Noise Caused by GaAs MESFETs in Optical Receivers

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 Γ 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.