One type of this device relies on an overlap between the valence and conduction bands to keep the Fermi level constant and operates by inducing a current across a narrow depletion region. Four of these elements are connected in a bridge to produce a full-wave rectifier. Voltage regulators are often 
One type of this device relies on an overlap between the valence and conduction bands to keep the Fermi level constant and operates by inducing a current across a narrow depletion region. Four of these elements are connected in a bridge to produce a full-wave rectifier. Voltage regulators are often designed using a specific type of this circuit element in which the current is independent of the applied voltage once the breakdown voltage is exceeded. The varactor type of these circuit elements is used to implement a variable capacitance. Seemingly negative resistance is exhibited by a variety of these devices named for Esaki; simpler ones consist of single PN junctions. For 10 points, identify these circuit elements which also exist in Schottky and Zener varieties and that typically pass current in only one direction.
One type of this device relies on an overlap between the valence and conduction bands to keep the Fermi level constant and operates by inducing a current across a narrow depletion region.
{diodes}