RF Reliability Comparison between DC Stressed and Non-DC-Stressed GaN-on-Si HEMTs in a 1GHz Class F Power Amplifier
Available at: https://digitalcommons.calpoly.edu/theses/876
Date of Award
MS in Electrical Engineering
Gallium Nitride (GaN) is a wide band gap semiconductor with an energy gap of 3.4eV. The large energy gap of this III-V semiconductor gives it the ability to withstand high electric fields, high operating temperatures, and allows for high power density. These properties of GaN maintain their advantage at high frequencies and are ideal for use in power amplifiers. This thesis discusses the reliability of a GaN high electron mobility transistor (HEMT) used in a class F power amplifier configuration. GaN HEMTs have only been commercially available since 2006 and as a result their long term reliability is still under investigation. The mechanisms that cause device failures and reduced performance are not fully understood and current work continues to produce new theories as to why they occur and how they affect device performance. The mechanisms that cause changes in device performance have been heavily explored in DC studies. These are useful in understanding the mechanisms that may lead to a change in device performance. However, there is less literature investigating how these mechanisms affect the radio frequency (RF) performance of devices. Using the information gathered from literature about DC studies this thesis launches an investigation to observe the RF behavior of GaN HEMT amplifiers that have been subjected to DC stressing. The DC stressing consists of applying a large negative voltage to the gate terminal while connecting the drain and source terminals to ground. The applied voltage is large enough to cause permanent damage to the device. To perform the investigation four GaN class F power amplifiers are subjected to DC stressing while another four GaN class F power amplifiers receive no DC stressing. The performance of the two groups of amplifiers is compared and evaluated. The eight power amplifiers are constructed to output 4 watts of power. The output power and DC bias are continuously monitored for each test. After a DC stress test, consisting of -70V applied between the gate and drain/source terminals, four observations are noted. One, the drain current, and two, the output powers are initially reduced and recover after roughly a 3 hour period of time. Three, when operated for periods of time greatly exceeding the recovery period there are no differences seen in behavior between stressed and non-stressed amplifiers. These three observations happened in all the test runs. Four, a permanent gate leakage current is observed in all stressed amplifiers.