Low frequency 10-100 kHz shot noise due to charge carriers photogenerated in planar Mo/n-Si/Mo structures leaving undepleted region has been observed under different bias and optical intensity conditions. The measurements revealed that the current noise observed depends not only on the illumination intensity levels as ordinal (not planar but vertical) photodetectors but also bias voltages. If the current noise spectrum is expressed as S(f) = 2qIGamma(2) (q is the elementary charge, I is the photoinduced current), the experimental noise ratio Gamma(2) lies between approximately 0.01 and 1.0 depending on the applying bias. The case Gamma(2) = 1.0 corresponds to full shot noise widely seen in many two terminal electronic devices carrying a dc current, I. This large change in the noise level and thus its noise ratio cannot be completely understood by the spatially uniform carrier generation and small cross-correlation between the drift current due to carriers emerged from the depleted region and the diffusion current due to carriers emerged from the undepleted region. To explain the behavior of observed noise more properly, we propose, in addition to the mechanisms above, that the reduction in autocorrelation effect of each current component plays an important role to decrease the relevant noise to such extremely low levels. (C) 2003 Elsevier Ltd. All rights reserved.