Patent ID: 12224339

DETAILED DESCRIPTION

FIG.1depicts an HEMT according to a first preferred embodiment of the present invention.

As shown inFIG.1, an HEMT100includes a substrate10. The substrate10may be a silicon substrate. A nucleation layer12is disposed on the silicon substrate10. A transition layer14is disposed on the nucleation layer12. A superlattice16is disposed on the transition layer14. A zinc oxide layer18is disposed on the superlattice16. A gallium nitride (GaN) layer20is disposed on the zinc oxide layer18, and the gallium nitride layer20contacts the zinc oxide layer18. An aluminum gallium nitride (AlxGa1-xN) layer22is disposed on the gallium nitride layer20and the aluminum gallium nitride layer22contacts the gallium nitride layer20. A P-type gallium nitride layer24is disposed on the aluminum gallium nitride layer22. A gate electrode26is disposed on the P-type gallium nitride layer24. A source electrode28and a drain electrode30are disposed on the aluminum gallium nitride layer22and between the source electrode28and the drain electrode30.

The gallium nitride layer20serves as a channel layer. A two-dimensional electron gas (2DEG)32generates in the gallium nitride layer20closed to the aluminum gallium nitride layer22. When the HEMT100tunes off, there is no 2DEG32directly below the P-type gallium nitride layer24. That is, the HEMT100is a normally-off transistor. The gate electrode26, the drain electrode30and the source electrode28may be titanium, aluminum, titanium nitride or other conductive materials.

The substrate10may be doped by P-type dopants such as boron, aluminum, gallium or indium. The nucleation layer12, the transition layer14and the superlattice16can be used to transition the lattice mismatch between the substrate10to zinc oxide layer18. The nucleation layer12, the transition layer14and the superlattice18may be a multiple-layer material. Moreover, the nucleation layer12, the transition layer14and the superlattice16may be doped with P-type dopants. The P-type dopants can trap the electrons diffusing from the substrate10so as to prevent the 2DEG32from being influenced by the electrons. The nucleation layer12, the transition layer14and the superlattice16includes aluminum nitride, aluminum gallium nitride or a combination thereof. According to a preferred embodiment of the present invention, the nucleation layer12is aluminum nitride. The transition layer14is aluminum gallium nitride. The superlattice16is formed by stacking aluminum nitride and aluminum gallium nitride alternately and repeatedly. The aluminum concentration of the aluminum gallium nitride in the transition layer14and the superlattice16may be the same as or different from each other.

According to a preferred embodiment of the present invention, a thickness of the zinc oxide layer18is 5 to 10 times greater than a thickness of the gallium nitride layer20. However, the thickness of the zinc oxide layer18and the thickness of the gallium nitride layer20can be altered based on different requirements. Moreover, the gallium nitride layer20is Ga-polarity. The direction [0001] of a Wurtzite hexagonal structure of the gallium nitride layer20is perpendicular to a top surface of the substrate10.

FIG.3depicts directions of piezoelectric polarization of a gallium nitride layer and an aluminum gallium nitride layer, wherein elements which are substantially the same as those in the first preferred embodiment are denoted by the same reference numerals; an accompanying explanation is therefore omitted.

As shown inFIG.3, the gallium nitride layer20is Ga-polarity. The direction [0001] of the Wurtzite hexagonal structure of the gallium nitride layer20is perpendicular to the top surface of the substrate10. On the occasion of the bottom of the gallium nitride layer20does not contact the zinc oxide layer18and the aluminum gallium nitride layer22is stacked on the gallium nitride layer20, the gallium nitride layer20contains compressive stress C and the aluminum gallium nitride layer22contains tensile stress T. At this point, the direction of piezoelectric polarization P2of the gallium nitride layer20and the direction of piezoelectric polarization P1of the aluminum gallium nitride layer22are opposite. In detail, the direction of piezoelectric polarization P2is the same as the direction [0001]. The direction of piezoelectric polarization P1is opposite to the direction [0001]. The direction of piezoelectric polarization P2decreases the current density of the 2DEG32.

FIG.4depicts directions of piezoelectric polarization of a gallium nitride layer, an aluminum gallium nitride layer and a zinc oxide layer inFIG.1, wherein elements which are substantially the same as those in the first preferred embodiment are denoted by the same reference numerals; an accompanying explanation is therefore omitted.

As shown inFIG.4, the gallium nitride layer20is Ga-polarity. The direction [0001] of the Wurtzite hexagonal structure of the gallium nitride layer20is perpendicular to the top surface of the substrate10. When the zinc oxide layer18is disposed below and contacts the gallium nitride layer20and the aluminum gallium nitride layer22is stacked on the gallium nitride layer20, the gallium nitride layer20contains tensile stress T2, the aluminum gallium nitride layer22contains tensile stress T1and the zinc oxide layer18contains compressive stress C1. At this point, the direction of piezoelectric polarization P4of the gallium nitride layer20and the direction of piezoelectric polarization P3of the aluminum gallium nitride layer22are same. The direction of piezoelectric polarization P5of the zinc oxide layer18and the direction of piezoelectric polarization P3of the aluminum gallium nitride layer22are opposite. In detail, the direction of piezoelectric polarization P5is the same as the direction [0001]. Both of the direction of piezoelectric polarization P3and the direction of piezoelectric polarization P4are opposite to the direction [0001]. In other words, both of the piezoelectric polarization P3and the piezoelectric polarization P4point to the zinc oxide layer18. Because the direction of piezoelectric polarization P3is the same as the direction of piezoelectric polarization P4, the current density of the 2DEG32is increased.

FIG.2depicts an HEMT according to a second preferred embodiment of the present invention, wherein elements which are substantially the same as those in the first preferred embodiment are denoted by the same reference numerals; an accompanying explanation is therefore omitted.

As shown inFIG.2, the HEMT200has a magnesium oxide layer118disposed under the gallium nitride layer20. The magnesium oxide layer118contacts the gallium nitride layer20. The thickness of the magnesium oxide layer118is 5 to 10 times greater than the thickness of the gallium nitride layer20. However, the thickness of the magnesium oxide layer118and the thickness of the gallium nitride layer20can be altered based on different requirements.

FIG.5depicts directions of piezoelectric polarization of a gallium nitride layer, an aluminum gallium nitride layer and a magnesium oxide layer inFIG.2, wherein elements which are substantially the same as those in the second preferred embodiment are denoted by the same reference numerals; an accompanying explanation is therefore omitted.

As shown inFIG.5, the gallium nitride layer20is Ga-polarity. The direction [0001] of the Wurtzite hexagonal structure of the gallium nitride layer20is perpendicular to the top surface of the substrate10. Similar to the case inFIG.4, when the magnesium oxide layer118is disposed below and contacts the gallium nitride layer20and the aluminum gallium nitride layer22is stacked on the gallium nitride layer20, the gallium nitride layer20contains tensile stress T3, the aluminum gallium nitride layer22contains tensile stress T4and the magnesium oxide layer118contains compressive stress C2. At this point, the direction of piezoelectric polarization P7of the gallium nitride layer20and the direction of piezoelectric polarization P6of the aluminum gallium nitride layer22are same. The direction of piezoelectric polarization P8of the magnesium oxide layer118and the direction of piezoelectric polarization P6of the aluminum gallium nitride layer22are opposite. In detail, the direction of piezoelectric polarization P8is the same as the direction.

Both of the direction of piezoelectric polarization P6and the direction of piezoelectric polarization P7are opposite to the direction [0001]. In other words, both of the piezoelectric polarization P6and the piezoelectric polarization P7point to the magnesium oxide layer118. Because the direction of piezoelectric polarization P6is the same as the direction of piezoelectric polarization P7, the current density of the 2DEG32is increased.

FIG.6depicts an HEMT according to a third preferred embodiment of the present invention.FIG.7depicts an HEMT according to a fourth preferred embodiment of the present invention, wherein elements inFIG.6andFIG.7which are substantially the same as those in the first preferred embodiment are denoted by the same reference numerals; an accompanying explanation is therefore omitted.

As shown inFIG.6andFIG.7, the zinc oxide layer18or the magnesium oxide layer118can not only be used in the HEMTs100/200with P-type gallium nitride layer24, but also can be utilized in the HEMT300with fluorine-doped region34in the aluminum gallium nitride layer22or the HEMT400with the gate electrode26recessed in the aluminum gallium nitride layer22. The HEMT300and the HEMT400are both normally-off transistors. The embodiments inFIG.6andFIG.7take the zinc oxide layer18as an example. However, based on different requirements, the zinc oxide layer18can be replaced by the magnesium oxide layer118.

The zinc oxide layer or the magnesium oxide layer of present invention is specially disposed under and contacts the gallium nitride layer. Because the lattice constant of zinc oxide and the lattice constant of magnesium oxide are both larger than that of gallium nitride, the gallium nitride layer contacting the zinc oxide layer or contacting the magnesium oxide layer generates tensile stress. Comparing to the situation inFIG.3, the zinc oxide layer and the magnesium oxide layer respectively in the first preferred and the second preferred embodiment can change the direction of piezoelectric polarization of the gallium nitride layer and therefore increase the current density of the 2DEG. Moreover, the band gap of zinc oxide and the band gap of magnesium oxide are higher than the band gap of gallium nitride; therefore the HEMT with the zinc oxide layer or the magnesium oxide layer has a higher breakdown voltage.

Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.