GaN Reliability Testing Beyond AEC Proves Robustness for Lidar
The superior performance of GaN power transistors and integrated circuits enable groundbreaking laser driver performance. provide the accuracy and robustness needed to ensure the performance and safety required for autonomous navigation.
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Gallium nitride (GaN) power devices have been in volume production since March 2010 and have established a remarkable field reliability record. An automotive application using GaN power devices in high volume is lidar (light detection and ranging) for autonomous vehicles. Lidar technology provides information about a vehicle’s surroundings, thus requiring high accuracy and reliability to ensure safety and performance. This article will discuss a novel testing mechanism developed by Efficient Power Conversion (EPC) to test eGaN devices beyond the qualification requirements of the Automotive Electronics Council (AEC) for the specific use case of lidar.
Gallium Nitride for Lidar Applications
eGaN FETs and ICs are widely implemented in lidar circuits for autonomous vehicles, where they offer several key benefits:
- Faster switching for shorter pulses and better range resolution,
- Smaller footprint which enables high power density, low inductance, and compact solutions,
- Higher efficiency at higher pulse repetition rates.
In a lidar application, the GaN device delivers short, high-current pulses, on the order of 1−5 ns, which drive a laser diode to generate narrow optical pulses. The peak currents are usually substantially greater than 50% of the FET pulse-current rating. The pulse duty cycle is typically low, and the pulse repetition frequency is in the range of 10 to 100 kHz. When not being pulsed, the GaN device is in the OFF state, exposed to a certain drain bias.
This stress condition is somewhat unusual for a power device, making it difficult to predict lifetime in operation by projecting conventional DC reliability tests such as HTGB or HTRB. Even GaN-specific tests, like the hard-switching reliability testing employed by EPC, do not effectively emulate the stress conditions in a lidar circuit.
Long-Term Stability Under High Current Pulses
From the standpoint of physics of failure, the simultaneous high current and voltage during a pulse raises concerns about hot-carrier effects, potentially leading to VTH or RDS(on) shifting within the device. In addition, the cumulative effect of repetitive high current pulses raises the specter of electro-migration leading to degradation of the solder joints.
To address these concerns in lidar applications, EPC initiated a novel test method in collaboration with key customers. This lidar reliability testing is part of EPC’s Beyond AEC Initiative, a series of GaN specific stress tests that go beyond the conventional reliability tests developed for MOSFETs as part of AEC-Q101 standard.
The concept of this test method is to stress parts in an actual lidar circuit for a total number of pulses commensurate with their ultimate mission profile. The mission profiles for automotive lidar vary from customer to customer. A typical automotive profile would call for a 15-year life, with two hours operation per day, at 100 kHz pulse repetition frequency (PRF). This corresponds to approximately four trillion total lidar pulses. Some worst-case scenarios might call for ten to twelve trillion pulses in service life. By testing a population of devices well beyond the end of their full mission profile while verifying the stability of the system performance and the device characteristics, this test method directly demonstrates the lifetime of eGaN devices in a lidar mission. Note that this direct approach eliminates the need for an acceleration factor or activation energy of dubious validity. It also removes the need to somehow project lifetime estimates from standard reliability tests to the unique stress conditions of lidar.
The Test Methodology and The Results
To achieve the large number of pulses, parts are stressed continuously at a pulse repetition frequency (PRF) much higher than in typical lidar circuits. The test circuit is based on EPC’s popular EPC9126 lidar application board. Experimental details of the testing procedures are provided in Appendix B of EPC’s Phase 11 Reliability Report.
For this study, two popular AEC grade parts were put under test: EPC2202 (80V) and EPC2212 (100V). Four parts of each type were tested simultaneously. During the stress, two key parameters were continuously monitored on every device: (i) peak pulse current and (ii) pulse width. These parameters are both critical to the range and resolution of a lidar system. Figure 1 shows the results of this test over the first 4.2 trillion pulses. Note that there is no observed degradation or drift in either the pulse width or height. The cumulative number of pulses corresponds to a typical automotive lifetime. While this is an indirect monitor of the health of the eGaN device, it indicates that no degradation mechanisms have occurred that would adversely impact circuit performance.
To gain better visibility into the eGaN device parametric stability over time, the test system interrupts lidar stress every six hours to measure the RDS(on) and device threshold VTH. After this brief parametric measurement, the parts are returned quickly to lidar stress mode. The results are shown in Figure 2. Both parameters show excellent stability over the duration of the test. The stability indicates that lidar stress is relatively benign to eGaN devices.
Summary: Short, high-current pulse (lidar) testing of eGaN devices shows they are very reliable in a lidar application over a typical automotive lifetime. As of the publishing of the EPC’s Phase 11 Reliability Report in March of 2020, no failure modes or parametric degradation have been observed. GaN power devices, already in volume production in lidar applications, provide the accuracy and robustness needed to ensure the performance and safety required for autonomous navigation.