Power Electronics Europe February/March 2022

26 INDUSTRY NEWS Issue 1 2022 Power Electronics Europe www.power-mag.com www.power-mag.com intercept (or additive vertical offset) of the line will increase like the logarithm of the switch current. For the same reason, the fine details of the switching loci have almost no impact, and inductive and resistive hard switching are equally valid methods to characterize dynamic R DS(on) . While equally valid to an inductive test circuit, a resistive circuit presents several practical advantages when it comes to evaluating dynamic R DS(on) . For one, the circuit is simpler and more compact, allowing it to be integrated on probe cards for wafer-level characterization. For another, the lack of voltage overshoot during turn off allows for testing at voltages closer to the breakdown voltage, achieving operating points in the switching loci even more severe than possible with an inductive switching circuit. The model is predicated on the assumption that hot electrons inject over a surface potential into the conduction band of the surface dielectric. Once inside, the electrons quickly fall into deep mid-gap states, where they are assumed to be trapped permanently (no de-trapping). Hot electrons are created during the switching transition, where the transient combination of high injection current and high electric field leads to a significant number of high energy carriers. Figure 7 shows a cross-section of an GaN transistor in the immediate vicinity of the drain contact. During a hard-switching transition, electrons rush toward the drain, and become highly accelerated by the electric fields there. Under the right conditions, some electrons gain sufficient kinetic energy to scatter into the conduction band of the dielectric above. To do so, they need kinetic energy > 2 eV. Once inside the dielectric, they trap in deep mid-gap states, and become permanently trapped. When the device is turned on, the trapped charge reduces the normal channel electron charge, leading to a rise in RDS(on). By expanding on this simple dynamical picture of charge trapping in the discussion to follow, the model explains all the observed characteristics. More details can be found in “GaN Reliability and Lifetime Projections: Phase 14”, https://epc-co.com/epc/DesignSupport/ eGaNFETReliability/ReliabilityReportPhase14.aspx Figure 7: Hot electron scattering into the surface dielectric near the drain contact. To enter this dielectric, electrons must have sufficient energy to surmount the potential barrier. Once in this dielectric, they fall into deep electron trap states and are trapped effectively indefinitely REGISTRATION IS NOW OPEN