The bulk trap-induced component of current collapse (CC) in GaN/AlGaN heterojunction field-effect transistors is studied in drift diffusion simulations, distinguishing between acceptor traps situated in the top and the bottom half of the bandgap, with Fe and C used as specific examples. It is shown that Fe doping results in an inherent but relatively minor contribution to dispersion under pulse conditions. This simulation is in reasonable quantitative agreement with double pulse experiments. Simulations using deep-level intrinsic growth defects produced a similar result. By contrast, carbon can induce a strong CC which is dependent on doping density. The difference is attributed to whether the trap levels, whether intrinsic or extrinsic dopants, result in a resistive n-type buffer or a p-type floating buffer with bias-dependent depletion regions. This insight provides a key design concept for compensation schemes needed to ensure semi-insulating buffer doping for either RF or power applications.

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Buffer Design to Minimize Current Collapse in GaN/AlGaN HFETs

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

This paper reports on an extensive analysis of the trapping processes and of the reliability of experimental AlGaN/GaN MIS-HEMTs, grown on silicon substrate. The study is based on combined pulsed characterization, transient investigation, breakdown, and reverse-bias stress tests, and provides the following, relevant, information: 1) the exposure to high gate-drain reverse-bias may result in a recoverable increase in the on-resistance (RON), and in a slight shift in threshold voltage; 2) devices with a longer gate-drain distance show a stronger increase in RON, compared to smaller devices; 3)current transient measurements indicate the existence of one trap level, with activation energy of 1.03 ± 0.09 eV; and 4) we demonstrate that through the improvement of the fabrication process, it is possible to design devices with negligible trapping. Furthermore, the degradation of the samples was studied by means of step-stress experiments in off-state. Results indicate that exposure to moderate-high reverse bias (<; 250 V for LGD = 2 μm) does not induce any measurable degradation, thus confirming the high reliability of the analyzed samples. A permanent degradation is detected only for very high reverse voltages (typically, VDS = 260-265 V, on a device with LGD = 2 μm stressed with VGS = - 8 V) and consists of a rapid increase in gate leakage current, followed by a catastrophic failure. EL measurements and microscopy investigation revealed that degradation occurs close to the gate, in proximity of the sharp edges of the drain contacts, i.e., in a region where the electric field is maximum.

Trapping and Reliability Assessment in D-Mode GaN-Based MIS-HEMTs for Power Applications

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.

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

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

The two-dimensional analysis of breakdown characteristics in field-plate AlGaN/GaN high electron mobility transistors with a relatively short gate length and short gate-to-drain distances is performed by considering a deep donor and a deep acceptor in a buffer layer. It is shown that when the acceptor density in the buffer layer is high, the breakdown voltage is determined by the impact ionization of carriers, and it can decrease with increasing the field-plate length. This is because the distance between the field-plate edge and the drain becomes very short and the electric field there becomes very high. On the other hand, when the acceptor density in the buffer layer is relatively low, the buffer leakage current becomes very large and this can determine the breakdown voltage, which becomes very low. In this case, the breakdown voltage increases with increasing the field-plate length.

https://iopscience.iop.org/article/10.1088/0268-1242/27/8/085016/pdf

Analysis of buffer-impurity and field-plate effects on breakdown characteristics in small-sized AlGaN/GaN high electron mobility transistors

Two-dimensional transient analysis of gate-field-plate AlGaN/GaN high electron mobility transistors with a backside electrode is performed by considering a deep donor and a deep acceptor in a buffer layer. Effects of introducing a field plate and a backside electrode on buffer-related lag and current collapse are studied. It is shown that gate field plate introduction is effective in reducing lag and current collapse when the acceptor density in the buffer layer is high. On the other hand, backside electrode introduction is shown to be effective in reducing drain lag and current collapse, particularly when the acceptor density in the buffer layer is relatively low, because the fixed potential at the backside electrode reduces electron injection into the buffer layer and the resulting trapping effects.

Analysis of backside-electrode and gate-field-plate effects on buffer-related current collapse in AlGaN/GaN high electron mobility transistors

Two-dimensional analysis of lag phenomena, current collapse and breakdown voltages in source-field-plate AlGaN/GaN HEMTs is performed by considering a deep donor and a deep acceptor in a buffer layer, and the results are compared with those for the case of gate-field-plate structure. It is shown that the reduction rate of drain-lag is similar between the two structures, but the reduction rates of gate lag and current collapse are smaller for the source-field-plate structure. This is because the electric field at the drain edge of the gate becomes higher in the off state and the trapping effects become more significant. For this reason, an off-state breakdown voltage is a little lower in the source-field-plate structure. It is suggested that there is an optimum thickness of SiN passivation layer to minimize the buffer-related current collapse in both structures.

Similarities of lags, current collapse and breakdown characteristics between source and gate field-plate AlGaN/GaN HEMTs

A two-dimensional transient analysis of source-field-plate AlGaN/GaN high-electron-mobility transistors (HEMTs) is performed by considering a deep donor and a deep acceptor in a buffer layer, and the results are compared with those in the case of gate-field-plate structures. It is shown that the reduction rate of drain lag obtained by introducing a field plate is quantitatively similar between source- and gate-field-plate structures. However, the gate-lag rate is rather higher in the source-field-plate structure because the electric field at the drain edge of the gate is higher in the off state, and hence electron injection into the buffer layer and the resulting trapping effects are more significant. Hence, current collapse is slightly larger in the source-field-plate structure. It is also shown that an optimum SiN passivation layer thickness is required to minimize buffer-related current collapse in source-field-plate AlGaN/GaN HEMTs.

https://iopscience.iop.org/article/10.7567/JJAP.52.08JN21/pdf

H. Onodera

Papers

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

Analysis of buffer-impurity and field-plate effects on breakdown characteristics in small-sized AlGaN/GaN high electron mobility transistors

Analysis of backside-electrode and gate-field-plate effects on buffer-related current collapse in AlGaN/GaN high electron mobility transistors

Similarities of lags, current collapse and breakdown characteristics between source and gate field-plate AlGaN/GaN HEMTs

Analysis of lags and current collapse in source-field-plate AlGaN/GaN high-electron-mobility transistors