A closed-form charge control model (CCM) for threshold voltage (Vth) of single-heterojunction (AlGaN/GaN) devices is developed. The CCM reveals the mechanisms of how the space charges across the barrier of a heterojunction deplete or enhance 2-D electron gas. The Vth of both depletion- and enhancement-mode heterostructure field-effect transistors may be calculated through this model. Considering the strained and relaxed states, and surface states, the calculated results for Vth are in good agreement with simulation results. The generalized form for CCM may have wide applications for theoretical-based design for Vth of AlGaN/GaN devices.

A Closed-Form Charge Control Model for the Threshold Voltage of Depletion- and Enhancement-Mode AlGaN/GaN Devices

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.

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Characteristics of AlGaN/GaN/AlGaN double heterojunction HEMTs with an improved breakdown voltage

A novel source-connected field plate structure, featuring the same photolithography mask as the gate electrode, is proposed as an improvement over the conventional field plate (FP) techniques to enhance the frequency performance in GaN-based HEMTs. The influences of the field plate on frequency and breakdown performance are investigated simultaneously by using a two-dimensional physics-based simulation. Compared with the conventional T-gate structures with a field plate length of 1.2 μm, this field plate structure can induce the small signal power gain at 10 GHz to increase by 5-9.5 dB, which depends on the distance between source FP and dramatically shortened gate FP. This technique minimizes the parasitic capacitances, especially the gate-to-drain capacitance, showing a substantial potential for millimeter-wave, high power applications.

http://iopscience.iop.org/article/10.1088/1674-1056/22/9/097303/pdf

Field plate structural optimization for enhancing the power gain of GaN-based HEMTs

In this paper, we demonstrate that a Schottky drain can improve the forward and reverse blocking voltages (BVs) simultaneously in AlGaN/GaN high-electron mobility transistors (HEMTs). The mechanism of improving the two BVs is investigated by analysing the leakage current components and by software simulation. The forward BV increases from 72 V to 149 V due to the good Schottky contact morphology. During the reverse bias, the buffer leakage in the Ohmicdrain HEMT increases significantly with the increase of the negative drain bias. For the Schottky-drain HEMT, the buffer leakage is suppressed effectively by the formation of the depletion region at the drain terminal. As a result, the reverse BV is enhanced from −5 V to −49 V by using a Schottky drain. Experiments and the simulation indicate that a Schottky drain is desirable for power electronic applications.

https://iopscience.iop.org/article/10.1088/1674-1056/23/10/107303/pdf

Mechanism of improving forward and reverse blocking voltages in AlGaN/GaN HEMTs by using Schottky drain

A circuit includes a first field effect transistor having a gate, a first drain-source terminal, and a second drain-source terminal; and a second field effect transistor having a gate, a first drain-source terminal, and a second drain-source terminal. The second field effect transistor and the first field effect transistor are of the same type, i.e., both re-channel transistors or both p-channel transistors. The second drain-source terminal of the first field effect transistor is coupled to the first drain-source terminal of the second field effect transistor; and the gate of the second field effect transistor is coupled to the first drain-source terminal of the second field effect transistor. The resulting three-terminal device can be substituted for a single field effect transistor that would otherwise suffer breakdown under proposed operating conditions.

Breakdown voltage multiplying integration scheme

This paper investigates the impact of electrical degradation and current collapse on different thickness SiNx passivated AlGaN/GaN high electron mobility transistors. It finds that higher thickness SiNx passivation can significantly improve the high-electric-field reliability of a device. The degradation mechanism of the SiNx passivation layer under ON-state stress has also been discussed in detail. Under the ON-state stress, the strong electric-field led to degradation of SiNx passivation located in the gate-drain region. As the thickness of SiNx passivation increases, the density of the surface state will be increased to some extent. Meanwhile, it is found that the high NH3 flow in the plasma enhanced chemical vapour deposition process could reduce the surface state and suppress the current collapse.

/pdf/electric-stress-reliability-and-current-collapse-of-c8d52le00r.pdf

Electric-stress reliability and current collapse of different thickness SiN x passivated AlGaN/GaN high electron mobility transistors

The fabrication of enhancement-mode AlGaN/GaN HEMTs by fluorine plasma treatment on sapphire substrates is reported. A new method is used to fabricate devices with different fluorine plasma RF power treatments on one wafer to avoid differences between different wafers. The plasma-treated gate regions of devices treated with different fluorine plasma RF powers were separately opened by a step-and-repeat system. The properties of these devices are compared and analyzed. The devices with 150 W fluorine plasma treatment power and with 0.6 μm gate-length exhibited a threshold voltage of 0.57 V, a maximum drain current of 501 mA/mm, a maximum transconductance of 210 mS/mm, a current gain cutoff frequency of 19.4 GHz and a maximum oscillation frequency of 26 GHz. An excessive fluorine plasma treatment power of 250 W results in a small maximum drain current, which can be attributed to the implantation of fluorine plasma in the channel.

https://iopscience.iop.org/article/10.1088/1674-4926/30/12/124002/pdf

Enhancement-mode AlGaN/GaN HEMTs fabricated by fluorine plasma treatment

DC I—V output, small signal and an extensive large signal characterization (load-pull measurements) of a GaN HEMT on a SiC substrate with different gate widths of 100 μm and 1 mm have been carried out. From the small signal data, it has been found that the cutoff frequencies increase with gate width varying from 100 μm to 1mm, owing to the reduced contribution of the parasitic effect. The devices investigated with different gate widths are enough to work in the C band and X band. The large signal measurements include the load-pull measurements and power sweep measurements at the C band (5.5 GHz) and X band (8 GHz). When biasing the gate voltage in class AB and selecting the source impedance, the optimum load impedances seen from the device for output power and PAE were localized in the load-pull map. The results of a power sweep at an 8 GHz biased various drain voltage demonstrate that a GaN HEMT on a SiC substrate has good thermal conductivity and a high breakdown voltage, and the CW power density of 10.16 W/mm was obtained. From the results of the power sweep measurement at 5.5 GHz with different gate widths, the actual scaling rules and heat effect on the large periphery device were analyzed, although the effects are not serious. The measurement results and analyses prove that a GaN HEMT on a SiC substrate is an ideal candidate for high-power amplifier design.

http://iopscience.iop.org/article/10.1088/1674-4926/31/11/114004/pdf

Nonlinear characterization of GaN HEMT

The fabrication of AlGaN/GaN double-channel high electron mobility transistors on sapphire substrates is reported. Two carrier channels are formed in an AlGaN/GaN/AlGaN/GaN multilayer structure. The DC performance of the resulting double-channel HEMT shows a wider high transconductance region compared with single-channel HEMT. Simulations provide an explanation for the influence of the double-channel on the high transconductance region. The buffer trap is suggested to be related to the wide region of high transconductance. The RF characteristics are also studied.

AlGaN/GaN double-channel HEMT

We present an AlGaN/GaN metal–insulator–semiconductor high electron mobility transistor (MIS-HEMT) with an NbAlO high-k dielectric deposited by atomic layer deposition (ALD). Surface morphology of samples are observed by atomic force microscopy (AFM), indicating that the ALD NbAlO has an excellent-property surface. Moreover, the sharp transition from depletion to accumulation in capacitance–voltage (C–V)curse of MIS-HEMT demonstrates the high quality bulk and interface properties of NbAlO on AlGaN. The fabricated MIS-HEMT with a gate length of 0.5 μm exhibits a maximum drain current of 960 mA/mm, and the reverse gate leakage current is almost 3 orders of magnitude lower than that of reference HEMT. Based on the improved direct-current operation, the NbAlO can be considered to be a potential gate oxide comparable to other dielectric insulators.

http://www.xidian.edu.cn/hyjsktz//docs/20110222172648195130.pdf

Quan Si

Papers

Electric-stress reliability and current collapse of different thickness SiN x passivated AlGaN/GaN high electron mobility transistors

Enhancement-mode AlGaN/GaN HEMTs fabricated by fluorine plasma treatment

Nonlinear characterization of GaN HEMT

AlGaN/GaN double-channel HEMT

AlGaN/GaN MIS-HEMT using NbAlO dielectric layer grown by atomic layer deposition