This application claims priority to UK Patent Application No. 1800900.1 entitled OHMIC CONTACTS IN SEMICONDUCTOR DEVICES and filed on Jan. 19, 2018, the entire contents of which are hereby incorporated by reference.

FIELD OF DISCLOSURE

The invention relates to ohmic contacts in semiconductor devices.

BACKGROUND

A high-electron-mobility transistor (HEMT) is a field-effect transistor incorporating a junction between two materials with different band gaps and lattice constants as the channel instead of a doped region. Such a junction is known as a hetero-junction. Thanks to the spontaneous polarization jump and/or piezoelectric field induced by crystal strain at the hetero-junction, a potential distortion occurs which forms a channel. In the case that the potential distortion is sufficient to form localization potential, the electrons are accumulated at the hetero-junction interface. These electrons form a layer known as a two dimensional electron gas. Within this region the electrons are able to move freely because there are no intentionally introduced doping or other scattering centers with which electrons will collide with significant impact into mobility and the mobility of the electrons in the gas is very high. In a GaN HEMT, doping is usually not used, and instead electrons are attributed to the states at the barrier surface.

A bias applied to the gate formed as a Schottky barrier diode is used to modulate the number of electrons in the channel formed from the 2D electron gas and in turn this controls the conductivity of the device. This can be compared to the more traditional types of FET where the width of the channel is changed by the gate bias.FIG. 1ais an energy momentum diagram100of a typical GaAs based HEMT. There is illustrated a larger bandgap semiconductor101, a smaller bandgap semiconductor102, donors103and electron gas104.FIG. 1bshows a band diagram of a GaN HEMT.

FIG. 2is a perspective view of an arrangement of a typical HEMT200. There is a source201, gate202and drain203as in any field effect transistor. The body204of the device comprises a substrate205, a two-dimensional electron gas (2DEG)206, a spacer207, a barrier layer208and capping layers209,210.

Although they give good performance, the manufacture of HEMTs, and particularly the provision of low resistance ohmic contacts, is challenging. The channel layer, which enables the formation of the 2DEG, lies typically in the region of 1 to 50 nm below the surface of the barrier layer. This distance is too high to provide tunnelling carrier transport via an isolated barrier layer placed between the ohmic contact metal and channels. Special treatment is required involving creating a recess in the barrier layer and/or high temperature annealing. During annealing a chemical reaction between the ohmic metal stack and semiconductor takes place providing a low resistance ohmic contact. In order to implement such an approach, well-defined metals stacks thicknesses have to be controlled to provide a correct metal alloy composition, so that during annealing the required chemical reaction occurs. It limits freedom for designing the Front End of Line (FEOL) and Back End of Line (BEOL). In particular, a specific alloy composition desired for a metal contact may require extensive and expensive redesigning and testing to comply with the new metal deposition process.

SUMMARY

The invention provides semiconductor arrangements and methods of fabricating such, as set out in the accompanying claims. In one embodiment the invention is a semiconductor arrangement including a substrate, a dielectric layer, and a semiconductor layer disposed between the substrate and the dielectric layer. The arrangement further includes an ohmic contact including a plurality of metal contact fragments located in a plurality of trenches formed in the dielectric layer, and a metallic connector layer electrically connecting the metal contact fragments. The ohmic contact electrically connects the metallic connector layer to the semiconductor layer.

Certain embodiments will now be described, by way of example only, with reference to the accompanying figures.

