||In this thesis, two kinds of punch-through phototransistors were studied. GaAs-AlGaAs heterojunction punch-through phototransistors (HPTPTs) were realized by employing Molecular Beam Epitaxy (MBE) growth and standard wet etching processing technology. Dark I-V characteristics were experimentally studied. Optical conversion gains as high as 1,400 were measured. The device also showed low output shot noise, which is only 10% of that of conventional 3-terminal phototransistor at the same bias condition. This is the first experimental proof that the output noise of phototransistors can be significantly reduced through punch-through operation. Lateral Si phototransistors with a punch-through base were fabricated with regular planar technology. A theoretical analysis of the dark I-V characteristics and gain of the devices, which was confirmed experimentally, was given. Ultra-high optical conversion gains were obtained under extremely low illumination level. The gain was higher than 100,000 when the incident light power is 0.19 nW. When the illumination level as low as 0.1 nW observed optical gain exceeded 150,000, which is the highest one ever reported in silicon material system. The noise characteristics of the devices were also discussed. Very low output noise was obtained. In comparison to either conventional three terminal phototransistors or p-i-n photodiodes, improvement of SNR beyond 40 dB has been achieved. The device also showed a fast transient response. The measured full-width-at-half-maximum (FWHM) of the device transient response is 1.2ns and a -3 dB bandwidth of 300 MHz. Currently, Silicon punch-through photodetectors (Si-PTPTs) can be used for detection through the visible and near infrared region out to 1.1 μm with a peak at 0.9 μm. However, the preliminary results predicted a bright future of the application of Si-PTPTs to diversified fields.