The negative differential resistance (NDR) effect and magnetoresistance (MR) effect attract a lot of attention since they have been widely applied in fields such as circuit amplifiers and information storage, respectively. In general, the NDR and MR effects derive from different physical mechanisms, which makes it difficult to obtain both an NDR effect and a large unsaturated MR effect in a device based on the same physical mechanism. In this paper, the NDR and unsaturated MR effects were observed simultaneously in In/SiO2/p-Si/SiO2/In hetero junction devices, and their physical mechanisms were investigated. It is found that under zero magnetic field conditions, the NDR effect can be observed up to 25 K which is much higher than the temperature of NDR obtaining in intrinsic silicon. The NDR effect is significantly enhanced by applying a small magnetic field of 0.1 T. Meanwhile, the unsaturated positive MR effect is obtained when an external magnetic field is applied. Our analysis shows that the NDR effect and the unsaturated MR effect can be explained on the basis of the same physical mechanism of the avalanche breakdown in the In/SiO2/p-Si hetero junction. The NDR effect mainly results from complex effects containing the carrier injection after avalanche breakdown in the hetero junction, the reduction of the Schottky barrier height and the Joule heating effect. The large unsaturated MR effect derives from the suppression of the plasma due to avalanche breakdown by the applied magnetic field. The largest value of MR based on avalanche breakdown can reach about 530%@1 T at 10 K, and MR sensitivity can be as high as about 29.4 T-1 under 0.05 T.