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Zener Diode

A Zener diode is a p-n junction operated in the reverse biased mode to take advantage of its sharply defined breakdown voltage. The Zener voltage VZ is specified at some test value of current I_ZT, at which the diode will exhibit some dynamic impedance   which depends upon the zener voltage of the diode and the level of Zener current.

Operation: A Zener diode may be used to regulate the load voltage at the value V_Z by acting as a bypass value to counteract line voltage or load current variations. Diodes having a breakdown voltage below about 6V rely on the true Zener effect (high electric field moves electrons from bonds), while the avalanche effect is responsible for reverse current above 6 V. Zener diodes have a temperature coefficient, α_Z which generally is negative for V_Z below about 6 V but positive above 6V, and is expressed in per cent of V_Z per °C, with the change in Zener voltage given by the equation.

Zener diode Characteristics

Zener Regulator

When Zener diode is forward biased, it works as a diode and drop across it is 0.7 V. When it works in breakdown region, the voltage across it is constant (V_Z) and the current through diode is decided by the external resistance. Thus, Zener diode can be used as a voltage regulator in the configuration shown in figure.
The load line of the circuit is given by:

V_s = R_sI_s + V_z

Zener diode as regulator

V-I Characteristics of Zener diode

To operate the zener in breakdown region V_s should always be greater than V_Z. R_s is used to limit the current
The Zener on state resistance produces more IR drop as the current increases. As the voltage varies from V₁ to V₂ the operating point shifts from Q₁ to Q₂,
The voltage at Q₁ is V₁= I₁ R_Z + V_Z
and at Q₂, V₂ = I₂ R_Z + V_Z
Thus, change in voltage is
V₂ – V₁ = (I₂ – I₁)R_Z; ΔV_Z = ΔI_ZR_Z

Operating point in V-I Characteristics of Zener diode

• Reference Zener diodes are available with α_z as low as O.0005 % °C.
• The admission of a small amount of mercury gas increases the current capability of a hot cathode gas filled tube.
• A cold cathode or glow discharge diode may be used as a DC voltage regulator in a similar manner to a Zener diode.

Optoelectronic Devices

Modern solid state devices, which include emitters, sensors and collectors, usually are known as optical electronic devices or optoelectronics.

• The radiations from a tungsten lamp is mostly in the infrared region, with relatively little in the visible region. It is often used as a source of infrared radiation.
• Illumination is a measure of the visible radiation on a surface and is measured in foot candles or lumens/sq-ft.
• Irradiance is a measure of the total radiation on a surface and is measured in watt/cm2.

1. Light Absorption and Emission: Light absorption and emission in a semiconductor is known to be heavily dependent on the detailed band structure of the semiconductor.

2. Direct Bandgap Semiconductors: Direct bandgap semiconductors, i.e., semiconductors for which the minimum of the conduction band occurs at the same wave vector k, as the maximum of the valence band, have a stronger absorption of light as characterized by a larger absorption coefficient.

3. Indirect Bandgap Semiconductors: Indirect bandgap semiconductors, i.e., semiconductors for which the minimum of the conduction band does not occur at the same wave vector as the maximum of the valence band, are known to have a smaller absorption coefficient and are rarely used in light emitting devices.

Photon absorption in a direct bandgap semiconductorIndirect bandgap semiconductor assisted photon absorption Indirect bandgap semiconductor assisted by photon emission

4. Photo-diode: The photo conductors which are junction type, are photo-diodes or photo transistors where in one or more p-n junction are used under reverse bias. Electron hole pairs resulting from photons incident on the p-n junction add to the minority carriers due to the reverse bias.

Symbol of Photo-diodeNote: Silicon and germanium photo sensors have their peak spectral response in the infrared region, with their response in the visible only approximately 40% of maximum.

• The inherent current gain of a transistor provides the photo transistor with a high current gain in the region of 1 mA/mW/cm2.
• By adding a second conventional transistor, a photo Darlington amplifier results, with even higher sensitivity.
• Photo-diodes and photo transistors may be operated as switching devices in light-operated relays, shaft encoders, paper tape readers, brush less DC motors, etc.

Variations of the basic p-n photo-diode are

• PIN photo-diode (Ultrafast response),
• Avalanche photo-diode (Higher current sensitivity and fast response),
• Photo-diode (Inexpensive higher current sensitivity but slower response)

5. Photovoltaic Sensors: Photovoltaic sensors, usually either silicon or selenium, generate a voltage under open circuit conditions of typically 0.5V. When operated in the short-circuit mode, the current is proportional to the illumination and can be used to provide a direct reading foot-candle metre. Photovoltaic sensors may be series parallel interconnected to provide large area solar power cells.

6. Light Emitting Diodes (LEDs): It is a special type of p-n junction device that under forward conditions can emit external spontaneous radiation in ultraviolet, visible and infrared regions of electromagnetic spectrum. Light Emitting Diodes may emit visible radiation when the p-n junction is diffused in GaAsP, infrared if constructed from GaAs, and a laser beam if sufficiently highly pulsed current is applied to a GaAs junction. A popular alphanumeric display is the LED seven segment device, which can provide high brightness for only a few milliwatts of input power per segment.

Basic Structure of light Emitting Diode (LED)

7. Photovoltaic (Solar Cells)

The solar cells are semiconductor junction devices which are used for converting optical radiation (sunlight) into electrical energy. The generated electric voltage is proportional to the intensity of incident light. Due to their capability of generating voltage they are called as photovoltaic cells. p-n junction solar cells are currently used to supply electrical power for many space satellites.
When a photon of light energy collides with the valence electron either in p-type material or n-type material, it imparts sufficient energy to the electron to leave its parent atom. As a result, free electrons and holes are generated on each side of the junction. In p-type material, the newly generated electrons are minority carriers.

Circuit diagram of photo voltaic cell

These electrons move freely across the junction with no applied bias. Similarly, in n-type material, the newly, generated holes are minority carriers
These holes move freely across the junction with no applied bias. The result is an increase in minority carrier flow. In this way, the depletion region potential causes the photocurrent to flow through the external load.

• The solar cell is a self-generating device i.e., it does not require any external power source.
• The internal emf and current generated by the solar cell is enough which can be measured by the galvanometer.
• A solar cell can't convert all solar radiation into electric energy.