Double Heterostructure High Electron Mobility Transistors Research Articles

Effects of Si-doped GaN insert layer in AlGaN/GaN/GaN:Si/AlN DH-HEMT structure

In this study, we investigate the effects of Si-doped GaN insert layer on the structural and electrical characteristics of AlGaN/GaN/GaN:Si/AlN double-heterostructure high-electron-mobility transistor (DH-HEMT). Compared with an AlGaN/GaN/AlN HEMT without a Si-doped GaN layer, the AlGaN/GaN/GaN:Si/AlN DH-HEMT showed an improved root-mean-square roughness of 0.57 nm and a decreased GaN channel compressive stress of –1.44 GPa. Also, an increased two-dimensional electron gas mobility of 1845 cm2/V∙s and a reduced specific contact resistance of 1.49 × 10−6 Ω∙cm2 were obtained. The direct-current output results showed an increased maximum drain current of 600 mA/mm and transconductance of 130 mS/mm, which was attributed to the enhanced 2DEG mobility and reduced contact resistance by the insertion of Si-doped GaN layer in the DH-HEMT.

Direct Current and Radio Frequency Characterizations of AlGaN/AlN/GaN/AlN Double‐Heterostructure High‐Electron Mobility Transistor (DH‐HEMT) on Sapphire

The AlGaN/AlN/GaN/AlN double‐heterostructure high‐electron mobility transistor (DH‐HEMT) on sapphire substrate is introduced, and its direct current (DC) and radio frequency (RF) characteristics to the conventional GaN‐based single‐heterostructure HEMT (SH‐HEMT) on SiC substrate are compared. The devices having the two‐finger gate are fabricated with gate width of 200 μm and gate length of 500 nm. The DC performance of the DH‐HEMT shows a transconductance of 0.233 S mm−1 and a maximum drain current density of 0.93 A mm−1, comparable with that of the SH‐HEMT. There is less‐pronounced kink‐effect in the DC I–V characteristics, whereas the off‐state subthreshold current is approximately four orders of magnitude higher than that of SH‐HEMT. A pulsed I–V shows a greatly suppressed slump ratio Z1 and Z2 of 1.6% and 4.3% for the DH‐HEMT. It is shown that the performances of a small‐ and a large‐signal characteristics of the DH‐HEMT are equivalent to the GaN SH‐HEMT: the current gain cutoff frequency (fT) and the maximum oscillation frequency (fmax) are 20.1 and 47.6 GHz, and the output power density and the power added efficiency (PAE) at the peak PAE, at 20 V drain voltage and 3.5 GHz frequency, are 3.83 W mm−1 and 57.2%, respectively.

The Effect of AlN Buffer Layer on AlGaN/GaN/AlN Double‐Heterostructure High‐Electron‐Mobility Transistor

Herein, the effect of crystal quality of AlN buffer layer on AlGaN/GaN/AlN double‐heterostructure high‐electron‐mobility transistor (DH‐HEMT) is investigated. The material quality of the GaN channel and the AlGaN barriers, such as the dislocation density and the interface roughness, deteriorates, and the 2D electron gas (2DEG) mobility decreases as the threading dislocation density (TDD) of the AlN buffer increases. It is also revealed that the thickness and the Al mole fraction of the AlGaN barrier are affected by the strain variation of the GaN channel depending on the TDD of the AlN buffer. The variation of the compressive strain of the GaN channel is responsible for the 2DEG density change by affecting the barrier condition and the piezoelectric polarization charge. Low‐temperature Hall effect measurement reveals that the interface roughness scattering is a dominant factor for the mobility of the DH‐HEMT, which is ≈2–6 × 103 cm2 (V s)−1.

Analysis of Al0.15Ga0.85N/GaN/Al0.15Ga0.85N DH-HEMT for RF and Microwave Frequency Applications

A charge control based analytical model is followed to study the impact of donor-layer doping and gate-length on microwave frequency performance of AlGaN/GaN/AlGaN double heterostructure high electron mobility transistor (DH-HEMT). DH-HEMT is observed to be more sensitive to gate-length and doping variation as compared to single heterostructure high electron mobility transistor (SH-HEMT). The effect of gate-length and doping on various performance parameters, i.e., transconductance, drain conductance, cut-off frequency and maximum oscillation frequency has been analysed. The results so obtained are compared with simulation results and are found to be in good agreement.

Optimization of enhancement mode P-type Mg-doped In0.2Ga0.8N cap gate DH-HEMT for low-loss high power efficient boost converter circuits

In this paper, Al0.23Ga0.77N/GaN/Al0.05Ga0.95N Double Heterostructure-High Electron Mobility Transistor targeting a low loss and high power efficient boost converter circuit was simulated using a thin P+In0.2Ga0.8N cap under the gate. The charge balancing concept of the polarization induced field of the device was due to the Mg-doped P+In0.2Ga0.8N which shows normally-off characteristics. The high positive threshold voltage VT shift achieved with the device was correlated with the obtained dynamic RON of the device. This dynamic RON was characterized 19μs immediately after the device was turned-on and goes up to a maximum of 550 V. Furthermore, this Al0.23Ga0.77N/GaN/Al0.05Ga0.95N demonstrates low dynamic RON of ~6% with increment of efficiency of up to 40% when measured in the booster converter circuit. For power switching applications, the result of this P+In0.2Ga0.8N cap having a back-barrier/buffer of AlGaN achieved a dynamic RON performance with excellent breakdown Voltage VBR.OFF and much lower OFF-state gate/drain leakage as compared to the conventional GaN buffer structures.

1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400100), the National Natural Science Foundation of China (Grant Nos. 11435010, 61474086, and 61804125), and the Natural Science Basic Research Program of Shaanxi Province, China (Grant No. 2016ZDJC-02).

In this paper, we present a 1.8-kV circular AlGaN/GaN/AlGaN double-heterostructure high electron mobility transistor (DH HEMT) with a gate-drain spacing . Compared with the regular DH HEMT, our circular structure has a high average breakdown electric-field strength that increases from 0.42 MV/cm to 0.96 MV/cm. The power figure of merit for the circular HEMT is as high as . The divergence of electric field lines at the gate edge and no edge effect account for the breakdown enhancement capability of the circular structure. Experiments and analysis indicate that the circular structure is an effective method to modulate the electric field.

Polarization dependent charge control model for microwave performance assessment of AlGaN/GaN/AlGaN double heterostructure HEMTs

An accurate polarization dependent charge control-based analytical model is proposed for microwave performance assessment of $$\hbox {Al}_{0.15}\hbox {Ga}_{0.85}\hbox {N/GaN/Al}_{0.15}\hbox {Ga}_{0.85}\hbox {N}$$ double heterostructure high electron mobility transistors (DH-HEMTs) in terms of current, transconductance, gate capacitances and cutoff frequency. An analytical expression correlating the sheet carrier concentration in the two 2DEGs formed at the upper and lower heterointerfaces of a DH-HEMT is obtained. AlGaN/GaN/AlGaN DH-HEMTs are found to exhibit superior RF performance as compared to its single heterostructure counterpart in terms of higher drain current, improved transconductance, higher gate capacitance and higher unity-gain cutoff frequency. This improvement in the DH-HEMT is mainly attributed to the formation of two 2DEGs (at top and the bottom heterointerface) as compared to the single 2DEG in a SH-HEMT. The variation of drain current with drain voltage and with gate voltage of AlGaN/GaN SH-HEMTs and AlGaN/GaN/AlGaN DH-HEMTs is obtained analytically and found to agree reasonably well with that obtained using ATLAS 2D device simulation, thereby validating the proposed model.

Trap state analysis in AlGaN/GaN/AlGaN double heterostructure high electron mobility transistors at high temperatures

In this work, frequency-dependent capacitances and conductance measurements are adopted to investigate high temperature characteristics of trap states in AlGaN/GaN/AlGaN double heterostructure high electron mobility transistors (DH-HEMTs). It is found that fast and slow trap states are present in DH-HEMTs, while only fast traps exist in AlGaN/GaN single heterostructure (SH) HEMTs. In the former, the fast trap state density ranges from 4.6 × 1012 cm−2 eV−1 to 1.9 × 1013 cm−2 eV−1 located at an energy below the conduction band between 0.273 eV and 0.277 eV, and the slow deep trap state density decreases from 2.4 × 1013 cm−2 eV−1 to 8.7 × 1012 cm−2 eV−1 located at an energy ranging from 0.384 eV to 0.423 eV in DH-HEMTs with a 14 nm GaN channel layer. These active trap energy levels in DH-HEMTs become deeper as the thickness of the channel layer decreases. In addition, the active trap energy levels in SH- and DH-HEMTs gradually become deeper as the measurement temperature increases. Also, the change in amplitude of the active trap energy levels in DH-HEMTs is larger than that in SH-HEMTs, which indicates that DH is efficient in suppressing the reverse gate leakage current at high temperatures.

Field‐effect transistors based on AlGaN/GaN/AlGaN double‐heterostructures grown by MBE

The double heterostructure (DHS) high-electron mobility transistor (HEMTs) grown by ammonia molecular beam epitaxy (MBE) is investigated both experimentally and theoretically. The band diagram and the sheet 2D electron concentration are computed, predicting alternating electron and hole slabs to co-exist in the structure. The DC characteristics of the DHS HEMT with the 1 µm gate are measured to asses the device performance. A breakdown threshold up to ∼200 V is demonstrated for the source/drain voltage in DC operation. The parameters of the DHS and conventional AlGaN/GaN single-heterostructure HEMTs are compared. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analytical non-linear charge control model for InAlAs/InGaAs/InAlAs double heterostructure high electron mobility transistor (DH-HEMT)

An analytical charge control model for sheet carrier density is presented for InAlAs/InGaAs/InAlAs double heterostructure high electron mobility transistor (DH-HEMT). The model uses a polynomial dependence of sheet carrier concentration on position of quasi-Fermi energy level to successfully predict the gradual saturation of charge in the device and is valid from subthreshold region to high conduction region for both the channels. The model has been extended to calculate I d– V d characteristics, transconductance and cut-off frequency of the device.