How do Diodes Work?

A diode allows current to flow in only ONE direction.

If the cathode end (marked with a stripe) is connected so it is more negative than the anode end, current will flow.

The picture shows three types of diode: Small signal diode Rectifier diode Soft fast recovery diode

A diode has a forward voltage drop. That is to say, when current is flowing, the voltage at the anode is always higher than the voltage at the cathode. The actual Forward Voltage Drop varies according to the type of diode. For example:

Silicon diode = 0.7v Schottky diode = 0.3v Germanium diode = 0.2v In addition, the voltage drop increases slightly as the current increases so, for example, a silicon rectifier diode might have a forward voltage drop of 1 volt when 1 Amp of current is flowing through it.

A ZENER diode allows current to flow in both directions. In the "forward" direction, no current will flow until the voltage across the diode is about 0.7 volts (as with a normal diode). In the reverse direction (cathode more positive than the anode) no current will flow until the voltage approaches the "zener" voltage, after which a LOT of current can flow and must be restricted by connecting a resistor in series with the zener diode so that the diode does not melt!

Within a certain supply voltage range, the voltage across the zener diode will remain constant. Values of 2.4 volts to 30 volts are common. Zener diodes are not available in values above around 33 volts but a different type of diode called an AVALANCHE diode works in a similar way for voltages between 100v and 300v. (These diodes are often called "zener" diodes since their performance is so similar.)

Zener diodes are used to "clamp" a voltage in order to prevent it rising higher than a certain value. This might be to protect a circuit from damage or it might be to "chop off" part of an alternating waveform for various reasons. Zener diodes are also used to provide a fixed "reference voltage" from a supply voltage that varies. They are widely used in regulated power supply circuits.

• A semiconductor based laser that generates coherent light at a characteristic wavelength through edge emission
  
• All laser diodes are based on an LED (an LED is basically a laser without a feedback cavity)
  
• CD-ROM and DVD-ROM, CD_RW, printers, bar code scanners, laser pointers, DWDM lasers

• Mass production techniques allow very low per unit costs – far cheaper than other non-semiconductor lasers available
  
• Current injection allows the diode to be rapidly modulated – perfect for data transfer in communications

• All diode lasers are based on LEDs, where the underlying structure is a p-n junction operating under a positive bias. As electrons from the n-type semiconductor combine with the holes in the p-type semiconductor they release energy in the form spontaneous emitted light (photons)
  
• Diodes differ in that light is generated by stimulated emission rather than spontaneous emission
   - Therefore the energy from the electrons dropping from the conduction band must be efficiently transferred to the generated photons so that these photons may have enough energy to stimulated emission from another electron. Therefore semiconductors with direct band-gaps are required (GaAs)
  
• Lasers also require a higher current to ensure that population inversion occurs (the excited states are more populated than the relaxed states).
   - Now one photon produces on average more than one stimulated photon before leaving the laser or being absorbed.