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    Diodes are nothing more than PN junctions, but they come in many different styles and with many different uses. Besides the regular diodes, which can be used in rectifiers that convert alternating currents (AC) into direct current (DC), there are also LEDs (Light Emitting Diodes), Zener diodes (also very useful for rectification), Photodiodes (which detect light), and solar cells.

   The picture on the right shows three diodes. The top one is the 1N4001. The silver band marks the cathode (negative) side. This diode is used in rectification. Most circuits require DC, like you get from a battery, so if the device is to be plugged into AC, like you get from the electric company, it must first be rectified. These diodes rectify the alternating current by allowing current to flow in only one direction. Of course, this is only the first step in converting the AC into a pure direct current. The middle diode is a 1N4148. It is similar to the 1N4001, but it is used only for low power applications (as you can guess from its small size). The bottom diode is a special type of diode called a Zener. Zeners are meant to be connected in reverse bias. When they break down, they do so only temporarily, but because they break down at a specific voltage that does not depend on the amount of current that flows through them, they are the perfect choice when you need a stable voltage, as is needed in the final step of a full regulated power supply. Note that the black band still refers to the negative terminal, but now it is meant to be connected in reverse. In forward bias, the Zener acts like a regular diode (although you would not use it for that purpose since a cheaper regular diode would do in that case).

   The picture shown at right shows various LEDs (Light Emitting Diodes). The first three are obvious - a red, green and yellow LED. All diodes actually emit light, although it is not visible. LEDs, although they are still PN junctions, are not made from silicon (which will emit infrared light), and therefore can be made to emit visible light. What most people do not realize, however, is that the color of the light emitted by an LED has nothing to do with the color of the plastic casing. Often, the plastic case is made of the same color as the light the actual PN junction will emit, and is made to diffuse the light in order to make it easier to see from any angle. But the color of the light emitted actually depends on the material used in the PN junction (which is why blue LEDs are so expensive - they are made from rare materials). The fourth LED shown is white, but it does not emit white light. It actually has two separate PN junctions in opposite polarity so that in one direction it will emit green light, and in the other red. You have probably seen these types of LEDs in many modern electronic devices. The last one shown has a clear case, but it actually emits red light. These don't diffuse (spread out) the light. Instead they produce a tighter beam that is useful when you want to transmit information through light. You simply can not tell what color will be emitted by just looking at the LED until it is plugged in.

   The diodes shown at right are not LEDs at all, although they may look like them. These are photodiodes (actually, the third one is a phototransistor, but it is used in the same way). These PN junctions detect light. Actually all diodes (including LEDs) will detect light, but these are made to optimize that property. The photodiodes shown will detect mainly infrared light (such as the light emitted by remote controls), although they will pick up other wavelengths to a smaller degree.

   Solar cells, such as the one shown at left are also PN junctions (made of silicon). They are like photodiodes, except the PN junction area is made incredibly larger so that it can be used not just to detect light, but to produce electrical energy from it.
Image taken from Osaka Gas company.