Diodes were formed by intermediating a thin film of PEDOT:PSS between aluminum and heavily doped silicon. Both p-type and n-type Si substrates were used. Hysteresis loops were observed in their current-voltage (I-V) characteristics. A ‘state’ can be written by applying a voltage pulse to aluminum electrode. The state of the device can be read out from the current under a small probe voltage (0.3-0.6 V, to Al electrode). Appling +4.0 V induces a ‘low’ conductance state while applying -4.0 V switches the device back to the ‘high’ conductance state. The current difference between two states is up to 3 orders of magnitude. The space charge storage in the polymer is believed to be responsible for the memory effect. Upon positive voltage bias the charges are injected into the place near the Al/PEDOT:PSS interface. The charges are stored there and will resist subsequent charge injection, resulting in the ‘low’ conduction state. The redox reactions further reduce the conductance of the device. Negative bias can remove the stored charges and reverse the redox reactions, thus recover the device back to ‘high’ conduction state.