Semiconductor device

Abstract

An object of the present invention is to improve, in a group III nitride semiconductor device, the productivity, heat radiation characteristic and performance in the element high speed operation; upon a sapphire substrate in which an A plane (an (11-20) plane) is set to be the basal plane, an epitaxial growth layer of a group III nitride semiconductor is formed and, thereon, a gate electrode 16, a source electrode 15 and a drain electrode 17 are formed; these electrodes are disposed in such a way that a direction along which they are laid makes an angle within 20° with respect to a C axis of sapphire.

Description

[0001] The present invention relates to a field effect transistor (FET) with a sapphire substrate, in particular to a field effect transistor utilizing a group III nitride semiconductor material such as GaN.BACKGROUND TO THE INVENTION[0002] The group III nitride semiconductors including GaN have carrier transport characteristics close to that of GaAs, together with high breakdown electric fields due to their wide band gaps. They are, thus, regarded as strong candidate materials for high frequency, high power transistors.[0003] When a device is manufactured making use of a GaN based semiconductor material, because it is difficult to obtain a bulk GaN based substrate, there is normally employed a method of fabricating a device wherein a GaN based semiconductor layer is formed by epitaxial growth on a substrate of a different material. For the substrate of a different material, sapphire or SiC is utilized. SiC has an excellent thermal conductivity but also drawbacks of high cost and di...

AI Technical Summary

Problems solved by technology

When a device is manufactured making use of a GaN based semiconductor material, because it is difficult to obtain a bulk GaN based substrate, there is normally employed a method of fabricating a device wherein a GaN based semiconductor layer is formed by epitaxial growth on a substrate of a different material.

SiC has an excellent thermal conductivity but also drawbacks of high cost and difficulty to attain a large wafer area.

First, attempts to obtain a wafer with a larger diameter are limited to a certain extent. In recent years, from the point of view of improving productivity, there have been demands that wafers should have larger diameters. Yet, the sapphire whose C plane is chosen for the crystal growth plane cannot be readily made to have a larger diameter, because of its low workability through surface polishing due to its poor mechanical processing feasibility and little ability to grow the crystal to have a large width by the ribbon crystal method or the like. A substrate with the largest diameter attained so far is 4 inches in diameter.

Secondly, a heat radiation characteristic thereof is difficult to improve. Since sapphire has a low thermal conductivity, improvements on the heat radiation characteristic have been sought after for some time and, for this purpose, thinner substrates have been looked for. Nevertheless, sapphire has insufficient feasibility in mechanical processing as described above so that a reduction in thickness is hard to achieve and, thus, the heat radiation characteristic is difficult to improve.

Thirdly, parasitic capacitances generated in the substrate are relatively large and act as an inhibitory factor to the improvement of device performance. Especially, in the case of a C plane sapphire, it is necessary to make the substrate have a certain thickness from the point of mechanical processing feasibility, which results in generation of large parasitic capacitances in the substrate.

Furthermore, the growth of defects in the group III nitride semiconductor layer leads to inefficient piezoelectric effects through lattice relaxation so that defects such as dislocations need to be reduced.

Yet, conventional techniques have not given any clear guide leading to a process for forming a group III nitride semiconductor layer stably in the direction of a C axis while reducing defects.

However, such examinations have been hardly made so far.

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