperatures reduce device performance and raise reliability concerns. Understanding the temperature and power dissipation dependence of the thermal resistance of GaN HFETs is also important for accelerated life testing. This work employs an accurate channel temperature measurement technique to extract the thermal resistance of the GaN device structures. Three dimensional numerical modeling using temperature dependent thermal conductivities was also performed. Simulation results agree well with measured thermal resistance. Further simulations were used to design low thermal resistance device structures which exploit the high thermal conductivity properties of diamond.

The Development of a High Power SP4T RF Switch in GaN HFET Technology

The development of a high power single-pole four throw (SP4T) hybrid switch using AlGaN/GaN heterostructure field effect transistors (HFETs) on Si substrate is reported for the first time for applications up to 1.5 GHz. The off-state capacitance of a single-gate GaN based HFET is 250 fF and the on-state resistance is 4.1 at a gate length of 0.7 m and a width of 1 mm. The hybrid SP4T switch with a size of 44 mm was implemented for system applications of three transmit paths and a receiver, of which each was configured with a series-shunt self-biased configuration. The switch has achieved an insertion loss of 1.4 dB with power handling of 43 dBm at the transmitter paths and an optimized isolation of better than 25 dB at the receiver path at 1.5 GHz. In addition, a high voltage switch driver using the GaN HFET technology was designed with an input control voltage of 0/4 V to provide an output voltage of 0/26 V. This development provides a baseline design for our next generation monolithic microwave integrated circuit switches in GaN technology.

Electron mobility related to scattering caused by the strain variation of AlGaN barrier layer in strained AlGaN∕GaN heterostructures

Using the measured capacitance-voltage curves of Ni Schottky contacts with different areas on strained AlGaN∕GaN heterostructures and the current-voltage characteristics for the AlGaN∕GaN heterostructure field-effect transistors at low drain-source voltage, we found that the two-dimensional electron gas (2DEG) electron mobility increased as the Ni Schottky contact area increased. When the gate bias increased from negative to positive, the 2DEG electron mobility for the samples increased monotonically except for the sample with the largest Ni Schottky contact area. A new scattering mechanism is proposed, which is based on the polarization Coulomb field scattering related to the strain variation of the AlGaN barrier layer.

High electron mobility in nearly lattice-matched AlInN∕AlN∕GaN heterostructure field effect transistors

High electron mobility was achieved in Al1−xInxN∕AlN∕GaN (x=0.20–0.12) heterostructure field effect transistors (HFETs) grown by metal-organic chemical vapor deposition. Reduction of In composition from 20% to 12% increased the room temperature equivalent two-dimensional-electron-gas density from 0.90×1013to1.64×1013cm−2 with corresponding electron mobilities of 1600 and 1410cm2∕Vs, respectively. The 10K mobility reached 17600cm2∕Vs for the nearly lattice-matched Al0.82In0.18N∕AlN∕GaN heterostructure with a sheet carrier density of 9.6×1012cm−2. For comparison, the AlInN∕GaN heterostructure without the AlN spacer exhibited a high sheet carrier density (2.42×1013cm−2) with low mobility (120cm2∕Vs) at room temperature. The high mobility in our samples is in part attributed to ∼1nm AlN spacer which significantly reduces the alloy scattering as well as provides a smooth interface. The HFETs having gate dimensions of 1.5×40μm2 and a 5μm source-drain separation exhibited a maximum transconductance of ∼200mS∕mm with good pinch-off characteristics and over 10GHz current gain cutoff frequency.

GaN-based FETs using Cat-CVD SiN passivation for millimeter-wave applications

We have found that SiN passivation by catalytic chemical vapor deposition (Cat-CVD) can significantly increase an electron density of an AlGaN/GaN heterostructure field-effect transistor (HFET). This effect enables thin-barrier HFET structures to have a high-density two-dimensional electron gas and leads to suppression of short-channel effects. We fabricated 30-nm-gate Al 0.4Ga 0.6N(8 nm)/GaN HFETs using Cat-CVD SiN. The maximum drain current density and extrinsic transconductance were 1.49 A/mm and 402 mS/mm, respectively. Current-gain cutoff frequency and maximum oscillation frequency of the HFETs were 181 and 186 GHz, respectively. These high-frequency device characteristics are sufficiently high enough for millimeter-wave applications.

Enhancement-Mode AlN/GaN HFETs Using Cat-CVD SiN

High-performance enhancement-mode (E-mode) AIN (2.5 nm)/GaN heterostructure field-effect transistors (HFETs) were fabricated with a novel method using SiN passivation by catalytic chemical vapor deposition (Cat-CVD). We found that the formation of a 2-D electron gas (2DEG) in the AIN/GaN heterostructure can be controlled by the presence of the Cat-CVD SiN on the barrier layer. Before SiN deposition, the 2DEG at the AIN/GaN heterointerface was completely depleted because of the extremely thin barrier layer. On the other hand, after SiN deposition, the decrease in AIN surface barrier height induced a high-density 2DEG. The E-mode HFETs with gate lengths of 100-180 nm and threshold voltages from +0.14 to +0.55 V showed a maximum drain-current density of 0.70-0.92 A/mm and a maximum extrinsic transconductance of 362-400 mS/mm. A current-gain cutoff frequency of 87 GHz and maximum oscillation frequency of 149 GHz were obtained for the 100-nm-gate devices.

Development of millimeter-wave GaN HFET technology

This paper describes approaches that we have used to improve the high-frequency device characteristics of GaN-based heterostructure field-effect transistors (HFETs). We developed three novel techniques to suppress short-channel effects: high-Al-composition and thin barrier layers, SiN passivation by catalytic chemical vapor deposition, and sub-100-nm Ti-based gates. These techniques were used to enhance the high-frequency device characteristics of Al 0.4 Ga 0.6 N/GaN HFETs, which had a record current-gain cutoff frequency of 181 GHz. In addition, high-performance depletion- and enhancement-mode AIN/GaN HFETs, which had DC and RF device characteristics fully comparable with those of state-of-the-art AlGaN/GaN HFETs, were also fabricated with these techniques.

Anomalous behavior of AlGaN∕GaN heterostructure field-effect transistors at cryogenic temperatures: From current collapse to current enhancement with cooling

While current collapse affects AlGaN∕GaN heterojunction field-effect transistors (HFETs) around room temperature, it gradually gives place to a current enhancement with cooling—below 200K, electrically stressed devices do show higher currents than in their prestressed state. This behavior can be explained by increased levels of channel impact ionization at lower temperatures. The positive hole charge generated by impact ionization compensates (and eventually dominates) the parasitic negative charge that is responsible for current collapse at higher temperatures (i.e., the “virtual gate charge”). Cooling of AlGaN∕GaN HFETs may potentially alleviate some of the nonidealities that have so far plagued this technology.

DC characteristics improvement of recessed gate GaN-based HFETs grown by MOCVD

The effect of the recessed gate structure on DC characteristics of n-channel depletion mode heterostructure field effect transistors (HFETs) grown by metalorganic chemical vapor deposition (MOCVD) was demonstrated. The recessed gate process was carried out by using photo-enhanced chemical (PEC) wet etching. There were improvements on the electrical characteristics after using the recessed gate AlGaN/GaN and AlGaN/GaN/InGaN/GaN HFET structures because of the better gate controllability to the drain current. It was also found that the gate voltage, V GS, of the transconductance peak value shifted toward positive bias for the recessed gate structure. The leakage current, the transconductance ( G m) and the saturation current of the recessed gate AlGaN/GaN/InGaN/GaN HFET with a 30-nm-thick Si-doped GaN cap layer are 1.95 mA/mm, 94.82 mA/mm and 230 mA/mm, respectively. This performance is quite better than that of the device with no recessed gate structure.

Self-heating in a GaN based heterostructure field effect transistor: Ultraviolet and visible Raman measurements and simulations

We report direct self-heating measurements for AlGaN∕GaN heterostructure field effect transistor grown on SiC. Measurements are carried out using micro-Raman scattering excited by above band gap ultraviolet and below band gap visible laser light. Ultraviolet excitation probes the GaN near the AlGaN∕GaN interface region of the device where the two-dimensional electron gas carries the source-drain current. The visible excitation probes the entire ∼1μm thick GaN layer and the SiC substrate near the interface with GaN. These results thus provide a measure of the average temperature throughout the GaN and of the substrate. Results are backed by combined electrical and thermal simulations. We find that the immediate hot spot region of the device, at the edge of the gate electrode, rises by up to ∼240°C over ambient under the most aggressive drive conditions examined.

Chemically gated AlGaN∕GaN heterostructure field effect transistors for polar liquid sensing

Al Ga N ∕ Ga N heterostructure field effect transistors are investigated to monitor the polarity of liquid molecules by exposing the unmetallized gate region between source and drain contacts to chemicals with dipole moments (μ) in the range of 1.6–3.96D. The results show that the channel current can be quantitatively modulated by the magnitude of μ of tested polar molecules. At room temperature, in comparison with the value in air, the channel current density drops from 13.9to7.15mA∕mm with chlorobenzene (μ=1.6D) or to 1.35mA∕mm with diemthyl sulfoxide (μ=3.96D) at a source-drain bias (VDS) of 10V. A linear fitting shows that the current slope is around −1.8mA∕(mmD). Dynamic response of the channel current indicates excellent reversibility of devices. The response time of devices is in the range of 0.1–16s for the chemicals tested in this work.

High-Efficiency Envelope-Tracking W-CDMA Base-Station Amplifier Using GaN HFETs

A high-efficiency wideband code-division multiple-access (W-CDMA) base-station amplifier is presented using high-performance GaN heterostructure field-effect transistors to achieve high gain and efficiency with good linearity. For high efficiency, class J/E operation was employed, which can attain up to 80% efficiency over a wide range of input powers and power supply voltages. For nonconstant envelope input, the average efficiency is further increased by employing the envelope-tracking architecture using a wide-bandwidth high-efficiency envelope amplifier. The linearity of overall system is enhanced by digital pre-distortion. The measured average power-added efficiency of the amplifier is as high as 50.7% for a W-CDMA modulated signal with peak-to-average power ratio of 7.67 dB at an average output power of 37.2 W and gain of 10.0 dB. We believe that this corresponds to the best efficiency performance among reported base-station power amplifiers for W-CDMA. The measured error vector magnitude is as low as 1.74% with adjacent channel leakage ratio of -51.0 dBc at an offset frequency of 5 MHz

Effects of SiN passivation by catalytic chemical vapor deposition on electrical properties of AlGaN∕GaN heterostructure field-effect transistors

We investigated the effects of SiN passivation by catalytic chemical vapor deposition (Cat-CVD) on the electrical properties of AlGaN∕GaN heterostructure field-effect transistors. The two-dimensional electron density (Ns) greatly increased after the Cat-CVD SiN deposition, and the tendency of the increase was enhanced with decreasing AlGaN barrier thickness. As a result of the large increase in Ns, the sheet resistance (Rsh) significantly decreased after the deposition, and it had low values of 320–460Ω∕◻ for extremely thin AlGaN barriers of 4–10nm. The increase in Ns showed little dependence on SiN thickness, indicating that the stress applied to the AlGaN barrier by SiN cannot be the origin of the increase. Cat-CVD SiN also improved the in-plane uniformity of mobility for extremely thin-barrier structures, which in turn improved the uniformity of Rsh. Moreover, we found that Cat-CVD was more effective than plasma-enhanced chemical vapor deposition in increasing Ns. A comparison of theoretical calculations and experimental results indicated that these behaviors can be explained by a decrease in the AlGaN surface barrier height due to the SiN deposition.

Novel Quaternary AlInGaN/GaN Heterostructure Field Effect Transistors on Sapphire Substrate

Undoped quaternary AlxIn0.02Ga0.98-xN/GaN heterostructure field effect transistors (HFET) with different Al mole fractions were fabricated on sapphire substrate. An enhancement in two-dimensional electron gas density with increasing the Al mole fractions was observed. The properties of HFETs revealed that, with the Al incorporation, both maximum transconductance (gmmax) and drain current (Idmax) increased up to 138 mS/mm and 1230 mA/mm, respectively, for the device with 2 µm gate length. In addition, the threshold voltage strongly depended on Al mole fraction, by which normally-off HFET was demonstrated on a low Al-contained sample. No breakdown was observed for all the AlxIn0.02Ga0.98-xN/GaN HFETs when the gate–drain reverse bias was applied up to 100 V. The results indicate that quaternary AlInGaN is a promising candidate for high power and high frequency device applications.

Surface states and persistent photocurrent in a GaN heterostructure field effect transistor

AlGaN/GaN heterostructure field effect transistors subject to UV illumination exhibit persistent photoconductivity both in the gate leakage and in the drain current. Time decay modelling of the gate photocurrent enables us to extract the surface potential transient. Simultaneous measurements of drain and gate currents indicate that (i) the gate leakage current negatively charges surface states, (ii) the large change in surface potential induced by UV illumination is responsible for the change in drain current in these devices. We present a detailed analytical model describing the gate and drain current decay transients, following UV illumination, in terms of surface potential modulation due to charging and discharging of surface traps. It is also found that for constant illumination and drain bias voltage conditions the change in the two-dimensional electron gas density is nearly constant independent of the gate bias. Using a previously measured mobility–carrier density relation we determine quantitatively the drain current transient.

Self-heating and the temperature dependence of the dc characteristics of GaN heterostructure field effect transistors

Self-heating is an important issue for GaN heterostructure field effect transistors (HFETs), especially in high power applications. Here we report the temperature dependence of the dc characteristics of some GaN HFETs including the variation of the transconductance. We present the characteristics as a function of added power, instead of voltage bias, and use the temperature data to transform the power dependence into a dependence on the average device temperature. For similar devices on sapphire and SiC, at 20 V VDS and 0 V VG, the temperature increase for the same added power is ∼2.7 times greater in the sapphire-based device.

Characterization and modeling of AlGaN∕GaN heterostructure field effect transistors for low noise amplifiers

A simple equivalent circuit model including noise sources for the GaN heterostructure field effect transistor is presented and analyzed. The model is used to determine optimum source impedance for low noise amplifier applications. A good agreement between the model and measured NFmin of experimental devices is shown. Prototype transistors with a 0.75μm gate length and 80μm width delivered a power-added efficiency of 45.6% and a minimum noise figure of 2.1dB at 2.4GHz when operated from a 48V supply. Gate leakage current was found to be an important noise source.

Thermally stable AlGaN∕GaN heterostructure field-effect transistor with IrO2 gate electrode

A thermally stable AlGaN∕GaN heterostructure field-effect transistor (HFET) using an iridium oxide (IrO2) gate contact was demonstrated, compared with conventionally used Pt Schottky contact. The Schottky barrier height of the Pt contact significantly decreased from 0.71to0.52eV and the reverse leakage current at −20V increased by two orders of magnitude when the device was annealed at 450°C for 24h. This was due to the indiffusion of the Pt atoms into the AlGaN layer during the annealing. However, no electrical degradation of the contact was found in the IrO2 Schottky contact. This was due to the fact that the IrO2 suppressed the indiffusion of the contact metals into the AlGaN∕GaN heterostructure. As a result, the sheet-carrier concentration at which the two-dimensional electron gases are confined was not degraded at high temperature. It was found that the electrical properties of the HFET using the IrO2 gate contact are thermally stable and no distinct change of device performances was observed even after annealing at 450°C for 24h. It is suggested that IrO2 is a promising candidate as gate electrode for a high-temperature AlGaN∕GaN HFET.