Table of Contents
SEMICONDUCTORS
Those materials which acts as insulators at low temperature & behave as conductors at room temperature or at higher temperature, are known as semiconductor.
Semiconductors could be:
(i) Elemental semiconductors: Si and Ge
(ii) Compound semiconductors: Examples are:
• Inorganic: CdS, GaAs, CdSe, InP, etc.
• Organic: anthracene, doped pthalocyanines, etc.
• Organic polymers: polypyrole, polyaniline, polythiophene, etc.
Most of the currently available semiconductor devices are based on elemental semiconductors Si or Ge and compound inorganic semiconductors.
However, after 1990, a few organic semiconductors and semiconducting polymers have been developed. This indicates the birth of a future technology of polymerelectronics and molecular-electronics.
There are only two materials which act as semiconductor in pure form.
- Si – 1s2 2s2 2p6 3s2 3p2
- Ge – 1s2 2s2 2p6 3s2 3d10 4s2 4p2
Both the semi conducting materials are having 4 valence electrons in outermost orbit of their atoms.
BAND THEORY OF SOLIDS
According to the Bohr atomic model, in an isolated atom the energy of any of its electrons is decided by the orbit in which it revolves. But when the atoms come together to form a solid they are close to each other. So the outer orbits of electrons (valance electrons) from neighbouring atoms would come very close or could even overlap. This would make the nature of electron motion in a solid very different from that in an isolated atom.
Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges (Charges of nuclei of surrounding atoms and other electrons). Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form what are called energy bands. The energy band which includes the energy levels of the valence electrons is called the valence band. The energy band above the valence band is called the conduction band. With no external energy (at very low temperature, ~0K), all the valence electrons will reside in the valence band. Normally the conduction band is empty at very low temperature.
Let us consider what happens in the case of Si or Ge crystal containing N atoms. For Si, the outermost orbit is the third orbit (n = 3), while for Ge it is the fourth orbit (n = 4). The number of electrons in the outermost orbit is 4 (2s and 2p electrons). Hence, the total number of outer electrons in the crystal is 4N. The maximum possible number of electrons in the
outermost orbit is 8 (2s + 6p electrons). So, for the4N valence electrons there are 8N available energy states. These 8N discrete energy levels can either form a continuous band or they may be grouped in different bands depending upon the distance between the atoms in the crystal.
For Si and Ge crystal lattices the distance between the atoms is such that the energy band of these 8N states is split apart into two which are separated by an energy gap Eg ( See Fig.1). The lower band which is completely occupied by the 4N valence electrons at temperature of absolute zero is the valence band. The other band consisting of 4N energy states, called the conduction band, is completely empty at absolute zero.
The lowest energy level in the conduction band is shown as EC and highest energy level in the valence band is shown as EV. Above EC and below EV there are a large number of closely spaced energy levels (as shown in Fig.1).
The gap between the top of the valence band and bottom of the conduction band
is called the energy band gap (Energy gap Eg). It may be large, small, or zero, depending upon the nature of the material.
Basic Definitions
- Valence band: The energy band in a solid, which is completely filled by valence electrons at low temperature (at zero Kelvin).
- Conduction band – Energy band in a crystal, which is empty at low temperature (at zero Kelvin). But when an electron goes to this band, it freely participates in conduction process.
- Forbidden gap – This is an energy gap below the conduction band & above the valence band such that no electrons can posses these energies while present in the crystal.
Band Theory of Solids
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Question.1 Distinguish between conductors, insulators or semiconductor, on the basis of energy band theory of solids?
1) Insulator
In a insulator the inter-atomic separation is such that, at low temperature, we get a narrow valance band, a narrow conduction band & a wide forbidden gap (Eg< 3 eV). In this case the energy gap is so large that electrons cannot be excited from the valence band to the conduction band by thermal excitation.
2) Conductors
In a conductor the inter-atomic separation is such that there is a wide V.B. (which is completely filled by valence electrons at low temperature) and a wide conduction band, such that the two bands overlap each other. In conductor there is no forbidden gap. So in conductor even at low temperature we get free electrons present in conduction band, which can participate in conduction process. (VB & CB overlap to each other, thus Eg disappears)
3) Semiconductor
In Semiconductor the inter-atomic separation is such that there is a wide V.B. (Completely filled by valence electrons at low temperature) and a wide conduction band is separated by a narrow forbidden gap.( Eg< 3 eV) [Eg for Si ⇒ 1.1 eV ; Eg for Ge ⇒ 0.74 eV]
Because of the small band gap, at room temperature some electrons from valence band can acquire enough energy to cross the energy gap and enter the conduction band. These electrons (though small in numbers) can move in the conduction band.
INTRINSIC SEMICONDUCTORS
A pure (99.9%) Si & Ge crystal is known as intrinsic semiconductors. In a Si crystal, every Si atom shares its four valence electrons with 4 neighboring atoms and thereby forms 4 covalent bonds. So at low temperature, every valence electrons present in the crystal is in the bond. In the equivalent band theory, these electrons are said to be present in the valence band.
When such a crystal is brought to the room temperature, some of the electrons originally present in the bond get enough energy [more than bond energy], so that they can break the band and become free. These electrons leave behind a positively charged region in the bond, which is called ‘hole’ and acts as an electrons trap. The hole behaves as an apparent free particle with effective positive charge. Thus in intrinsic semiconductors free electrons & holes are generated in pair.
This phenomenon is shown as a transition of electron from Valence Band to the Conduction Band, in energy band diagram and it is called intrinsic transition. So, intrinsic transition generates a ‘free electron’ in Conduction Band and a ‘hole’ in Valence Band.
So, at any instant and at any temperature.
The number of holes per unit volume (nh)i is equal to number of free electrons (ne)i
i.e. (ne)i = (nh)I = ni
where ni is called intrinsic carrier concentration.
Whenever a potential difference is applied across the ends of a semiconductor, an electric field is generated inside it. Due to this electric field, every electron experience an electrostatic force in the direction opposite to the electric field and the free electrons move with drift velocity in the direction opposite to the field and thereby generates current Ie ,in the direction of field. The holes also effectively move in the direction of the field, jumping from one bond to another bond. Due to this movement of holes, a hole current (Ih) is generated, in the direction of the field. So, in semiconductors both free electrons as well as holes act as charge carriers.
Also the total current flowing through the semiconductor is the sum of the current generated by the electrons and by the holes.
I = I e + I h
It may be noted that, apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination of the electrons with the holes occurs. At equilibrium, the rate of generation (of electron and hole pair) is equal to the rate of recombination of charge carriers. The recombination occurs due to an electron colliding with a hole.
EXTRINSIC SEMICONDUCTORS
Doping – “The deliberate addition of impurity atom (3rd group or 5th group) element atoms into an otherwise pure Si or Ge crystal, to increase its conductivity, is called doping”.
The impurity atoms are called dopants. The material that is formed after doping is called a doped semiconductor or an extrinsic semiconductor.
[The dopant has to be such that it does not distort the original pure semiconductor lattice. It occupies only a very few of the original semiconductor atom sites in the crystal. A necessary condition to attain this is that the sizes of the dopant and the semiconductor atoms should be nearly the same.]
n – type extrinsic semiconductor:
When 5th group element atoms [like Arsenic (As), Antimony (Sb), Phosphorous
(P), etc.] are introduced (dropped) into an otherwise pure semiconductor, the crystal thus formed is called n-type semiconductor.
When an antimony (or arsenic) atom is introduced in an otherwise pure semiconducting crystal, the antimony atom replaces one of the Si atom from its lattice point. The antimony atom shares four of its valence electrons with four neighboring Si atom. Due to this the fifth valence electron of the Sb atom becomes very weakly bound with its parent atom.
The semiconductor’s energy band structure is affected by doping. In the case of n –type extrinsic semiconductors, additional energy states due to donor impurities (ED) , called ‘donor level’, is formed at slightly below the bottom EC of the conduction band and electrons from this level can move into the conduction band with very small supply of energy (about 0.01eV energy is required).
When such a crystal is brought to the room temperature all these fifth electrons of Sb atoms, after getting a small energy (0.01 electrons volt), leaves the atom and becomes free. Due to this the antimony atoms become positively charged immobile atoms (ions). In the energy band diagram this process in shown by extrinsic transition between the donor level and the conduction band. So the conduction band will have most electrons coming from the donor impurities.
Besides this there are a few electrons, originally present in the bond, which after getting sufficient energy breaks the bonds & causes generation of free electron – hole pair. This is shown by the intrinsic transition between the valence band & the conduction band.
So at room temperature in a n-type semiconductor we get
- A large number of free electrons – (majority charge carriers)
- A few holes – (minority charge carriers)
- Negatively charge immobile doped atoms fixed at their lattice points.
So at any instant in a n-type semiconductor the free electron density is much higher than hole density i.e. ne >> nh
P- type semiconductor
When the third group element atom (Indium, Boron, Al) are doped in an otherwise pure Si or Ge crystal, the crystal thus formed is known as p-type semiconductor.
When an Aluminum (Al) atom is introduced in Si crystal, it replaces one of the Si atoms and gets its position in the lattice. The Al atom shares all its three valance electrons with three neighboring Si atoms & there by formed three covalent bonds. But it doesn’t satisfy the Al atom and also one neighboring Si atom. These two atoms together form a positively charged region, which attracts the electrons from the neighboring band. Due to this the electrons present in the neighboring atoms become weekly bound.
In the case of p-type extrinsic semiconductors, additional energy state due to acceptor impurities (EA), called ‘acceptor level’ is formed. The acceptor energy level EA is slightly above the top EV of the valence band as shown in Fig. With very small supply of energy an electron from the valence band can jump to the level EA and thereby ionise the acceptor atom negatively.
When such a crystal is brought to the room temperature every Al atom present in it receives one electron from some neighboring bonds. Due to this transfer of electrons a hole is created in the bond. In this process Al atom becomes negatively charged immobile atom. In energy band diagram this process is shown by extrinsic transition between the valence band and the acceptor level, which is present just above the valence band at an energy difference of about 0.01 – 0.05 eV.
Besides this there are a few electrons originally present in the bond, after receiving sufficient energy breaks the bonds and generate electrons -hole pair.(Intrinsic transition). Thus at room temperature in a p-type semiconductor we get
- A large number of holes – (majority charge carriers.)
- A few free electrons – (minority charge carriers)
- Negatively charged immobile Al atoms.
So, for p-type semiconductors nh >> ne
Note that the crystal maintains an overall charge neutrality as the charge of additional charge carriers is just equal and opposite to that of the ionised cores in the lattice.
Extrinsic Semiconductors: n-type and p-type semiconductors.
(In Hindi + English mix Language)
Note:
- Donor level – The additional energy level is generated by the donor atoms (fifth group element atoms) in a n-type semiconductor is known as donor level. This level is present just below the conduction band & thereby it is present in forbidden gap. At low temperature it contains all the fifth electrons (additional electrons) of the donor atoms.
- Acceptor level – The additional energy level formed by the acceptor atoms (3rd group element atoms) in a p-type semiconductor is known as acceptor level. This level is present in the forbidden gap just above the V.B. At low temperature this energy level is empty & it accepts electrons from the valence band whenever the crystal is brought to the room temperature.
- At any temperature, a semi conducting crystal is always electrically neutral this is because the number of free electrons and the number of holes & the immobile charges is such that the crystal is electrically neutral,
- At any temperature in a semi conducting crystal continuously the process of recombination of electrons and hole as well as the generation of electron hole pair take place simultaneously. In equilibrium the rate of recombination of electron- hole pair is equal to the rate of generation of electro- hole pair.
CONDUCTION OF CURRENT THROUGH A SEMICONDUCTOR
Whenever a potential difference is applied across a semiconducting block an electric field is generated inside it.
………………….. (1)
Due to this electric field the electrons as well as hole move with drift velocity 
and 
in opposite direction.
This cause generation of electronic current (Ie) and hole current (Ih), both in the direction of electric field. So the total current flowing through the semiconductor.
………………(2)
Where me and mh are the mobility of free electron and hole respectively.
Note 1:
From equation (2) we get that the conductivity of a semiconductor depends only upon the charge carrier density (number of free electrons and hole per unit volume). The conductivity of the material increases with increase in the charge carrier density.
Since in extrinsic semiconductor, the number of majority charge carrier per unit volume is directly determined by doping concentration (number of dopped atoms per unit volume), the conductivity of extrinsic semiconductor increases with increases in the dopping concentration.
Note 2:
As the temperature of semiconductor is increased, more and more electron- hole pair are generated due to the breaking of bond (breaking of bonds). Thus as temperature is increased, the number of charge carriers per unit volume in the semiconductor increases and there by its conductivity increase. So resistivity of semiconducting materials decreases with rise in temperature.
So for T > T0 , 
.
Since, 
So for semiconductor the temperature coefficient of resistance ( 
) is a negative quantity.
Conduction of Current in Semiconductors
(In Hindi + English mix Language)
Law of mass action:
At a given temperature, in thermal equilibrium, the product of free electron density and the hole density in any semiconducting material is constant i.e. at a given temperature.
……………..(1)
or 
…………………..(2)
So, in general we can say (ne)i = (nh)i = ni2 (let)
Question: Suppose a pure Si crystal has 5 Χ 1028 atoms m-3. It is doped by 1ppm concentration of pentavalent As. Calculate the number of electrons and holes. Given that ni = 15 × 1016 m-3.
Solution: Total no of Si atom in 1 m3 = 5 Χ 1028
So, number of As atom in 1 m3 = 
ND = 5 Χ 1022 atom/m3
In n-type material ne ≈ ND = 5 Χ 1022 electrons/m3
= 0.45 Χ 1010 holes/m3.
So, 
p-n JUNCTION
‘p-n junction (diode)’ is a single semi conducting crystal with one of its end p-type & other end n-type material.
Whenever a p-n junction is formed in a crystal, following the diffusion process the electrons start entering into the p-type material. These electrons neutralize holes present closer to the junction. This leaves the positively charged immobile atoms in n-type region & negatively charged immobile atoms in p-type region, un-neutralised. These un-neutralised immobile charges present at both the sides of the junction, generate an electric field, which opposes the flow of electrons through the junction. But, still, those free electrons which have sufficiently large amount of energy to overcome this opposing force and cross the junction. This causes the increases in the immobile charged atoms & thereby increases the opposing electric field. This process continuous until even the most energetic electrons present in the n-type region, fails to overcome the opposing force & thereby fail to cross the junction. This brings the equilibrium state of the p-n junction.
Thus in equilibrium state in p-n junction diode we get
- A p-type material at one end, in which a large number of holes (majority charge carriers) are present.
- A n-type material at the other end, in which a large number of free electrons are present.
- A region across the junction on either side of the junction which contain only the immobile charges. There is no charge carrier present in the region. This region is called charge depletion region.
p-n junction and junction diode
(In Hindi + English mix Language)
The immobile charges present in the charge depletion region generate an opposing potential difference across the junction, which acts as a barrier for the majority charge carriers. This potential difference across the junction is known as junction potential barriers (VB or Vo)
[An unbiased diode has a depletion layer at the pn junction. The ions in this depletion layer produce a barrier potential. At room temperature, this barrier potential is approximately 0.7 V for a silicon diode and 0.3 V for a germanium diode.]
The value of VB (also known as height of junction potential barrier) and it depends upon the maximum energy of free electrons present in the n-type region. So its value depends upon the temperature of the material. Higher the temperature of the material more is the value of VB.
The width of the charge depletion region depends upon the doping concentration of the crystals. Higher the doping concentration lesser is the width of charge depletion region.
BIASING OF A p-n JUNCTION DIODE
Applying a potential difference across a p-n junction diode is known as its biasing.
Forward biasing
When the p-type material in a p-n junction diode it kept at higher potential than the n-type region, it is said to be in forward bias condition.
Whenever a p-n junction diode is put in forward bias condition, due to the repulsion experienced from the terminal the majority charge carriers enter into the charge depletion region. Due to this, width of charge depletion region decreases and the effective value of junction potential barrier also decrease. Now, a few free electrons, which have sufficiently high value of energy, cross the junction and enter into p-type region following the diffusion process. In p-type region these electrons combine with the holes and get neutralized.
The cell (its negative terminal) compensates the loss of electrons by n-type region i.e. sends electrons there. In the p- region, the cell uses its energy and breaks equal number of bonds and then takes away the free electrons generated there. This completes the process
and causes flow of current in the circuit.
If we increase forward bias voltage, the charge depletion region decreases further. This causes decrease in junction potential barrier and causes increase in current.
If we go on increasing the forward bias voltage, we get a particular value of voltage above which the current through junction diode increases fast with increase in voltage. This forward bias voltage is called threshold voltage, knee voltage or cut-in voltage .
Forward biasing of pn junction diode
(In Hindi + English mix Language)
Reverse biasing
When p-type material in a p-n junction diode is put at a lower potential in comparison to n-type material, it is said to be in reverses bias condition.
When a reverse bias is applied across a p-n junction diode, due to attraction felt by the majority charge carriers from the terminals, the width of charge depletion region increases. Due to this the junction potential Barriers increases and it becomes (VB +V).
Now it becomes impossible for the majority charge carriers to cross the junction & there by participate in conduction process. But the increase in junction potential barrier is favorable for minority charge carriers to cross the junction. As the free electrons, originally present in p-type region, enter into n-type region, it join with holes present there. The negative terminal of cell sends electrons to the p-type material and removes equal number of electrons from the n-type material. This causes a small current in the circuit. As the reverse bias voltage is increased, initially the current increases a little and then it achieves a saturation value (constant value). However when reverse bias voltage becomes very high, suddenly current is found to increase rapidly. This is called Breakdown. The reverse bias voltage above which the breakdown occurs is called Breakdown Voltage.
Note: If we consider both the characteristics curve simultaneously in same graph paper we find that diode is a special type of device which allows current to pass through it only in one direction. (From p to n-type during F.B.) whereas it doesn’t allow current to flow through it in opposite direction.
In other words diode offers low (R) in F.B. condition and offers very high resistance in R.B. condition.
RECTIFIER
Rectifier is a device which converts alternating current into unidirectional current. There are two types of rectifiers
Half Wave Rectifier
The secondary coil of transformer is connected across the p-n junction diode through a load resistance (RL). The input voltage is generated across secondary coil of the transformer where as output voltage is received across (RL) (V0 = IRL)
In the positive half cycle of input voltage, let P-type material is at higher potential than n-type i.e., the diode is in forward bias condition. In this case diode conducts current and generates output voltage, which is proportional to the current. So current, as will as the output Voltage, increases and then decreases along with the input voltage (Vi). In the negetive half cycle of (Vi), the diode is in reverse bias condition. So it doesn’t conduct current. So, in this part output voltage remains zero.
Thus the circuit remains operative only for half of cycle of input voltage, due to this it is called half wave rectifier.
Full Wave Rectifier :
In this circuit there is a centrally tapped transformer (A, B, C). C is the central terminal. In the positive half cycle of the input voltage let terminal A it at higher potential than terminal B.
The diode D1 is in forward bias where D2 is in reverse bias condition. D1 conduct current and the current increases and then decreases with increase & decreases of input voltage Vi. So the output voltage V0 also varies accordingly. In negetive half cycle of Vi let the terminal B is at higher potential than the terminal A. In this case D1 is in reverse bias & D2 is in forward bias. So D2 conducts current & this current flows through the load resistance RL in the same direction as it was flowing during the positive half cycle Vi . So for this current also the output voltage obtained, which is of same nature as it was earlier. So the circuit remains operative throughout the whole cycle of input voltage.
Half Wave & Full Wave Rectifiers
(In Hindi + English mix Language)
Note : The output of rectifier in unidirectional but it is not constant. This variation in current & output voltage is known as occurrence of ripple. It is due to the a.c. component that is remaining in the output of rectifier.
To eliminate the ripple factor from output of a rectifier, a circuit, called filter, is connected after the rectifier. The ripple factor can also be significantly eliminated if capacitor of high capacitance is connected in parallel to RL. The effect of capacitor is that when output voltage increases and attains the max. value, it gets charged. Now when the output voltage tends to decrease, the capacitor starts discharging through the resistance and thereby tries to maintain current and voltage constant.
LIGHT EMITTING DIODE (LED)
Light Emitting Diode or LED, is a heavily doped p-n junction which under forward bias emits spontaneous radiation. The diode is put inside a transparent cover so that emitted light can come out.
Working of LED:
When the diode is forward biased, electrons are sent from n →p (where they are minority carriers) and holes are sent from p → n (where they are minority carriers). At the junction boundary the concentration of minority carriers increases compared to the equilibrium concentration (i.e., when there is no bias).
Thus at the junction boundary on either side of the junction, excess minority carriers are there which recombine with majority carriers near the junction. On recombination, the energy is released in the form of photons. Photons with energy equal to or slightly less than the band gap are emitted (i.e. hν ≤ Eg). When the forward current of the diode is small, the intensity of light emitted is small. As the forward current increases, intensity of light increases and reaches a maximum.
- LEDs are biased such that the light emitting efficiency is maximum.
- The V-I characteristics of a LED is similar to that of a Si junction diode. But the threshold voltages are much higher and slightly different for each colour. The reverse breakdown voltages of LEDs are very low, typically around 5V. So care should be taken that high reverse voltages do not appear across them.
- LEDs that can emit red, yellow, orange, green and blue light are commercially available.
- The semiconductor used for fabrication of visible LEDs must at least have a band gap of 1.8 eV (spectral range of visible light is from about 0.4 μm to 0.7 μm, i.e., from about 3 eV to 1.8 eV).
- The compound semiconductor Gallium Arsenide – Phosphide (GaAs1–xPx) is used for making LEDs of different colours. GaAs6 P0.4 (Eg ~ 1.9 eV) is used for red LED. GaAs (Eg ~ 1.4 eV) is used for making infrared LED.
Uses of LEDs:
These LEDs find extensive use in remote controls, burglar alarm systems, optical communication, etc. Extensive research is being done for developing white LEDs which can replace incandescent lamps.
Advantages of LEDs over conventional incandescent low power lamps:
(i) Low operational voltage and less power.
(ii) Fast action and no warm-up time required.
(iii) The bandwidth of emitted light is 100 Å to 500 Å or in other words it is nearly (but not exactly) monochromatic.
(iv) Long life and ruggedness.
(v) Fast on-off switching capability.
PHOTODIODE
A Photodiode is a special purpose p-n junction diode which used for detecting optical signal (photo-detectors). It is the semiconductor equivalent of ‘photocell’. It works on the same principle of ‘photovoltaic effect’.
Construction of Photodiode:
A Photodiode is a pn-jnction diode, constructed with a transparent window to allow light to fall on the diode (at its charge depletion region). It is operated under reverse bias.
Working of Photodiode:
The photodiode works when it is illuminated with light (photons) with energy (hν) greater than the energy gap (Eg) of the semiconductor. When an electron, originally present in the covalent bond, receives a photon of energy more than Eg , it breaks the bond and cause the generation of e-h pairs takes place in or near the depletion region of the diode of electron-hole pair.
Due to electric field of the junction, electrons and holes are separated before they recombine. This causes the generation of extra charge carriers in the material and thereby increases the current. The magnitude of the photocurrent depends on the intensity of incident light (photocurrent is proportional to incident light intensity).
Question: What is the reason to operate the photodiodes in reverse bias?
Answer:
The reason is, for a given intensity of light, the percentage change in the number of minority charge carrier is more significant than that for the majority charge carriers. As, in case of reverse bias current is determined by minority charge carriers, the change in current is more apparent in reverse bias condition. Hence, photodiodes are preferably used in the reverse bias condition for measuring light intensity.
- It is easier to observe the change in the current with change in the light intensity, if a reverse bias is applied.
- Thus photodiode can be used as a photo-detector to detect optical signals.
SOLAR CELL
A solar cell is basically a p-n junction which generates emf when solar radiation falls on the p-n junction. It works on the same principle (photovoltaic effect) as the photodiode, except that no external bias is applied and the junction area is kept much larger for solar radiation to be incident because we are interested in more power.
Working of Solar Cell:
When light falls on it the generation of emf by a solar cell is due to the following three basic processes: generation, separation and collection —
(i) generation of e-h pairs due to light (with hν > Eg) close to the junction – When an electron, originally present in the covalent bond, receives a photon of energy more than Eg , it breaks the bond and cause the generation of e-h pairs takes place in or near the depletion region of the diode of electron-hole pair.
(ii) separation of electrons and holes due to electric field of the depletion region – Due to electric field of the charge depletion region, electrons and holes are separated before they recombine. The direction of the electric field in the charge depletion region is such that electrons reach n-side and holes reach p-side.
(iii) generation of emf : the electrons reaching the n-side are collected by the front contact and holes reaching p-side are collected by the back contact. Thus p-side becomes positive and n-side becomes negative giving rise to photovoltage (e.m.f).
When an external load is connected across it, current flows.
- A typical I-V characteristic of a solar cell is shown in the Fig. (b). Note that the I – V characteristics of solar cell is drawn in the fourth quadrant of the coordinate axes. This is because a solar cell does not draw current but supplies the same to the load.
Question: How a simple p-n junction solar cell is constructed (fabricated)?
Solution: A p-Si wafer of about 300 μm is taken over which a thin layer (~0.3 μm) of n-Si is grown on one-side by diffusion process. The other side of p-Si is coated with a metal (back contact). On the top of n-Si layer, metal finger electrode (or metallic grid) is deposited. This acts as a front contact. The metallic grid occupies only a very small fraction of the cell area (<15%) so that light can be incident on the cell from the top.
Materials for solar cell fabrication:
Semiconductors with band gap close to 1.5 eV are ideal materials for solar cell fabrication. Solar cells are made with semiconductors like Si (Eg = 1.1 eV), GaAs (Eg = 1.43 eV), CdTe (Eg = 1.45 eV),
CuInSe2 (Eg = 1.04 eV), etc.
Note:
- 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.
- Production of low-cost photovoltaic cells for large-scale solar energy is a topic for research.
