A survey of Gallium Nitride HEMT for RF and high power applications

This paper reviews the physical mechanisms responsible for breakdown current in AlGaN/GaN high electron mobility transistors (HEMTs). Through a critical comparison between experimental data and previously published results we describe the following mechanisms, which can be responsible for the increase in drain current at high drain voltage levels, in the off-state: (i) source–drain breakdown, due to punch-through effects and/or to a poor depletion of the buffer; (ii) vertical (drain-bulk) breakdown, which can be particularly prominent when the devices are grown on a silicon substrate; (iii) breakdown of the gate–drain junction, due either to surface conduction mechanisms or to conduction through the (reverse-biased) Schottky junction at the gate; (iv) impact ionization triggered by hot electrons, that may induce an increase in drain current due to the lowering of the barrier for the injection of electrons from the source.

http://iopscience.iop.org/article/10.7567/JJAP.53.100211/pdf

Breakdown mechanisms in AlGaN/GaN HEMTs: An overview

As a promising ultra-wide bandgap semiconductor, the β-phase of Ga
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 has attracted more and more interest in the field of power electronics due to its ultra-wide bandgap (4.8 eV), high theoretical breakdown electric field (8 MV/cm), and large Baliga's figure of merit, which is deemed as a potential candidate for next generation high-power electronics, including diodes, field effect transistors (FETs), etc. In this article, we introduce the basic material properties of Ga
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, and review the recent progress and advances of β-Ga
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 based metal-oxide-semiconductor field-effect transistors (MOSFETs). Due to the problematic p-type doping technology up to now, the enhancement-mode (E-mode) β-Ga
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 FETs face more difficulties, compared with depletion mode (D-mode). This article focuses on reviewing the recent progress of E-mode Ga
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 MOSFETs, summarizing and comparing various feasible solutions when p-type doping is absent. Furthermore, the device fabrication and performances of state-of-art β-Ga
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Progress of Ultra-Wide Bandgap Ga 2 O 3 Semiconductor Materials in Power MOSFETs

We make a 2-D analysis of breakdown characteristics of field-plate AlGaN/GaN HEMTs with a high- ${k}$ passivation layer, and the results are compared with those having a normal SiN passivation layer. As a result, it is found that the breakdown voltage is enhanced particularly in the cases with relatively short field plates because the reduction in the electric field at the drain edge of gate effectively improves the breakdown voltage in the case with the high- ${k}$ passivation layer. In the case with the moderate-length field plate, the enhancement of breakdown voltage due to the high- ${k}$ passivation layer occurs because the electric field profiles between the field-plate edge and the drain become more uniform. It is also studied how the breakdown voltage depends on a deep-acceptor density in the Fe-doped semi-insulating buffer layer when a high- ${k}$ passivation layer is used. It is shown that the breakdown voltage increases with increasing the relative permittivity of the passivation layer $\varepsilon _{\text{r}}$ and with increasing the deep-acceptor density NDA. When $\varepsilon _{\text{r}} = 60$ and $N_{\mathrm {DA}} = 2$ – $3 \times 10^{17}$ cm−3 at the gate length of $0.3~\mu \text{m}$ , the breakdown voltage becomes about 500 V at a gate-to-drain distance of $1.5~\mu \text{m}$ , which corresponds to an average electric field of about 3.3 MV/cm between the gate and the drain.

/pdf/enhancement-of-breakdown-voltage-in-algan-gan-hemts-field-3v836at2dn.pdf

Enhancement of Breakdown Voltage in AlGaN/GaN HEMTs: Field Plate Plus High- ${k}$ Passivation Layer and High Acceptor Density in Buffer Layer

In this paper, the RF and DC characteristics of AlGaN/GaN High electron mobility transistor is analysed using discrete field plate technique. Surprisingly, it reduces the device parasitic capacitance exhibiting very low CGS and CGD of 5.8 × 10−13 F/mm and 4.2 × 10−13 F/mm respectively to improve the cut off frequency (fT) from 17.5 GHz to 20 GHz. The discrete field plate suppresses the maximum electric field between gate and drain region to achieve the high breakdown voltage of 330 V. The maximum transconductance (gm) achieved is 275 mS/mm, ensuring the better DC operation of the device. The simulated results clearly show that, the discrete field plate HEMTs are superior in performance over conventional GaN FP-HEMTs for future high frequency and high power applications.

Analysis of AlGaN/GaN HEMT using discrete field plate technique for high power and high frequency applications

2-D analysis of breakdown characteristics in AlGaN/GaN high electron mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer. The dependence of the OFF-state breakdown voltage on the relative permittivity of the passivation layer er and the thickness of the passivation layer d are studied. It is shown that as er increases, the OFF-state breakdown voltage increases. This is because the electric field at the drain edge of the gate is weakened as er increases. This occurs because in the insulator the applied voltage tends to drop uniformly in general, and hence when the insulator is attached to the semiconductor, the voltage drop along the semiconductor becomes smoother at the drain edge of the gate if the er of the insulator is higher. It is also shown that the OFF-state breakdown voltage increases as d increases because the electric field at the drain edge of the gate is weakened as d increases. It is concluded that AlGaN/GaN HEMTs with a high- k and thick passivation layer should have high breakdown voltages.

Numerical Analysis of Breakdown Voltage Enhancement in AlGaN/GaN HEMTs With a High- $k$ Passivation Layer

A two-dimensional analysis of breakdown characteristics in AlGaN/GaN high-electron-mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer. The dependence of an off-state breakdown voltage on the relative permittivity of the passivation layer ϵr is studied. It is shown that as ϵr increases, the off-state breakdown voltage increases. This is because the electric field at the drain edge of the gate is weakened as ϵr increases. This occurs because in the insulator the applied voltage tends to drop uniformly in general, and hence when the insulator is attached to the semiconductor, the voltage drop along the semiconductor becomes smoother at the drain edge of the gate if ϵr of the insulator is higher and the effect of the insulator becomes more significant. It is concluded that AlGaN/GaN HEMTs with a high-k passivation layer should have high breakdown voltages.

Increase in breakdown voltage of AlGaN/GaN HEMTs with a high‐k dielectric layer

2-D analysis of off-state drain current–drain voltage characteristics in AlGaN/GaN HEMTs is performed; where three cases with single passivation layers (SiN or high- ${k}$ dielectric) and double passivation layers (first layer: SiN, second layer: high- ${k}$ dielectric) are compared. The passivation layer’s thicknesses and relative permittivity of high- ${k}$ dielectric are varied as parameters. It is shown that in the case of double passivation layers, the breakdown voltage is enhanced significantly compared to the case of the SiN single passivation layer when the second high- ${k}$ layer becomes thick. This occurs because the electric field at the drain edge of the gate is reduced. However, in the case of a relatively thin second high- ${k}$ layer, the breakdown voltage can be lowered remarkably compared to the case with a high- ${k}$ single passivation layer even if the first SiN layer is rather thin. Also, when the first SiN layer is thick ( ${\sim } 0.1~{\mu }\text{m}$ ), the improvement of the breakdown voltage by increasing the thickness of the second layer is rather limited. However, it is also shown that in the case of double passivation layers, the breakdown voltage becomes close to the case of the high- ${k}$ single passivation layer when the relative permittivity of the second passivation layer becomes high and the SiN layer is relatively thin.

/pdf/analysis-of-breakdown-voltages-in-algan-gan-hemts-with-low-k-2euqer9nug.pdf

Analysis of Breakdown Voltages in AlGaN/GaN HEMTs With Low- ${k}$ /High- ${k}$ Double Passivation Layers

We analyze off-state breakdown characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) with a Fe-doped buffer layer where a deep acceptor located above the midgap is included. It is shown that by introducing a high-k passivation layer, the breakdown voltage V br improves as in a case with an undoped semi-insulating buffer layer. In the Fe-doped case, V br becomes a little higher in the case where the passivation layer's relative permittivity er is rather higher when the energy levels determining the Fermi level are set equal in the two buffers. It is also shown that when the energy level of deep acceptor is deeper, V br becomes higher in the region where er is high. This occurs because the leakage current via the buffer layer becomes smaller.

https://iopscience.iop.org/article/10.7567/JJAP.56.108003/pdf

Effects of acceptors in a Fe-doped buffer layer on breakdown characteristics of AlGaN/GaN high electron mobility transistors with a high-k passivation layer

We analyze breakdown characteristics and current collapse in AlGaN/GaN HEMTs, with the passivation layer’s relative permittivity er as a parameter. It is shown that the off-state breakdown voltage is considerably enhanced by introducing a high-k passivation layer because the electric field at the drain edge of the gate is weakened and the buffer leakage current is reduced. The breakdown voltage in the high-er region increases when the gate voltage is changed from −8 to −10 V, because the buffer leakage current is reduced. It is also shown that drain lag and current collapse in AlGaN/GaN HEMTs could be reduced by introducing a high-k thick passivation layer, because the electric field at the drain edge of the gate is reduced, leading to less electron injection into the buffer layer and weaker buffer trapping effects.

http://iopscience.iop.org/article/10.7567/JJAP.54.031002/pdf

Hideyuki Hanawa

Papers

Numerical Analysis of Breakdown Voltage Enhancement in AlGaN/GaN HEMTs With a High- $k$ Passivation Layer

Increase in breakdown voltage of AlGaN/GaN HEMTs with a high‐k dielectric layer

Analysis of Breakdown Voltages in AlGaN/GaN HEMTs With Low- ${k}$ /High- ${k}$ Double Passivation Layers

Effects of acceptors in a Fe-doped buffer layer on breakdown characteristics of AlGaN/GaN high electron mobility transistors with a high-k passivation layer

Analysis of high-k passivation-layer effects on buffer-related breakdown and current collapse in AlGaN/GaN HEMTs