1. Field of the Invention
The present invention relates to a field-effect transistor and a method of manufacturing the field-effect transistor, and more particularly, to a field-effect transistor containing gallium arsenide (GaAs) or gallium nitride (GaN) as a main component, which is suitable for use in a microwave band and a millimeter wave band of several GHz or more, and a method of manufacturing the field-effect transistor.
2. Description of the Related Art
Wireless communication markets using a microwave band and a millimeter wave band are expanding along with the diffusion of satellite communications, high-speed and large-capacity communication systems of a 60 GHz band, on-vehicle radar systems of a 70 to 80 GHz band, and the like. For a transmitting/receiving portion of a signal in those high frequency bands, a field-effect transistor (hereinafter, referred to as FET) such as a metal semiconductor FET (MESFET), a heterostructure FET (HFET), or a high electron mobility transistor (HEMT), which is obtained by lamination on a compound semiconductor epitaxial layer substrate formed of a compound semiconductor, in particular, GaAs or GaN, is mainly used.
In order to achieve cost reduction of transmitting/receiving devices, which is requisite for expanding the wireless communication markets, it is effective to change a hermetically sealed package for a non-hermetically sealed type or a mold sealed type. Meanwhile, a field-effect transistor using the compound semiconductor is generally vulnerable to moisture, and is required to attain high moisture resistance of a chip itself.
In order to enhance moisture resistance, there is an effective measure in which a moisture-resistant protective film is laminated on a surface of a semiconductor to thereby prevent moisture from reaching the surface of the semiconductor. However, in general, the covering by the moisture-resistant protective film accompanies deterioration of transistor characteristics. In a case of the field-effect transistor, an insulating protective film is interposed in a space between a gate electrode and the surface of the semiconductor or between the gate electrode and source and drain electrodes, and therefore a gate capacitance is increased, with the result that the device characteristics are deteriorated.
In general, a FET for a high-power amplifier often adopts a T-shaped gate structure which is effective in reducing a gate resistance. In the T-shaped gate structure, a cavity is formed between a portion below a canopy of the gate and a semiconductor layer. When the cavity below the canopy is filled by lamination of an insulating protective film, the above-mentioned gate capacitance is further increased, and gain characteristics in the micro wave and the millimeter wave are reduced.
An attempt to simultaneously achieve protection of a highly-moisture-resistant film and suppression of an increase in gate capacitance is proposed in JP 2008-98400 A, for example. In the field-effect transistor described in JP 2008-98400 A, the increase in gate capacitance is suppressed by forming the cavity below the canopy of the T-shaped gate.
In JP 2008-98400 A, the increase in gate capacitance is suppressed by forming the cavity below the canopy of the T-shaped gate. However, forming the cavity only below the canopy of the T-shaped gate cannot eliminate a constant increase in gate capacitance due to the highly-moisture-resistant insulating film deposited outside the canopy of the T-shaped gate. As a result, there arises a problem that a gain reduction is caused.
Table 1 below shows results of a gain (MSG) evaluated at 10 GHz in three types of GaAs-pHEMT structures. In a transistor structure with a cavity below a canopy of a gate, the gain is increased by about 0.5 dB compared with a structure in which a portion below the canopy is filled, whereas the gain is decreased by about 0.6 dB to 0.7 dB compared with a transistor without a highly-moisture-resistant film.
TABLE 1GaAs-pHEMT structure: Comparison of gain (MSG) at 10 GHzStructure in which aportion below aInsulating filmcanopy is filledprotection structurewith an insulatingwith a cavity below aStructure without afilmcanopyprotective film14.5 dB15.0 dB15.65 dB