The quarternary In-0.52(AlxGa1-x)(0.48)As compound on InP substrates is an important material for use in optoelectronic and microwave devices. We have systematically investigated the electrical properties of quarternary In-0.52(AlxGa1-x)(0.48)As layers, and found that a 10% addition of Ga atoms into the InAlAs layer improves the Schottky diode performance. The energy bandgap (E-g) for the In-0.52(AlxGa1-x)(0.48)As layer was (0.806 + 0.711x) eV, and the associated conduction-band discontinuity (Delta E-c), in the InAlGaAs/In0.53Ga0.47As heterojunction, was around (0.68 +/- 0.01)Delta E-g. Using this high quality In-0.52(Al0.9Ga0.1)(0.48)As layer in the Schottky and buffer layers, we obtained quarternary In-0.52(Al0.9Ga0.1)(0.48)As/In0.53Ga0.47As HEMT's. This quarternary HEMT revealed excellent dc and microwave characteristics. In comparison with the conventional InAlAs/InGaAs HEMT's, quarternary HEMT's demonstrated improved sidegating and device reliability.