Al.sub.x Ga.sub.1-x As is being used in LED (light emitting diode) to form a double heterostructure, which is the most efficient because carrier and optical confinement can be achieved. For efficient and reliable operation of devices, it is essential to have ohmic contacts with low contact resistance. Previous investigations of ohmic contacts to n type Al.sub.x Ga.sub.1-x As have primarily focused on alloyed Au-Ge-Ni contacts through GaAs cap layers. The formation of ohmic contacts directly on n-type Al.sub.x Ga.sub.1-x As has not been successful yet for Al.sub.x Ga.sub.1-x As where x is greater than 0.3 as described in the article of E. D. Marshall et al., entitled "Planar Ge/Pd and alloyed Au-Ge-Ni ohmic contacts to n-Al.sub.x Ga.sub.1-x As (0.ltoreq.x.ltoreq.0.3)", published in APPLIED PHYSICS LETTERS, Vol. 54, No. 8, pgs. 721-723, Feb. 20, 1989. The contact resistivity is relatively high in the range of 10.sup.-4 ohm-cm.sup.2, and vertical and lateral penetration of the contact is commonly observed on the order of 100 nm. With the recently developed Pd/Ge metallization scheme, the contact resistivity on Al.sub.0. 3 Ga.sub.0.7 As was approximately 1.times.10.sup.-4 ohm-cm.sup.2.
FIG. 1 to which it is now made reference illustrates the different contact resistivity on n-Al.sub.x Ga.sub.1-x As with different metallization schemes. The curve defined by the points referenced as o illustrates the contact resistivity obtained with the recently developed Pd/Ge metallization scheme. The curve defined by the points referenced as .dbd. illustrates the contact resistivity obtained with the AuGe/Ni/Au metallization scheme. The point referenced as .DELTA. illustrates the contact resistivity obtained with the Ni/AuGe/Ni/Au metallization scheme. From these curves it appears that the contact resistivity increases rapidly with increasing AlAs mole fraction (x). For example, with the Pd/Ge metallization scheme the contact resistivity is found to be 5.times.10.sup.-7 ohm-cm on GaAs and to increase to 1.times.10.sup.-4 ohm-cm on Al.sub.0.3 Ga.sub.0.7 As. With the AuGe/Ni/Au metallization scheme the contact resistivity increases from 3.times.10.sup.-6 ohm-cm.sup.2 on GaAs to 7.times.10.sup.-4 ohm-cm.sup.2 on Al..sub.35 Ga..sub.65 As. With the Ni/AuGe/Ni/Au metallization scheme the contact resistivity on Al.sub.0.4 Ga.sub.0.6 As is about 2.5.times.10.sup.-4 ohm-cm.sup.2. No data have been reported for x greater than 0.4.
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