^Zener diode

It is a specially designed junction diode, which can operate in the reverse breakdown voltage region continuously without being damaged.  In forward bias behaves like ordinary diode.  Zener diode does not gets damaged at breakdown voltage, but it does so at some higher reverse voltage, known as its ‘burn out value’. The magnitude of zener voltage V_Z can be decreased by increasing doping level in p and n type materials of zener diode.

I – V characteristics of a Zener diode are as shown. For a voltage more than zener breakdown voltage current through diode increases without increase in voltage across it. This feature is exploited to provide voltage stabilization across a circuit.

Suppose a load R_L connected between a & b is connected to a fluctuating dc voltage supply V_s between c & d. Let the load can’t tolerate a voltage VL but the supply V_s can be more than V_L. To protect the load from any increase voltage a zener diode of breaking voltage V_Z = V_L is connected to the input supply with a safety resistor R_S in the input loop.

If at any time V_Z increases than V_L the zener diode works in the breakdown region increasing the current through it without increasing the voltage across it & thus the load always remains protected.

^Solar or photovoltaic cells

A solar cell is a pn junction diode generally made from Si or GaAs. It works on following three basic processes: generation, separation and collection:

When sun light of energy   E > E_g falls near the junction (area large) they excite the electrons from VB to CB, leaving behind equal number of holes in the VB. These photo generated electron – hole pairs generated in the depletion region move in opposite directions due to the barrier electric field (electrons move towards n side and holes move towards p side of the junction) & are collected near the side of the diode, which makes the p – side a positive electrode & the n – side a negative electrode & thus a photovoltaic potential difference is developed across the junction. If external load is connected across a solar cell a photocurrent I_L starts flowing through it. This current is proportional to the intensity of the illumination falling on the diode.

Solar intensity received is maximum near 1.5 eV. As, for photo excitation E > E_g, so, semiconductor with band gap energy E_g ~ 1.5 eV or lower gives better solarconversion energy & thus are ideal materials for solar cell fabrication. Since Si has E_g ~ 1.1 eV while for GaAs it is ~1.43 eV, so they have relatively higher absorption coefficient than other  materials like CdS or CdSe. This is why Si and GaAs are preferred materials for solar cells. Note that sunlight is not always required for a solar cell.

Any light with photon energies greater than the band gap will do.

Solar cells are used to power electronic devices in satellites and space vehicles and also as power supply to some calculators.

^Ripple factor

The output obtained from a rectifier is the superposition of both ac & dc components. The ratio of ac component to the dc component in the rectified output is called the ripple factor it decides the effectiveness of a rectifier. i.e.

ac power input

It is the effective input power i.e.

dc power output

It is the effective output power across the load i.e.

Rectifier efficiency

It is defined as the ratio of dc output power to the applied ac input power i.e.