The present invention relates to a semiconductor device and, more particularly, to a photoelectric converting apparatus such as a radiation detector for detecting a radiation such as a X-ray or the like and converting the detected radiation into electric signals. In a photoelectric converting apparatus, particularly a radiation detector using amorphous silicon, charges are generated by a radiation which has been absorbed in a photoconductive layer, and the generated charges are stored as an electric signal, thereby detecting the radiation. A conventional radiation detector is described in detail in, e.g., "Philips Research Record", Vol. 29, No. 1, pp. 12-21 (1984). The principle of operation of the conventional radiation detector will be described with reference to FIG. 4. An electrode 4 is disposed in contact with a photoconductive layer 2 on an insulating substrate 1. A bias voltage for reading out stored charges is applied to the electrode 4, and the substrate 1 is grounded. A reverse bias voltage is applied to the photoconductive layer 2 to eliminate the surface charge. When a radiation 11 is applied from a side of the substrate 1, the radiation 11 is absorbed by the photoconductive layer 2, and charges 23 are generated as shown in FIG. 5(A). At the same time, in the vicinity of the surface of the substrate 1, charges 22 which are generated by a leakage current are stored. Since the charges 23 are generated only in the vicinity of the surface of the photoconductive layer 2, the charges 23 generated in the vicinity of the surface are called bulk charges 23B, while the charges 22 generated in the region deep from the surface of the substrate 1 are called depletion charges 22D. If an electric field caused by the charges 23B is applied between the electrode 4 and the photoconductive layer 2, the charges 23B migrate toward the electrode 4. If the charges 23B are attracted by the electrode 4, and a depletion region 21 is formed in the photoconductive layer 2, no current flows. When the photoconductive layer 2 is exposed to the radiation 11, charges 22D move toward the electrode 4. If charges 22D are attracted by the electrode 4, a depletion region 23D is formed in the vicinity of the surface of the photoconductive layer 2, and a current flows. The charges 23 which have been moved toward the electrode 4 form an electric current in proportion to the amount of radiation 11 applied, thereby obtaining an output. However, since the photoconductive layer 2 has high resistivity and since a large leakage current is generated in the region of the depletion region 23D under a high reverse bias voltage, the depletion region 23D is spread as shown in FIG. 5(B). On the other hand, the charges 22 which have been moved toward the electrode 4 are stored in a state where the charges 22 have been dispersed in the surface of the photoconductive layer 2 as shown in FIG. 5(C). Consequently, the surface charges 22 are gradually reduced and the electric field is weakened, and an image quality of a radiation image deteriorates. As described above, in the conventional radiation detector using amorphous silicon, the storage charges 22 are lost by a leakage current of the photoconductive layer 2 in an applied high reverse bias voltage region, and the stored charges 22 move to the electrode 4 and become a cause of spreading a depletion region. As a result, as the surface area of the photoconductive layer 2 is increased, the above-mentioned problem of deteriorated image quality becomes prominent. In a radiation detector using a glass tube, the depletion layer spreads, and the output value of the radiation detector changes depending on the pattern of the image.