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A diode conducts (offers 0 resistance ideally i.e., it acts as a short circuit) only when it is forward biased and it offers very high impedance (in other words, it acts as a open circuit) when reverse biased. Thus, when an input is given to the diode (to its anode), only when the input is positive, it forward biases the diode, and so there will be current flow in the circuit, else the diode will not conduct and so there will be no current flow. This way u can make diode work as a switch, (only when voltage is +ve, u get an output current and when voltage is -ve, the current flow will be zero).
Logic gates "AND" and "OR" can be constructed by diode switches. The diode logic is not used into modern ICs, but they are very easy circuits to implement and test by yourself!
The clear explanation you asked for, could be found in wikipedia:
A diode (ideal) does not have resistance . So when it is Forward biased(FB) ie the positive terminal of the voltage is connected to the P junction(anode) of the diode and the negative of the supply is connected to the N junction(cathode) , the diode acts as a closed switch
When the diode is reverse biased (RB) ie the supply polarity is interchanged the diode acts as an open switch.
( note that this is for an IDEAL diode) Practically a small current flows through the junction when the diode is in RB.
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