2, Technical University of Berlin, Berlin, , Germany
GaN devices for microwave and power switching are enabling for new and innovative system applications. The specific material properties of AlGaN/GaN or similar heterojunctions facilitate very compact and extremely fast devices. On system level these properties translate into advantages such as low weight and low volume for a given power handling. Most of the GaN devices implemented so far are relying on a lateral architecture. The device current flows in an infinitesimal small sheet layer located at the interface between two adjacent semiconducting materials with different degrees of spontaneous and piezoelectric polarization. Thus a 2-dimensional electron gas (2DEG) forms whose properties depend on the pairing of the adjacent semiconductor materials, their respective mechanical strain and on charged trap states in the vicinity. Traps located close to the 2DEG influence electron population and are thus affecting maximum device current and on-state resistance. In general, trap states are very dynamic in nature. Thus trapping and detrapping depends on specific device biasing conditions. Strictly spoken this means that at a given bias condition the trap population is practically frozen and cannot respond to fast device switching transients. Therefore, immediately after device switching the transition to the new bias point may be delayed as traps need to be charged or discharged according to their inherent time constants. For example, if negative charges are trapped in the vicinity of the channel the device cannot fully turn on immediately after trying to switch the device from an off-state to an on-state bias point. The new static bias point will be reached after a certain delay only. This effect is known as “dynamic on-state resistance” for power switching devices or gate or drain lagging in GaN microwave devices. It turns out that the properties of the dynamic on-state resistance strongly depend on history of device biasing, on the time elapsed since device switching, on local electric field in the active device and on temperature.
The invited presentation discusses the most important dynamic properties observed in lateral GaN devices and sheds light on the respective physical mechanisms. Furthermore, GaN devices form institutional and industrial vendors are compared against each other with respect to their dynamic switching properties. It can be shown that some of them but not all can be practically free of dynamic on-state resistance effects and show nearly ideal switching properties.