Special Purpose \(p-n\) Junction Diodes includes Zener Diode, Light-Emitting Diode, Photodiode, A solar cell or photovoltaic devices. Light-emitting diodes, photodiodes, and photovoltaic devices are known as optoelectronic junction devices.

The cameras we used today have auto-focus features. And this feature is possible because of Photodiodes. Because Photodiodes react when light reaches them, they are also used in barcode scanners to measure the intensity of the light reflected from the barcode. But what are Photodiodes? And How do they work? Nowadays, we are replacing Incandescent bulbs with LED light bulbs in our houses. But why? What do you think is an LED, and what are its advantages?  You may have seen a voltage regulator used to protect appliances and devices. Well, this Regulator uses a special diode called Zener diode. 

Did you know that the Zener diode was named after the Physicist Clarence Zener who first discovered its electrical properties? The remote controls are used for operating television sets; but, how is the signal emitted from a remote control?  On closely observing, we find a tiny bulb-like device at the end of the remote. Solar energy is Renewable energy that comes from the Sun. One way to utilise it is by using solar cells.

Types of Reverse Breakdown

 (1) Zener Breakdown 

When a reverse bias is increased, the electric field at the junction also increases. The electric field becomes so high that it breaks the covalent bonds creating electron-hole pairs at some stage. Thus a large number of charge carriers are generated. This generation of charge carriers causes a large current to flow. This mechanism is known as Zener breakdown.

(2) Avalanche Breakdown 

Due to the high electric field, the minority charge carriers, while crossing the junction, acquire very high velocities at high reverse voltage. These by collision breaks down the covalent bonds, generating more carriers. A chain reaction is established, giving rise to a high current. This mechanism is called avalanche breakdown.

(3) Optoelectronic Junction Devices
These are the junction devices in which light energy is converted into electrical energy or vice versa.

Zener Diode

It is a heavily doped \(p-n\) junction and can be used as a voltage regulator. It is designed to operate under reverse bias in the breakdown region. Due to the heavy doping of \(p\) and \(n\) sides, the depletion region formed is very thin, and the junction’s electric field is extremely high.

At the high value of breakdown voltage, the intensity of the electric field becomes high. When the charge carriers cross the depletion region (like the electron, for example), it gets accelerated due to the high electric field. This accelerated charge breaks many covalent bonds in the depletion region and creates electron-hole pairs. This newly created electron also accelerates and breaks many more covalent bonds to create more electron-hole pairs. This process keeps on repeating. This process increases the electric current flowing through the diode. We can say that the diode has reached a breakdown point. This type of breakdown is called the Avalanche effect. The diode in which breakdown is due to the Avalanche effect is known as the Avalanche diode.

The symbolic diagram of the Zener diode is shown-

The symbolic representation of the Zener diode is similar to the normal diode, the only difference being that the cathode has been shaped in the form of the alphabet \(‘Z’\).

Characteristics of the Zener Diode

The forward bias characteristic of the Zener diode is very similar to that of the \(P-N\) junction diode. Near the breakdown voltage \(({V_Z})\), this current suddenly increases to the order of milliampere. This current is called the Zener current \(({I_Z})\).

The breakdown obtained in this case is very sharp. It can be seen from the reverse bias characteristic that a small change in the voltage near the breakdown voltage produces a large change in the current. The voltage across the Zener diode remains constant for large changes in the current in this situation. Hence such a diode can be used as a voltage regulator.

Zener diode as a voltage regulator:  Zener diode can be used for regulating supply voltages so that they are constant. The circuit of the voltage regulator using a Zener diode is as shown.

The unregulated dc voltage is connected to the Zener diode through a series resistance \({R_S}\) such that the diode is reverse biased. If the input voltage increases, the current through \({R_S}\) and Zener diode also increase. This process increases the voltage drop cross \({R_S}\) without any change in the voltage across the Zener diode. ( in the breakdown region, the Zener voltage remains constant even though the current through it changes ). If the input voltage decreases, the current through \({R_S}\) and Zener diode decrease without changing the voltage across the Zener diode. Thus the Zener diode acts as a voltage regulator. One must select the Zener diode according to the output voltage required and the series resistance \({R_S}\) accordingly_.

Light Emitting Diode

We have observed the light indicators in various electrical appliances. All these devices are tiny light sources called light-emitting diodes or LEDs.

This is the symbol of an LED-

LED is a heavily doped \(p-n\) junction, which converts electrical energy into optical energy under forward bias and emits spontaneous radiation. Its default \(p\)-type semiconductor material is doped gallium aluminium arsenide (GaAlAs), while the \(n\)-type semiconductor is doped gallium arsenide (GaAs). When this diode is forward biased, the light of a particular colour is emitted. During the forward bias, the electrons from the \(n\)-type semiconductor combine with the holes in the \(p\)-type semiconductor at the junction. The electrons make the transition from the conduction band to the valence band. During this process, energy is released in the form of visible radiation.

This diode is placed in a small reflective cup connected to a lead frame bonded with two frame terminals through tiny bonding wires. The entire assembly is encased in a solid epoxy dome lens that enables the emitted light to be focused in a single direction. The longer terminal is the anode, and the shorter one is the cathode. When connected to a circuit in forward bias, this LED gives off light, but there is no light emission if the diode is reverse biased.

Depending on the semiconductor material used, LEDs emit light in various colours like orange, yellow etc. These LEDs emit light in the visible region, but few produce light in the infrared region, which is not visible to the human eye. The remote controls used for operating these appliances use some of these LEDs.  On pressing any key on the remote, the corresponding signal in infrared rays is emitted. This signal is received by the infrared receiver on the television, which then operates as per the commands.

This display is called the seven-segment display. Each segment is an LED. Seven-segment displays are widely used in electronic meters, digital clocks, basic calculators, and other electronic devices that display numerical information.

Compared to other filament lamps, LED has many advantages like its fast response, long life, reliability, and small current requirement.

The wavelength of radiation emitted by an LED is given by:

\(\lambda = \frac{{hc}}{{{E_g}}} = \frac{{1.24}}{{{E_g}(eV)}}\mu m\)

Silicon and germanium are heat (IR) producing semiconductors. For an LED to emit visible light, it must have an energy gap from \(1.8 eV\) to \(3 eV\). The compound semiconductor GaAsP is used for making LEDs of different colours.

LEDs are used in display devices, indicators and optical communication. LED’s emitting infrared radiation are used in remote controls and burglar alarm systems.

Photodiode

The construction of a photodiode is similar to the normal diode. The only difference between the two is a window in a photodiode through which the light enters and is incident on the diode. This diode is always connected in a reverse bias mode.

Reverse saturation current flows through the \(PN\) junction diode on connecting it in a reverse bias mode. The reverse saturation current can be increased by increasing the diode’s temperature or making more light incident over it. When the energy of the light incident on the junction \(\frac{{hc}}{\lambda }\) is more than \({{E_g}}\), many covalent bonds are broken near the junction.

This process further produces a large number of electron-hole pairs. Thus the increase in the minority charge carriers contributes towards an increase in the reverse current. This reverse current is of the order of \(\mu A\). The reverse current flowing through the diode in the absence of the incident light is known as a dark current. The electron-hole pair increases by increasing the intensity of the incident light. This process results in a proportionate increase in current.

Solar Cell

Solar cells, also called photovoltaic cells, are used in spacecraft, blinkers, solar calculators, solar watches etc. A solar cell is a \(p-n\) junction device that generates electromotive force when solar radiation falls on it. For this reason, it is also called an optoelectronic junction device as it is a combination of both optics and electronics. Solar cells, photodiodes and LEDs are some of the optoelectronic devices.  In this topic, we will study a \(p-n\) junction solar cell and its \({\rm{I – V}}\) characteristics.

To begin, let us see how a solar cell is constructed. A solar cell consists of a \(p-n\) junction.

The \(p\)-type -\(Si\) wafer is about \(300\,\mu m\) while the n type-\(Si\) wafer is a thin layer of about \(0.3\,\mu m\). One side of the \(p\)-type- \(Si\) layer is coated with metal and serves as the back contact. The top side of the n type-\(Si\) layer has a metallic grid deposited on it, also called the metal finger electrode.  This acts as a front contact. The space occupied by the metal grid is kept small, approximately less than/of the cell area, so that light can be incident on it from the top.

Now let us understand the working of a solar cell.

We know that for a \(p-n\) junction under equilibrium conditions, an electric field exists across the depletion layer of the junction. A solar cell operates on solar radiation when photons are incident near the junction. If the incident photons have energy greater than the energy gap of \({{E_g}}\) the semiconductor, the valence electrons become, and the electron-hole pairs are created near the junction. Due to the electric field in the junction, the electrons are swept towards the \(n\)- region and the holes towards the \(p\)–region.  The electrons that reach the \(n\)-side are collected by the front contact, while the holes reaching the \(p\)-region are collected by the back contact. Thus, the \(p\)-region becomes positive while the \(n\)-region becomes negative, giving rise to photo-voltage. If we connect an external load across the end of the solar cell, a photocurrent flows across the load.

A typical \({\rm{I – V}}\) characteristic of a solar cell is observed in the fourth quadrant because a solar cell does not draw any current but supplies the same to the load.

Thus, we can see that a solar cell works on the principle of photovoltaic effect as in the case of a photodiode. However, a solar cell is different from a photodiode as no external bias is applied across the junction, and the junction area is kept much larger for solar radiation.

Now let us look at the criterion of selection of semiconductors for solar cells. Semiconductors with an energy gap of about \(\left( {1.5{\rm{ }}eV} \right)\) are ideal materials for solar cells. Silicon with an energy gap of about \(\left( {1.1{\rm{ }}eV} \right)\) and gallium arsenide with an energy gap of  are preferable choices. The material used to make solar cells should have high optical absorption \(({10^4}/{\rm{cm)}}\), good electrical conductivity, availability, and cost-effectiveness.

Solar cells are one of the best renewable resources as well as eco-friendly.

Summary

1. Solar cells, also called photovoltaic cells, are used in spacecraft, blinkers, solar calculators, solar watches, etc.
2. A solar cell works on the principle of the photovoltaic cell. It generates electromotive force when solar radiation falls on the \(p-n\) junction.
3. Silicon with an energy gap of about \(\left( {1.1{\rm{ }}eV} \right)\) and gallium arsenide with an energy gap of \(\left( {1.43{\rm{ }}eV} \right)\) are preferable choices.
4. Zener diode is a heavily doped \(p-n\) junction and can be used as a voltage regulator.
5. A photodiode is one in which the reverse saturation current increases upon illumination of the light of suitable frequency.
6. LED is a heavily doped \(p-n\) junction that emits radiation when forward biased.

FAQs on Special Purpose p-n Junction Diodes

Q.1. What are special-purpose diodes?
Ans: Some of the more common special-purpose diodes are Zener diode, Light-emitting diode (LED), Photo-diode, Tunnel diode, Varactor diode and Schottky diode.

Q.2. What is the working principle of a light-emitting diode?
Ans: When a suitable voltage is applied to the leads, electrons can recombine with electron holes within the device, releasing energy in the form of photons.

Q.3. How is junction diode different from Zener diode?
Ans: The main difference between the \(p-n\) junction diode and the Zener diode is that the \(p-n\) junction diode allows the flow of electrons in one direction, whereas the Zener diode allows the flow of electrons in both directions.

Q.4. What are the uses of LED lights?
Ans: LED lights are mostly used in street lights, parking garage lighting, walkway and outdoor area lighting, refrigerated case lighting, modular lighting, and task lighting.

Q.5. Is Zener diode a PN junction diode?
Ans: A Zener diode is a silicon semiconductor device that permits current to flow forward or reverse direction. The diode consists of a special, heavily doped \(p-n\) junction designed to conduct in the reverse direction when a certain specified voltage is reached.

Q.6. Why are LED’s so efficient?
Ans: LED’s are efficient because of their fast response, long life, reliability, and small current requirement.

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