what is DIODE?

DIODE

• A diode is a 2 lead semiconductor that acts as a one way gate to electron flow.
• Diode allows current to pass in only one direction.
• A pn-junction diode is formed by joining together n-type and p-type silicon.
• In practice, as the n-type Si crystal is being grown, the process is abruptly altered to grow p-type Si crystal. Finally, a glass or plastic coating is placed around the joined crystal.
• The p-side is called anode and the n-side is called cathode.
• When the anode and cathode of a pn-junction diode are connected to external voltage such that the potential at anode is higher than the potential at cathode, the diode is said to be forward biased.
• In a forward-biased diode current is allowed to flow through the device.
• When potential at anode is smaller than the potential at cathode, the diode is said to be reverse biased. In a reverse-biased diode current is blocked.

Water Analogy of diodes

• When water pressure on left overcomes the restoring force of spring, the gate is opened and water is allowed to flow.
• When water pressure is from right to left, the gate is pressed against the solid stop and no water is allowed to flow.
• Spring restoring force is analogous to 0.6V needed to forward bias a Si diode.

Diode: How it Works —I

• When a diode is connected to a battery as shown, electrons from the n-side and holes from the p-side are forced toward the center by the electrical field supplied by the battery. The electrons and holes combine causing the current to pass through the diode. When a diode is arranged in this way, it is said to be forwardbiased.

Diode: How it Works—II

• A diode’s one-way gate feature does not work all the time.
• Typically for silicon diodes, an applied voltage of 0.6V or greater is needed, otherwise, the diode will not conduct.
• This feature is useful in forming a voltage-sensitive switch.
• I-V characteristics for silicon and germanium diodes is shown below.

Diode: How it doesn’t work

• When a diode is connected to a battery as shown, holes in the nside are forced to the left while electrons in the p-side are forced to the right. This results in an empty zone around the pn- junction that is free of charge carries creating a depletion region.This depletion region acts as an insulator preventing current from flowing through the diode. When a diode is arranged in this way, it is said to be reversebiased.

Diode Applications —Half Wave Rectifier

• Diode converts ac input voltage to a pulsed dc output voltage.
• Whenever the ac input becomes negative at diode’s anode, the diode blocks current flow.
• o/p voltage become zero.
• Diode introduces a 0.6V drop so o/p peak is 0.6V smaller than the i/p peak.
• The o/p frequency is same as the i/p frequency.

Diode Applications —Full Wave Rectifier

• A full-wave rectifier does not block negative swings in the i/p voltage, rather it transforms them into positive swings at the o/p.
• To gain an understanding of device operation, follow current flow through pairs of diodes in the bridge circuit.
• It is easily seen that one pair (D3-Rout-D2) allows current flow during the +ve half cycle of Vin while the other pair (D4-Rout-D1) allows current flow during the -ve half cycle of Vin.
• o/p voltage peak is 1.2V below the i/p voltage peak.
• The o/p frequency is twice the i/p frequency.

Diode Applications —AC2DC Power Supply

• An AC2DC power supply is built using a transformer and a full-wave rectifier.
• Transformer is used to step down the voltage i/p.
• Rectifier converts AC to pulsed DC.
• A filter capacitor is used to smooth out the pulses.
• Capacitor must be large enough to store sufficient charge so as to provide a steady current supply to the load:

 

f is rectified signal’s frequency (120Hz).