A rectifier is the simplist semiconductor device. It is simply a PN junction.
A rectifier diode has a low resistance in the forward direction, but a very high resistance in the other. The band on the device indicates the cathode leg, the another is the anode. If the voltage backwards on the device is high enough the high resistance will collapse in an "avalanche" and the diode will "break down". The diode is often used to rectify AC voltage in a power supply to create a DC current. This system when used with 4 diodes is called a bridge rectifier.