GaN, AlN, and InN, which are nitride semiconductors, or mixed crystals thereof have a large band gap, and are used for a high-power electronic device, a short-wavelength light emitting device, or other such devices. For the high-power electronic device of these devices, a technique concerning a field effect transistor (FET), particularly, a high electron mobility transistor (HEMT) has been developed. The HEMT made of such nitride semiconductors is used for a high-power and high-efficiency amplifier, a high-power switching device, or other such devices.
The HEMT used for this purpose needs to be normally off and have a high dielectric strength voltage. In particular, normally-off characteristics are important for safe operations, and hence various methods for achieving the normally-off characteristics have been studied. An example of the method for achieving the normally-off characteristics involves removing part of a semiconductor layer located immediately below a gate electrode to thereby form a gate recess. The gate recess structure formed according to this method is advantageous in that a threshold voltage may be positive without an increase in resistance components between electrodes. In addition, a normally-off semiconductor device used for the purpose of electric power is requested to have a high drain withstand voltage and a high gate withstand voltage. Hence, a metal insulator semiconductor (MIS) structure in which an insulating film as a gate insulating film is formed is adopted for lateral FETs and HEMTs. As described above, the gate recess structure and the MIS structure are adopted in combination for the HEMT made of a GaN-based semiconductor material, whereby the semiconductor device suitable for the purpose of electric power may be achieved.
[Patent Document]
Japanese Laid-Open Patent Publication No. 2002-359256.
In the HEMT having the gate recess structure and the MIS structure as described above, a gate leakage current during a transistor operation is suppressed by using aluminum oxide for the gate insulating film. In addition, in order to make the normally-off operation more reliable, fluorine may be implanted into the semiconductor layer in the region in which the gate recess is formed. These methods may enhance characteristics of the HEMT.
Unfortunately, in order to implant fluorine into the semiconductor layer in the region in which the gate recess is formed, a step of implanting fluorine by ion implantation or other such methods needs to be added after the formation of the gate recess, and hence the number of steps is increased, resulting in higher costs. In addition, in the gate recess structure, a trap level is easily formed between the semiconductor layer and the gate insulating film in the gate recess, and electrons are trapped by the trap level, so that the threshold value of a gate voltage fluctuates to be unstable. As a result, the uniformity of semiconductor devices to be produced is reduced, and the yield becomes lower.