DiodeEngr. Maria Romina P. Angustia

-Professor-

Once you graduate from the simple, passive components that

are resistors, capacitors and inductors, it’s time to step on up to

the wonderful world of semiconductors. One of the most

widely used semiconductor components is the diode.

Diode

A diode is a specialized electronic component with two electrodes called the anode and the

cathode. Most diodes are made with semiconductor materials such as silicon,

germanium, or selenium.The key function of a diode is to control

the direction of current-flow.

Diode Circuit Symbol

Every diode has two terminals – connections on each end of the component – and those

terminals are polarized, meaning the two terminals are distinctly different.

The positive end of a diode is called the anode, and the negative end is called the cathode. Current can flow from the anode end to the cathode, but not the

other direction.

Current passing through a diode can only go in one direction, called the forward direction.

Current trying to flow the reverse direction is blocked. They’re like the one-way valve of

electronics.

Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is

prohibited; the diode is reversed biased.

When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is “backward” and the diode blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch: “closed” when forward-biased and “open” when reverse-biased.

Oddly enough, the direction of the diode symbol’s “arrowhead” points against the

direction of electron flow. This is because the diode symbol was invented by engineers, who

predominantly use conventional flow notation in their schematics, showing current as a flow of charge from the positive (+) side of the voltage

source to the negative (-). This convention holds true for all semiconductor symbols possessing

“arrowheads:” the arrow points in the permitted direction of conventional flow, and against the

permitted direction of electron flow.

Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valve allows fluid flow through it in only one direction as in Figure below.

• If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it. (Figure below)

)

Depletion region expands with reverse bias.

• Conversely, if a forward-biasing voltage is applied across the P-N junction, the depletion region collapses becoming thinner. The diode becomes less resistive to current through it.

Kinds of Diodes• Normal Diodes - Standard Signal Diodes are

among the most basic, average, no-frills members of the diode family.

• A rectifier or power diode is a standard diode with a much higher maximum current rating.

• Light-Emitting Diodes (LEDs) - The flashiest member of the diode family must be the light emitting diode (LED). These diodes quite literally light up when a positive voltage is applied.

• Schottky Diodes - Schottky diodes are especially useful in limiting losses

• Zener Diodes - are the weird outcast of the diode family. They’re usually used to intentionally conduct reverse current.

• Photodiodes - are specially constructed diodes, which capture energy from photons of light to generate electrical current. Kind of operating as an anti-LED.

Some Diode Applications• Rectifiers - A rectifier is a circuit that converts

alternating current (AC) to direct current (DC). This conversion is critical for all sorts of household electronics.

• Reverse Current ProtectionEver stick a battery in the wrong way? Or switch up the red and black power wires? If so, a diode might be to thank for your circuit still being alive. A diode placed in series with the positive side of the power supply is called a reverse protection diode. It ensures that current can only flow in the positive direction, and the power supply only applies a positive voltage to your circuit. Schottky diodes an excellent choice for reverse protection diodes.

• Logic Gates

• Flyback Diodes and Voltage Spike Suppression - Diodes are very often used to limit potential damage from unexpected large spikes in voltage. Transient-voltage-suppression (TVS) diodes are specialty diodes, kind of like zener diodes.