Period-doubling bifurcation and the related noise effect have been investigated experimentally in the periodically driven instability of n-GaAs. An S-shaped I-V curve formed by the impact ionization avalanche of neutral shallow donors is modulated periodically by a DC+AC bias of the form V0+Vacsin(2 pi f0t). With f0 approximately 2 MHz, we observe a smooth period-doubling bifurcation as a function of V0. We have found a remarkable noise effect which appears as a deterministic noise amplification. The behaviour has been attributed to the sensitive nature of the NDR instability and thus the sensitivity of the filamentary current. In a theoretical frame, we have proposed some novel applications using the chaos system such as a deterministic noise amplifier and a chaos memory device.