This paper presents a comprehensive review of AlGaN/GaN high electron mobility transistor failure physics and reliability, focusing on mechanisms affecting the gate-drain edge, where maximum electric field and peak temperatures are reached. Physical effects at the origin of device degradation (inverse piezoelectric effect, time-dependent trap formation and percolative conductive paths formation, and electrochemical AlGaN and GaN degradation) are discussed on the basis of literature data and unpublished results. Thermally activated mechanisms involving metal-metal and metal-semiconductor interdiffusion at the gate Schottky junction are also discussed.

AlGaN/GaN-Based HEMTs Failure Physics and Reliability: Mechanisms Affecting Gate Edge and Schottky Junction

Enhanced performance of RF power modules is required in a next-generation information society. To satisfy these requirements, we designed a novel high-electron mobility transistor (HEMT) structure employing wider bandgap AlGaN for a channel layer, which we called AlGaN channel HEMT, and investigated it. The wider bandgap is more effective for higher voltage operation of HEMTs and contributes to the increase of output power in RF power modules. As a result, fabricated AlGaN channel HEMTs had much higher breakdown voltages than those of conventional GaN channel HEMTs with good pinchoff operation and sufficiently high drain current density without noticeable current collapse. Furthermore, specific on-state resistances of fabricated AlGaN channel HEMTs were competitive with the best values of reported GaN- and SiC-based devices with similar breakdown voltages. These results indicate that the proposed AlGaN channel HEMTs are very promising not only for an information-communication society but also in the power electronics field.

AlGaN Channel HEMT With Extremely High Breakdown Voltage

Transient currents in AlGaN/GaN High Electron Mobility Transistors (HEMTs) induced by focused ion beams are measured. Enhanced charge collection occurs in the on- and pinch-off states of AlGaN/GaN HEMTs. The first-order mechanism of enhancement is associated with bipolar and back-channel effects. In addition, we suggest that the temporal positive charge buildup contributes to the enhanced charge collection. The effect of displacement damage on the enhanced charge collection is also discussed.

Enhanced Charge Collection by Single Ion Strike in AlGaN/GaN HEMTs

This paper describes a high efficiency and high output power GaN power amplifier for C-band space applications. The amplifier uses on-chip harmonic tuned FETs to improve dc-to-rf conversion efficiency. A 2nd harmonic input tuning circuit is incorporated into each unit on-chip FET cell and realizes high precise control of 2nd harmonic input impedance. In addition, 2nd and 3rd harmonic output impedances are optimized with external output matching circuits. A 100 W power amplifier with 4-chips achieves a 67.0% PAE (72.4% drain efficiency) at 3.7 GHz under CW operating conditions. To the best of our knowledge, this is the highest efficiency of C-band power amplifiers ever reported with over 100 W output power.

A 67% PAE, 100 W GaN power amplifier with on-chip harmonic tuning circuits for C-band space applications

Charge collection mechanisms in AlGaN/GaN MOS-HEMTs are investigated. Device types include those with no gate oxide, and those with HfO2 and Al2O3 gate oxides. Simultaneous charge collection is observed at the gate and the drain or the source, depending on strike location. Heavy ion data coupled with device simulations show that the introduction of a thin HfO2 layer in the gate stack introduces only a small valence band barrier, reducing but not preventing collection of holes at the gate in HfO2-gate devices. Furthermore, using Al2O3 gate oxide increases the valence band barrier over that of the HfO2, to the point where the radiation-induced transient is not detectable.

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Charge Collection Mechanisms in AlGaN/GaN MOS High Electron Mobility Transistors

This paper describes a high efficiency (68%), high output power (100W), high reliability GaN HEMT amplifier for C-band space applications. The high efficiency is achieved by 2nd-harmonic frequency (2f o ) tuning circuits in the input and output matching circuits. The input circuit uses open-ended stubs located nearby FET gate terminals for setting the 2f o reflection-phase at the optimum phase. In the output circuit, the optimum 2f o reflection phase is realized using three transmission-line transformers while matching loss is kept low at fundamental frequency. In addition, a 3000 hour's RF overdrive life test reveals that an estimated mean time to failure (MTTF) is 1×107 hours at 150°C channel temperature, proving that the amplifier has sufficient reliability for space applications. To the best of our knowledge, the efficiency of 68% is the highest of 100-W class C-band amplifiers ever reported, and is also comparable to that of commercially available traveling wave tube amplifiers.

A 68% efficiency, C-band 100W GaN power amplifier for space applications

Over 65% efficiency 300MHz bandwidth C-band internally-matched GaAs FET has been developed by using a large-signal FET model. In order to achieve high efficiency in broad-band, optimum impedances at each frequency are estimated by using the large-signal FET model. The large-signal FET model is based on Angelov model. The capacitance equations of the model are improved to satisfy charge conservation. The model parameters are extracted for a unit cell FET with a small gate width. The unit cell FET model was scaled up to one chip FET model with a wide gate width, considering increase of source inductance by sharing via holes. The developed C-band internally-matched FET has achieved power-added efficiency (PAE) of over 65% across 300MHz bandwidth.

H. Minamide

Papers

A 68% efficiency, C-band 100W GaN power amplifier for space applications

Over 65% efficiency 300 MHz bandwidth C-band internally-matched GaAs FET designed with a large-signal FET model