The characteristics of InGaP-AlxGal-xAs-GaAs composite-emitter heterojunction bipolar transistors (CEHBTs) with different thickness of AlxGa1-xAs graded layer are comprehensively studied and demonstrated. The thickness of the graded AlxGa1-xAs layer is an important factor to affect device performances. In this work, it is found that CEHBTs with 70 Angstrom similar to 100 Angstrom AlxGa1-xAs graded layers exhibit better properties due to the absence of potential spike. For comparison, experimentally, two practical CEHBTs with 0 and 100 Angstrom AlxGa1-xAs graded layers are fabricated. Generally, good agreements between experimental results and theoretical simulations are found. It is, therefore, concluded that the CEHBT with an appropriate thickness of the AlxGa1-xAs graded layer offers the promise for analog, digital, and microwave device applications, especially for lower operation voltage and lower power consumption circuits. (C) 2004 American Vacuum Society.