An element using the wide-band-gap semiconductor attracts attention as a circuit element such as switching power supply and an inverter. Such an element includes a hetero-junction field effect transistor (HFET) having a hetero-structure of aluminum gallium nitride (AlGaN)/gallium nitride (GaN), for example, as a device easily to have a low ON-resistance. The HFET realizes the low ON-resistance by a high mobility in a hetero-interface channel and a high electron density generated by piezo-polarization.
However, when a high voltage is applied between a gate and a drain of the HFET, electric field concentration occurs at an edge part of the gate electrode. Electrons accelerated by this electric field concentration jump into a passivation film or an AlGaN layer. As a result, the electrons are trapped in the passivation film or the AlGaN layer. It is difficult to release the trapped electrons even when the HFET comes to have an ON-state and the applied voltage across the gate and the drain is reduced.
The electron trapping depletes the hetero-interface channel partially. As a result, there is a possibility that the ON-resistance is increased in the HFET. Such a phenomenon is referred to as a current collapse phenomenon. To suppress the current collapse phenomenon realizes the low ON-resistance effectively. Furthermore, when the electrons jump into the passivation film or the AlGaN layer, defects are generated in the passivation film or the AlGaN layer. Thereby, the variation in HFET characteristics, that is, reliability deterioration is caused.
A measure to reduce the electric field at the edge part of the gate electrode includes employment of a field plate (FP) structure. For example, a substrate FP electrode is formed by way of using a conductive substrate as a support substrate and connecting the conductive substrate to the source electrode. However, when the substrate FP electrode is employed, a high voltage is applied also across a semiconductor layer between the support substrate and the drain electrode. Accordingly, for realizing a high breakdown voltage, it is necessary to increase the thickness of a semiconductor layer between the support substrate and the drain electrode. Therefore, a spatial distance is increased between the support substrate and the gate electrode and a shield effect of the substrate FP electrode becomes weak. That is, in the HFET, when the electric field at the edge part of the gate electrode is increased, there is a possibility that the low ON-resistance and the high reliability are not obtained.