No abstract

We demonstrate by measurement and analysis that previously reported InP/InGaAs heterojunction phototransistors are promising photodetectors for long-wavelength optical receivers. The gain characteristics of these devices are shown to reflect the benefit of a wide band-gap emitter as well as the effects of recombination centers near the base-emitter heterojunction. In order to analyze the noise characteristics we derive a model which accounts for the three principal noise sources: the shot noise of the base current, the shot noise of the collector current, and the thermal noise of the load resistor. Using this model and experimental device parameters, we calculate the sensitivity of an optical receiver which utilizes a heterojunction phototransistor. This calculation indicates that the performance of this monolithic device can be as good as or exceed that of a hybrid p–i–n photodiode/field effect transistor-preamplifier combination.

We describe the fabrication and characteristics of a small-area (diameter ≃20 μm) InP/InGaAs heterojunction phototransistor, a promising photodetector/preamplifier for long-wavelength optical receivers. The high sensitivity (hfe ≃100 at 20-nW incident power) and small junction capacitance (≲0.2 pF) of the device combine to produce a gain-bandwidth product in excess of 1.7 GHz.

In the application of GaAs metal‐semiconductor field effect transistors (MESFETs) in ultra low‐noise lightwave receivers, the channel noise is often the dominant effect in determining sensitivity. This paper analyzes for the first time the excess channel‐noise factor Γ for GaAs by considering the effect of circuit capacitance, as well as gate‐to‐source capacitance on the correlation of gate and channel fluctuations, and derives a useful and analytic expression for Γ. For example, we find that T for practical GaAs MESFET amplifiers can be much larger than 1.1 as is usually assumed. The multiplication factor, Γ is approximately 1.75 for the practical GaAs MESFET with 1‐μm gate length, which explains the discrepancy between the optical sensitivity from the noise calculation and experiments.

At the wavelength of 1.3 μm, fiber loss and dispersion are sufficiently small that many lightwave communications applications can use simple and reliable LED transmitters and PIN photodiode receivers without avalanche gain. Bit rates can be as high as several hundred Mb/s. With new high‐speed devices based on III‐V semiconductors and microwave silicon IC technology, we have designed two fully retimed optical regenerators that operate at 1.3 μm and at bit rates of 44.7 and 274 Mb/s to study the potential of the LED‐PIN approach. A detailed analysis of the baseband characteristics of the LED, the fiber, and the receiver leads to an overall equalization approach that minimizes receiver noise. The success of this performance optimization is corroborated by bit‐error measurements under simulated system conditions. The results suggest repeaterless operation over distances up to 24 km at 44.7 Mb/s and 8 km at 274 Mb/s for a cable loss of 1 dB/km and a bandwidth of 1000 MHz·km. The use of lasers in such multimode fiber sytems would permit larger margin allocations and penalties than those chosen for LED systems, but would not lead to substantially longer repeater spacings.