The Photovoltaic Effect

Philip Said:

Alexandre-Edmon Becquerel – what exactly did he discover?

We Answered:

For one thing , don’t call him an old man!! He was only 20 when he showed that light can initiate chemical reactions that produce an electric current. His discovery opened up the idea that light energy might be transformed into electrical energy and led to the investigations that uncovered the photoelectric effect.
Don’t be too bothered about the different names used for the effect. If light energy is being transformed into electric energy then folk can call it what they like ( and obviously do ) but its all the same thing.

Helen Said:

What is the photovoltaic effect? What is doing? What is N-type and P-type material? Why aren’t solar cells?

We Answered:

Sci-Tech Encyclopedia: Photovoltaic effect

The conversion of electromagnetic radiation into electric power through absorption by a semiconducting material. Devices based on this effect serve as power sources in remote terrestrial locations and for satellites and other space applications. Photovoltaic powered calculators and other consumer electronic products are widely available, and solar photovoltaic automobiles and aircraft have been demonstrated.

The basic requirements for the photovoltaic effect are (1) the absorption of photons through the creation of electron-hole pairs in a semiconductor; (2) the separation of the electron and hole so that their recombination is inhibited and the electric field within the semiconductor is altered; and (3) the collection of the electrons and holes, separately, by each of two current-collecting electrodes so that current can be induced to flow in a circuit external to the semiconductor itself.…

A bipolar (junction) transistor (BJT) is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes, as opposed to unipolar transistors, such as field-effect transistors, in which only one carrier type is involved in charge flow.

Although a small part of the transistor current is due to the flow of majority carriers, most of the transistor current is due to the flow of minority carriers and so BJTs are classified as ‘minority-carrier’ devices.

An NPN transistor can be considered as two diodes with a shared anode region. In typical operation, the emitter–base junction is forward biased and the base–collector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or “diffuse”) through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base.

The base region of the transistor must be made thin, so that carriers can diffuse across it in much less time than the semiconductor’s minority carrier lifetime, to minimize the percentage of carriers that recombine before reaching the collector–base junction. To ensure this, the thickness of the base is much less than the diffusion length of the electrons. The collector–base junction is reverse-biased, so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base junction.…