Q. Explain about Diodes?

A single pn-junction with appropriate contacts for connecting the junction to external circuits is called a semiconductor pn-junction diode. The fundamental building block upon which all semiconductor devices are based is the pn-junction. The most common two-terminal nonlinear resistor is the semiconductor diode, whose symbol is shown in Figure (a). The terminal voltage and current are denoted by vD and iD, respectively. The physical structure of the pn junction is shown in Figure (b). The junction is made by doping the two sides of the crystal with different impurities. Figure (c) shows the volt-ampere curve (or static characteristic) of the ideal (or perfect) diode. Note that when vD is zero, iD is not and vice versa, a condition corresponding to a switch. A diode acts like a switch that closes to allow current flow in the forward direction, but opens to prevent current flow in the reverse direction. The diode thus acts like a unilateral circuit element providing an on-off characteristic.