||Photodetectors with large internal gain and low intrinsic device noise are desired in various applications, such as weak signal detection and high-speed imaging. Commonly used photodetectors with internal gain are avalanche photodiodes (APDs). Because of the random nature of the avalanche multiplication process, the excess noise generated in these devices is the limiting factor on the detectivity. Therefore, there is a trade-off between the gain and noise for APDs. The gain of APDs is generally limited at a level of 10 for InGaAs/InP long wavelength APDs. In addition to APDs, phototransistors are also capable of satisfying many of the detector requirements. One of the drawbacks of phototransistors is that the gain is usually small and the response speed is slow at low incident optical power. The conventional way to solve these problems is to provide a quiescent bias through a base terminal. However, the base bias current provides another noise source, which will be amplified and contributes to the most important noise component to the total collector noise. The punchthrough phototransistors studied in this experiment, with a completely depleted base under normal operating conditions, can intrinsically offer very high gain and extremely low noise. The collector bias current can be provided by emitter-collector voltage through static induction effect. Such bias scheme can improve both current gain and frequency response without degradation in the output noise figure. Detailed analysis shows that the output noise power of this kind transistor is approximately l/β of the conventional transistors with the same collector bias current.