Semiconductor Devices

Zener Breakdown

Observed in diodes with breakdown voltage $V_z$ of 5 to 8 volts.
Valence electrons are pulled into the conduction band due to the high electric field in the narrow depletion region.
Increase in temperature decreases the breakdown voltage.
VI characteristics have a sharp curve.
Occurs in highly doped diodes.
The depletion region is thin.
High doping concentration at the junction.
Occurs because of the high electric field.
Production of electron-hole pairs takes place.

Avalanche Breakdown

Observed in diodes with breakdown voltage $V_z$ greater than 8 volts.
Valence electrons are pushed to conduction due to energy imparted by accelerated electrons, which gain velocity by colliding with other atoms.
Increase in temperature increases the breakdown voltage.
VI characteristic curve is not as sharp as zener breakdown.
Occurs in lightly doped diodes.
The depletion region is thick.
Minimum doping at the junction.
Occurs because of the collision of free electrons.
The production of electrons takes place.

Tunnel Diode

A pn junction diode exhibiting negative resistance between peak point voltage and valley point voltage in forward biased condition.
Conventional diodes exhibit positive resistance in forward and reverse bias.
Heavily doped pn junction (approximately 1000 times more than conventional diodes) leads to negative resistance in some regions when forward biased.

Working

Heavy doping leads to a large number of majority carriers.
Most majority carriers are not used during initial recombination, producing a narrow depletion layer.
The working principle is based on the tunneling effect.
Large number of majority carriers causes drift activity in p and n regions.
Valence electrons have their energy levels raised closer to the conduction region.
Small forward voltage is required to start conduction.
Tunneling: Movement of valence electrons from valence band to conduction band with little or no forward bias voltage.
Electrons seem to tunnel through the forbidden energy gap.
As forward bias voltage increases, the diode current increases rapidly due to tunneling effect.
Tunneling effect reduces, and current starts to decrease as forward voltage increases further.
Peak Point: Voltage at which current starts to decrease.
Negative Resistance Region: Region where the tunnel diode exhibits negative resistance.
Valley Point: As voltage further increases, the effect of tunneling decreases until the valley point is reached.
After the valley point, the tunnel diode behaves like an ordinary diode: current increases with increase in forward voltage.

V-I Characteristics

Initially, as the forward bias voltage increases, electrons from the n region tunnel through the potential barrier to the p region, and the diode current increases until the peak point $V_p$ is reached.
When voltage increases beyond $Vp$ , the tunneling action decreases, and the diode current decreases as the forward voltage is increased until valley point $Vy$ is reached.
In the region between the peak and valley points, the diode exhibits negative resistance.
Negative Resistance Region: When operated in this region, the tunnel diode can be used as an oscillator or a switch.
If the forward bias voltage is increased beyond the valley point voltage, the tunnel diode acts as a normal diode, and the current increases with the forward voltage, exhibiting a positive resistance region.

Applications

Ultra-high speed switch due to tunneling (takes place at the speed of light), with switching time of nanoseconds or picoseconds.
Logic memory storage device.
Used in satellite communication.
Used in relaxation oscillator circuits due to its negative resistance feature.

Semiconductor Laser

Specifically made pn junction diode that emits coherent light under forward bias.
First semiconductor laser made in 1962 by R.N Hall and his coworkers.
Can emit light almost anywhere in the spectrum from UV to IR.

Working

When a pn junction diode is forward biased, electrons from the n region and holes from the p region cross the junction and recombine with each other.
During recombination, light radiation (photons) is released from direct band gap semiconductors like Ga-As.
This light radiation is known as recombination radiation.
The photon emitted during recombination stimulates other electrons and holes to recombine.
Stimulated emission takes place, producing laser light.

Construction

Consists of a pn junction diode.
Example: Gallium arsenide laser (GaAs).
Heavily doped semiconductor.
- n-region: heavily doped with tellurium (concentration of $3 \, x \, 10^{18}$ to $5 \, x \, 10^{18}$ atoms/cm³).
- p-region: doped with zinc (concentration around $10^{19}$ atoms/cm³).
Active medium: GaAs or the depletion region (thickness around 0.1μm).
Two faces perpendicular to the junction plane make a resonant cavity.
Top and bottom faces parallel to the junction plane are metallized for external connections.
Front and back faces are roughened to suppress oscillations in unwanted directions.

Heavily Doped p and n Regions

Used in making laser diodes.
High doping on the n side broadens donor levels, extending them into the conduction band.
Fermi level is pushed into the conduction band.
Electrons occupy the portion of the conduction band lying below the Fermi level.
Similarly, on the heavily doped p side, the Fermi level lies within the valence band, and holes occupy the portion of the valence band that lies above the Fermi level.
At thermal equilibrium, the Fermi level is uniform across the junction.

Forward Biasing

Electrons and holes are injected into the junction region in high concentrations.
Charge carriers are pumped by the DC voltage source.
When the diode current reaches a threshold value, the carrier concentration in the junction region rises to a very high value.
Region near the junction contains a large concentration of electrons within the conduction band and a large number of holes within the valence band.
Condition of population inversion is attained in the narrow junction region.
Narrow zone where population inversion occurs is called an inversion region or active region.
Chance recombination of electron and hole pairs emits spontaneous photons.
Spontaneous photons propagating in the junction plane stimulate conduction electrons to jump into vacant sites of the valence band.
This stimulated electron-hole recombination produces coherent radiation.
Gallium arsenide laser emits light of wavelength 9000 Å in the IR region.
When electrons recombine with holes in the junction region, energy is released in the form of photons.
Energy release in the form of photons happens only in special types of semiconductors like Gallium Arsenide (GaAs).
In semiconductors like silicon and germanium, energy is released in the form of heat; thus, Si and Ge cannot be used for laser production.
Spontaneously emitted photons during recombination in the junction region of GaAs trigger laser action near the junction diode.
Photons emitted have a wavelength from 8200 Å to 9000 Å in the infrared region.
Wavelength of laser light is given by the equation: $\lambda = \frac{hc}{E_g}$

Applications

Fiber optic communications.
Barcode readers.
Laser pointers.
Disc readers.
Laser printing and scanning.
Directional lighting sources.

Advantages

Small dimension.
Simple and compact arrangement.
High efficiency.
Laser output can be easily increased by controlling the junction current.
Operated with lesser power than ruby and CO2 laser.
Requires very little auxiliary equipment.
Can have a continuous wave output or pulsed output.

Disadvantages

Due to relatively low power production, these lasers are not suited to typical laser applications.
Temperature affects its output.
Beam divergence is much greater compared to other laser types.
Cooling system required in some cases.
The purity and monochromaticity are poorer than other types of lasers.

Photonics

Branch of science that deals with the production, control, and detection of photons.
Technology that combines optics and electronics.
Makes use of the ideas of optics, electromagnetism, and quantum mechanics.
Applications: communication, data processing, transportation, traffic, medicine, biotechnologies, lighting, etc.
Photons have a role similar to electrons in electronics.
Photonic devices have advantages over electronic devices due to the very high speed of light.
Information transmitted photonically can travel very long distances within a very short time.
Devices required for generating, switching, and amplifying light for transmission through optical fibers over long distances.

Solid State Lighting (SSL)

Type of lighting that uses mainly light emitting diodes (LEDs).
Higher efficiency, reliability, and environmentally friendly technology compared to conventional incandescent lighting.

Photo Detector

Device used to convert light signals into voltage or current.
Required at the receiving end of an optical communication link.

Essential Requirements

High sensitivity.
High reliability.
Short response time.
Low bias voltage.
High electrical response.

Examples

Photodiodes.
Phototransistors.

Similar Optical Devices

Solar cells: Absorb light and convert it into electrical energy.

Different Optical Device

LED: Converts voltage or current into light (inverse of a photodiode).

Photo Diode

Type of light detector used to convert light into current or voltage based on the mode of operation.
Response time decreases as the surface area increases.
Similar to regular semiconductor diodes but transparent enough to let light reach the device.

Types of Photodiode

PN Photodiode.
Schottky Photo Diode.
PIN Photodiode.
Avalanche Photodiode.

Junction Photodiode

Reverse biased pn junction embedded in clear plastic medium.
When exposed to light, the current varies linearly with the flux of light.
The unit is very small (order of 1/10th of an inch).

Construction

Pn junction diode formed by p-type (e.g., Boron) and n-type (e.g., Phosphorous) semiconductor materials.
Formed by diffusion of lightly doped p region into the heavily doped n region.
Depletion region: Space between p and n regions.
Active area: Portion of the front area coated with antireflection coating.
Non-active area: Portion coated with a layer of $SiO_2$ .
Thickness of non-active area controls the response and speed of a photodiode to convert light into current.

Working

Connected in a circuit in reverse biased condition.
If the reverse biased voltage is very low, a constant current flows through the diode (reverse saturation current).
Reverse Saturation Current: Flows due to thermally generated minority carriers (electrons in p-type and holes in n-type).
Motion of minority carriers forms a current known as leakage current or dark current.
The dark current depends on the reverse biased voltage, ambient temperature, and series resistance in the circuit.
Dark current does not allow majority carriers to cross the junction.
When a photon is absorbed from the incident light by the p or n region, an electron is released from the valence band and goes to the conduction band, creating a hole in the valence band.
Incident light causes the creation of a large number of electron-hole pairs (photo carriers) which produce a current known as photocurrent in the external circuit in addition to dark current.
The dark current should be minimized to increase the sensitivity of the device.
Width of the depletion region should be increased to absorb a large quantity of light.
The resulting output current or voltage can be measured.

Applications

Used in scintillators, charge-coupled devices, photoconductors, and photomultiplier tubes.
Used in consumer electronic devices like smoke detectors, compact disc players, televisions, and remote controls.
Frequently used for the exact measurement of the intensity of light.
Widely used in the medical field (e.g., instruments to analyze samples, detectors for computed tomography, blood gas monitors).
Used for lighting regulation and in optical communications.

PIN Photodiode

A type of photodiode in which a very lightly doped and wide intrinsic semiconductor is introduced between heavily doped p and n regions to improve sensitivity.
Width of the intrinsic region is much larger (10-200 microns) than the space charge region (depletion region) of a normal pn junction.
When reverse biased voltage is applied, the space charge region extends throughout the intrinsic region.

Working

When light is incident on the diode, electrons are excited from the valence band to the conduction band.
Produces a large number of electron-hole pairs.
The intrinsic layer absorbs a very large number of incoming photons compared to the p and n regions.
Increases the photocurrent, improves the efficiency, speed, and sensitivity compared to a pn junction photodiode.
Output can be measured and analyzed.

Applications

Used in RF and microwave switches and microwave variable attenuators since they have low capacitance.
Used for fiber optic network cards and switches.
Used to detect X-rays and gamma rays photons.

Solar Cell

Devices making use of the photovoltaic effect to convert solar energy into electrical energy.
Generates electric potential when irradiated by optical radiation.
Can be considered as a large area photodiode made to work in the photovoltaic mode and in zero bias.
No flow of current out of the diode, and a voltage develops inside the device.
Sunlight is trapped inside the device to produce an electric effect in the form of voltage.
Output voltage of about 0.6V.
Total output voltage can be increased by connecting a number of solar cells in series.
A solar panel or module is an array of solar cells connected together.
Materials: Silicon (Eg=1.11eV), GaAs(E,= 1.40eV), CdSe(Eg = 1.74) etc.

Construction

Heavily doped p-n junction.
Top layer (n region) is very thin to allow solar radiation to reach the p-n junction.
P-n junction is typically around 20nm because the doping level is extremely high.
Large surface area to receive a large amount of light.
Anode connection is made from the bottom (p layer) and the cathode from the top (n layer).
An antireflection coating is made on the top layer to prevent light losses due to reflection.

Working

Light strikes the top of the semiconductor wafer (n region).
Electrons are knocked from the material.
The electrons travel from n type to p type semiconductor through an external load, completing the electric circuit.
IV measurements are well-known procedure to characterize solar cells.
Most solar cell parameters can be obtained from I-V measurements.
Consider a solar cell connected with an ammeter, voltmeter, and a load resistance and it is exposed to sunlight.
If no load is connected, an open-circuit voltage $V_{oc}$ is produced without a current.
If the terminals are shorted, the short circuit current $I_{sc}$ flows without an output voltage.
No power is delivered in either case.
When a load is connected, a voltage is developed, a current flows, and there is an output power.
Output power is maximum for a specific load resistance.
$V{max}$ and $I{max}$ are the voltage and current corresponding to the maximum power point.
Maximum power point is the condition under which the solar cell generates its maximum power $P_{max}$ .
The load resistance is chosen to maximize the output power.

Efficiency of a Solar Cell

Ratio of total power converted by the solar cell to the total power available for energy conversion.
$\eta = \frac{maximum \, output \, electrical \, power}{input \, optical \, power}$
$\eta = \frac{P_{max}}{light \, intensity \, x \, area \, of \, the \, solar \, cell}$

Fill Factor

Gives the fraction of the maximum output power to the product of the open circuit voltage and short circuit current.
$f = \frac{maximum \, output \, power}{V{oc} \, x \, I{sc}}$
Fill factor lies in the range 0.65 to 0.8.
The higher the fill factor, the greater the power output.

Advantages

Do not use any fuels, so they are not dangerous.
Can be used for years without any maintenance expenditure.
Do not produce atmospheric and noise pollution.

Disadvantages

Very delicate, brittle, and may be broken into small pieces.
Due to the slow response, solar cells are not normally used as optical detectors.
Space consumption is very large.
Solar panels must be cleaned from time to time to get better results

Applications

Used in satellites and rockets as sources of power.
Used in telecommunication field at remote and difficult to access areas like mountain tops, islands, and deserts.
Used in defense equipment like remote instrumentations, remote radars etc.
Used for rural electrification, water pumping, domestic supply, health care, lighting, ocean navigation aids etc.
Used in pocket calculators, watches, torches, garden lights, portable fans for cars and houses, radios, toys, street lights, traffic signals, electric fences etc.

Stringing of Solar Cells to Solar Panel

Solar panels are made up of multiple solar cells.
Cells can be connected in series or in parallel.

Series Connection

Increases the voltage.
Each panel adds to the total voltage (V) of the string, but the current (I) remains the same.
If one cell's open circuit voltage is 0.6 V, a string of three cells will produce an open circuit voltage of 1.8 V.
The current of the whole string is determined by the cell that delivers the smallest current.
Total current is equal to the smallest current generated by one single solar cell.
Positive pole of one cell is connected to the negative pole of the next cell.
This series stringing is used to increase the voltage of the panel.
Battery charging panels typically have 32, 36, or 48 cells in the series string.
The 32-cell module has the lowest voltage rating of 14.7 volts, 36 cell module has 16.7 volts, and 48 cell panel has 22 volts.
Voltages add up while the current stays the same.
Resulting open circuit voltage is two times that of the single cell with two cells in series.
If we connect three solar cells in series, the open circuit voltage becomes three times as large, whereas the current still is that of one solar cell.

Parallel Connection

Joined in parallel, the current of the solar cells adds up, and the voltage is the same over all solar cells.
Positive terminal of the solar panel is connected to the positive terminal of the next one, and the negative terminal is connected to the negative terminal.
Results in a fixed voltage but increases the current of the circuit.
Used to power inverters with fixed/limited current.
If we connect e.g. four cells in parallel, the current becomes four times as large, while the voltage is the same as for a single cell

Light Emitting Diode (LED)

Heavily doped pn junction of suitable materials that emits light when forward biased.
In the forward biased state, electrons move from n region to p region, and holes move from p region to n region.
Electron-hole recombination takes place on either side of the pn junction.
The loss of energy in these recombinations appears as light.
The energy of the emitted photons is nearly equal to the band gap energy.
The intensity of the emitted light increases with the increase of forward current, reaches a maximum value, and then decreases.
The color or wavelength of the light depends on the band gap energy.
Wavelength of emitted light is given by: $\frac{hc}{Eg}$ where h is the Planck's constant, c is the velocity of light in free space and $Eg$ the band gap energy
Different materials are used to get red, green, blue, yellow, orange etc. light.
All semiconductor diodes produce radiation during electron-hole recombination, but the emitted radiation is absorbed by the semiconductor material.
In an LED, the band gap is wide, and the junction is constructed so the radiation can escape.
The semiconductor materials used for LEDs should have a band gap energy of about 2eV.

White Light Production

Produced from colour LEDs by phosphor conversion, RGB systems, or a hybrid method.
Phosphor Conversion: A phosphor is used to convert blue or near ultraviolet light from an LED into white light.
RGB Systems: Mix light from multiple monochromatic LEDs (red, green, and blue) to get white light.
Hybrid Method: Uses both phosphor-converted and monochromatic LEDs.

Nobel Prize

In 1994, Isamu Akasaki, Hiroshi Amano, and Shuji Nakamura developed the blue LED using Gallium Nitride (GaN).
Received the 2014 Nobel Prize in Physics for this achievement.

Materials

Indium Gallium Nitride InGaN is used for making violet, blue and green LEDs.
Aluminium Gallium Indium Phosphide AlGaInP is used for green, yellow, orange and red LEDs.
These two semiconductor materials with slight changes in their composition give different colors to the LEDs.
Zinc Selenide Zn.Se, Aluminium Gallium Phosphide AlGaP, Gallium Arsenide Phosphide GaAsP, Aluminium Gallium Arsenide AlGaAs etc. are also used
Organic Light Emitting Diodes (OLEDS) and Polymer Light Emitting Diodes (PLEDs).

Quantum Dot LEDs (QLEDs)

Quantum dots are tiny particles or nano crystals exhibiting quantum mechanical properties.
In QLEDs quantum dots are placed in between n-type and p-type semiconductors.
When an electric field is applied, electrons and holes recombine in the quantum dot layer to produce light.
QLEDs are reliable, energy efficient, low cost and tunable over the entire visible wavelength range.

LED Characteristics

The junction voltage current characteristic of an LED is similar to the V-I characteristics of diodes.
The knee voltage of a diode is related to the barrier potential of the material used in the device.
Silicon diodes and bipolar junction transistors whose knee voltage or junction voltage is about 0.7 V are commonly used.
LEDs junction voltage can be anywhere between 1.5 to 2.2 Volts depending on the material used.
When operating an LED from a DC voltage source greater than the LED's forward voltage, a series-connected "dropping" resistor must be included to prevent full source voltage from damaging the LED.
LED starts emitting light as its forward voltage reaches at a particular level, and its intensity will increase further with the increase in applied forward voltage.
LEDs emit no light when reverse biased; operating LEDs in reverse direction will quickly destroy them if the applied voltage is quite large.

Advantages

It helps in saving energy.
Reduction in costs.
Very low voltage and current are enough to drive the LED.
Total power output will be less than 150 milliwatts.
The response time is very less, only about 10 nanoseconds.
The device does not need any heating and warm up time.
Miniature in size and hence light weight.
Have a rugged construction and hence can withstand shock and vibrations.
An LED has a life span of more than 10 years

Disadvantages

A slight excess in voltage or current can damage the device.
High initial price; LEDs are currently more expensive (price/lumen) than more conventional light sources.
Efficiency of LED decreases as the electric current increases.

Applications

LEDs in conjunction with a photodiode or other photosensitive device can be used as light sources in optical fibre communication systems.
It is used in digital displays in all modern day electronic devices.
LED is used as a bulb in homes and industries.
The light emitting diodes are used in motorcycles and cars.
These are used in mobile phones to display the message.
At the traffic light signals LEDs are used.