Photosensitive Diodes

Virtually all P-N junctions exhibit conductivity that varies with exposure to radiant electromagnetic energy such as IR, visible light, and UV. The reason that conventional diodes are not affected by these rays is that they are enclosed in opaque packages. Some photosensitive diodes have variable dc resistance that depends on the intensity of the electromagnetic rays. Other types of diodes produce their own dc in the presence of radiant energy.

Silicon Photodiodes

A silicon diode, housed in a transparent case and constructed in such a way that visible light can strike the barrier between the P-type and N-type materials, forms a silicon photodiode. A reverse-bias voltage is applied to the device. When radiant energy strikes the junction, current flows. The current is proportional to the intensity of the radiant energy, within certain limits.
Silicon photodiodes are more sensitive at some wavelengths than at others. The greatest sensitivity is in the near infrared part of the spectrum, at wavelengths just a little bit longer than the wavelength of visible red light. When radiant energy of variable intensity strikes the P-N junction of a reverse-biased silicon photodiode, the output current follows the light-intensity variations. This makes silicon photodiodes useful for receiving modulated-light signals of the kind used in fiber-optic communications systems.

The Optoisolator

An optoisolator has an LED or IRED at the input and a photodiode at the output.
An LED or IRED and a photodiode can be combined in a single package to get a component called an optoisolator. This device, the schematic symbol for which is shown in above figure, creates a modulated-light signal and sends it over a small, clear gap to a receptor. An LED or IRED converts an electrical signal to visible light or IR; a photodiode changes the visible light or infrared back into an electrical signal.
When a signal is electrically coupled from one circuit to another, the two stages interact. The input impedance of a given stage, such as an amplifier, can affect the behavior of the circuits that feed power to it. This can lead to various sorts of trouble. Optoisolators overcome this effect, because the coupling is not done electrically. If the input impedance of the second circuit changes, the impedance that the first circuit sees is not affected, because it is simply the impedance of the LED or IRED. That is where the “isolator” in “optoisolator” comes from. The circuits can be electronically coupled, and yet at the same time remain electrically isolated.

Photovoltaic Cells

A silicon diode, with no bias voltage applied, can generate dc all by itself if enough electromagnetic radiation hits its P-N junction. This is known as the photovoltaic effect. It is the principle by which solar cells work.
Photovoltaic cells are specially manufactured to have the greatest possible P-N junction surface area. This maximizes the amount of light that strikes the junction. A single silicon photovoltaic cell can produce about 0.6 V of dc electricity. The amount of current that it can deliver, and thus the amount of power it can provide, depends on the surface area of the junction.
Photovoltaic cells can be connected in series-parallel combinations to provide power for solidstate electronic devices such as portable radios. These arrays can also be used to charge batteries, allowing for use of the electronic devices when radiant energy is not available (for example, at night!).
A large assembly of solar cells, connected in series-parallel, is called a solar panel. The power produced by a solar panel depends on the intensity of the light that strikes it, the sum total of the surface areas of all the cells, and the angle at which the light strikes the cells. Some solar panels can produce several kilowatts of electrical power in direct sunlight that shines in such a way that the sun’s rays arrive perpendicular to the surfaces of all the cells.