| 摘要 |
A novel field-effect semiconductor device having both low-noise and high-output operating characteristics has a first semiconductor buffer layer, an undoped second semi-conductor layer, an undoped third semiconductor layer the forbidden band gap of which increases from the substrate to the electrode side, a fourth semiconductor layer of one conductivity type, and a fifth semiconductor layer of undoped type or one conductivity type, formed one on top of another in this order on a semiconductor substrate. When the gate potential is deep, electrons mostly travel through the undoped second and third semiconductor layers, the device exhibiting superior low-noise characteristic; when the gate potential is shallow, electrons mostly travel through the highly doped fourth semiconductor layer, the device thus achieving high output characteristic.
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