Saturday, April 14, 2012

The metal oxide semiconductor FET (MOSFETS)

The metal oxide semiconductor FET (MOSFETS): MOSFETS are another type of unipolar transistors which are similar, in many respects, to JFETS. MOSFETS also have drain, source and gate terminals similar to JFETS. Here also, the voltage across gate-source terminals contains the drain current. The only difference is that the gate terminal in a MOSFET is insulated from its channel region by using metal oxide. There are two types of MOSFETS.  They are:
        I.            Enhancement type or E-MOSFETS
      II.            Depletion type or D-MOSFETS
Enhancement type MOSFETS: These types of MOSFETS do not have channel formed during its construction. The voltage applied to the gate develops a channel of charge carriers. So, that a current flows when a voltage is applied across the drain source terminals. A channel is said to be enhanced by the application of suitable gate-source bias voltage. So, it is called enhancement MOSFET. There can be both n-channel & p-channel in E-MOSFETS.
Depletion type MOSFETS: These types of MOSFETS already have a channel present in it even when no gate voltage applied. The voltage applied to the gate determines whether the channel is increased or decreased. For an n-channel D-MOSFETS, if the channel is increased by applying positive gate voltage, then the channel is said to be enhanced or enhancement mode. If the channel is decreased by applying negative gate voltage, then the channel is said to be depleted or depletion mode. Hence, a D-MOSFET can be operated in both enhancement as well as depletion mode.

Saturday, April 14, 2012 by Pooja · 74

Wednesday, April 11, 2012

Reverse and forward bias

Reverse bias: A diode is said to be reversed biased if the p-region is connected to the negative terminal and n-region is connected to positive terminal of the battery. In a reversed biased diode, the depletion region of the n-type material will increase due to the large number of free electrons drawn to the positive potential of the applied voltage. Similarly, the number of uncovered negative ions in the p-type material will increase. Due to this, the depletion region widens establishing a great barrier across the junction. The majority carriers cannot overcome the potential barrier resulting in no flow of current. However, the minority carriers can flow across the junction resulting in a flow of current even in reverse-bias conditions, known as reverse saturation current.
Forward bias: A diode is said to be forward biased if the positive terminal and n-region is connected to the negative terminal of the battery. When a diode is forward biased, the electrons in the p-type material will recombine with the ions near the boundary and hence reduce the width of the region results in a heavy majority flow across the junction. An electron of the n-type material has a strong attraction for the positive potential applied to the p-type material. Similar is the case for the holes present in p-type material. As the applied bias increases in magnitude, the depletion region will continue to decrease in width until, a flow of electrons can pass through the junction resulting in an exponential rise in current.

Wednesday, April 11, 2012 by Pooja · 9

Tuesday, April 10, 2012

Electromagnetic spectrum

After the experimental discovery of electromagnetic waves by hertz in 1888, many other electromagnetic waves were discovered by different ways of excitation. The orderly distribution of electromagnetic radiations according to their wavelength or frequency is called the electromagnetic spectrum. Electromagnetic spectrum has a very wide range with wavelength variation from 10⁻¹³ m to 6*10⁶m. The whole electromagnetic spectrum has been classified into different parts and sub parts, in the order of increasing wavelength and according to the type of excitation. There is over lapping in certain parts of the spectrum, showing that the particular radiation can be produced by two methods. However, the physical properties of electromagnetic waves are determined by their wavelength and not by the method of excitation. The most commonly encountered portion of the electromagnetic spectrum is shown in figure. Which gives approximate wavelength and frequency ranges for the various segments. Only a small part of the complete electromagnetic spectrum (wavelength from 400nm to 700nm) is visible to human eye, as the radiations of this part can produce some sensation on the retina of the eye. This part of the spectrum is called visible spectrum. The radiations of different wavelengths in the visible part of the spectrum produce different colors on the retina of our eye.
The usual classification of electromagnetic spectrum is explained below.
1.       Radio waves: these are the electromagnetic waves of frequency of frequency range from few  Hz to 10⁹ Hz. These waves which are used in television and radio broad casting systems, are generated by electronic devices, mainly oscillating circuits having and inductor and capacitor.
2.       Microwaves: the wavelength of microwaves is greater than 1.0 mm and less than 30 cm. the frequency range of microwaves is 10⁹ Hz to 3.0*10¹¹ Hz. They are used for atomic and molecular research. These are used for aircraft navigation.
3.       Infra-red (IR) rays: the wavelengths range of infra-red rays is 1 nm and the frequency range is 3.08*10¹¹ Hz to 4.3*10¹⁴ HZ. These rays are used to keep the green house warm. These are used to treat muscular strains. The infra-red rays from the sun keep the earth warm.
4.       Visible light: the range of visible light is 400 nm (violet) to 700 nm (red) and the frequency range is 4.3*10¹⁴Hz to 7.5*10¹⁴ Hz. It is useful in optical microscopy and astronomy. Visible light is useful in photography.
5.       Ultra-violet (UV) rays: the range of ultra violet rays is 400 nm to 60 nm and their frequency range is 7.5*10¹⁴Hz to 5.0*10¹⁵ Hz. The sun important source of ultra-violet rays. They are used for sterilizing the surgical instrument. They are used in detecting the invisible writings, forged documents and finger prints.
6.       X-rays: the range of the wavelengths of x-rays varies from 60nm to 10⁻⁸ nm. The frequency of these rays varies from 5.0*10¹⁵ Hz to 3.0*10¹⁸ Hz. X-rays can penetrate through the human flesh but bones or metallic materials block these rays. These are used in radio therapy.
7.       Gamma- rays: the wavelength range of gamma-rays varies from 0.1 nm to 10⁻⁵ nm. The frequency of γ-rays is the highest of all the electromagnetic waves. The range of the frequency of γ-rays varies from 3.0*10¹⁸ Hz to 3.0*10²² Hz. Γ-rays are used in the treatment of cancer and tumors. It is also used to produce nuclear reactions.

Tuesday, April 10, 2012 by Pooja · 17

Monday, April 9, 2012

Magnetic materials

A bar magnet strongly attracts an iron piece, but other materials are weakly attracted and some are actually repelled. We may use this response of material to the field of a bar magnet to broadly classify magnetic materials. All magnetic are classified into three categories. These are: diamagnetic, paramagnetic and ferromagnetic materials. The classification depends on the magnetic dipole moment of atoms of the material and on the interactions among the atoms. When the different magnetic materials are placed in a uniform magnetic field, the field lines are changed as shown in figure.

1.       Diamagnetic material: the diamagnetic materials are those substances which are feebly magnetized in the direction opposite to the applied field. So, they are weakly repelled by magnets as shown in figure. Examples of diamagnetic material are bismuth, copper, water, alcohol, mercury etc. the magnetic moment of atoms of a diamagnetic material is zero. But they acquire induced dipole moments when the material is placed in an external magnetic field. These moments, however, are opposite in direction to the applied field. So, the magnetization in a diamagnetic material always opposes the applied field. They are repelled by magnets. The diamagnetic materials move from a stronger to a weaker field. These materials are independent of temperature.
2.       Paramagnetic material: the paramagnetic materials have atoms that have permanent magnetic moments. These moments interact weakly with each other and randomly orient in different directions. When an external magnetic field is applied to the material, its atomic moments tend to line up with the field. The magnetic field inside it is the sum of the applied field and the induced field due to magnetization. These are found in solid liquid and gas. A paramagnetic rod, freely suspended in a magnetic field. The paramagnetic materials are temperature dependent and follow curie law.
3.       Ferromagnetic material: the ferromagnetic materials are highly magnetized in a magnetic field. The examples of ferromagnetic materials are iron, nickel and cobalt, and their alloys such as alnico. Godolinium and dysprosium are ferromagnetic at low temperature, and compounds such as CrO₂ used in a magnetic tap recording are also ferromagnetic materials though though neither chromium nor oxygen is ferromagnetic. They are highly attracted by magnets. The magnetic susceptibility is positive and very high, and varies with applied field.

Monday, April 9, 2012 by Pooja · 16

Free, forced and resonant oscillations

Free oscillations: when a body capable of oscillation is displaced from its equilibrium position and then left free, it begins to oscillate with a definite amplitude and frequency. If the body is not resisted by any kind of friction, the motion continues. Such oscillation is called the free oscillation. The frequency of vibration depends on the intrinsic properties (shape, elasticity etc) of the body which is called as the natural frequency. The force acting on the on the system is the restoring force. For example, when a simple pendulum is displaced from its mean position and then left free, it executes free oscillations. The natural frequency of the simple pendulum is f= (¹/₂π) {root (l/g)}. In absence of frictional forces, the amplitude of the free oscillations would remain constant. However, in actual practice, a complete free oscillation cannot be obtained in nature.
Forced oscillations: when a body is maintained in a state of oscillation by an external periodic force of frequency other than the natural frequency of the body, the oscillation is called the forced oscillation. In such oscillation is equal to the frequency of the periodic force. The external applied force on the body is called the driver and the body set into the oscillations is called driven oscillator.

    Resonant oscillations: when a body is maintained in state of oscillations by a periodic force having the same frequency as the natural frequency of the body, the oscillations are called resonant oscillations. The phenomenon of producing resonant oscillations is called resonance. Note that the resonance is a particular case of forced oscillations in which the two frequencies are equal.

by Pooja · 31

Sunday, April 8, 2012

Nature and source of light

Light is a form of energy which on entering our eyes gives the sensation of sight, and enables us to see various objects. Since the early times, scientists have made discussion on the true nature of light and have postulated various theories. These are discussed below:
A.      Newton’s corpuscular theory: Newton, in 1678, put forward this theory that light consists of tiny particles, called corpuscles, which are shot out by luminous objects such as an electric lamp, candle, etc. the corpuscles are tiny mass less particles, shot out at a speed of about 3*10⁸ ms⁻¹ from the lighted object. According to newton, these particles travel in straight lines and bounce off or pass into an object on striking it. On entering the eyes, these corpuscles cause the sensation of vision and the various colors are due to different sizes of the corpuscles. The phenomenon of refraction was explained by starting that these corpuscles are attracted by the material of the denser medium. Thus, the speed of light is increased in the denser medium and this resulted in a change in the direction. This theory is based on the fact that the speed of light must be greater in the denser medium. However, it was found later that light travels slowly in a denser medium than in rarer medium. This theory could not explain interference, diffraction and polarization of light also. Therefore, the corpuscular theory was discarded as an incomplete and inaccurate theory.
B.      Huygens wave theory: christian Huygens, a Dutch physicist, in 1690 proposed a new theory about the nature of light. According to this theory, light propagates from the source in the form of a wave. For the propagation of wave a medium is necessary. So, it was assumed that all space including vacuum is filled with medium called ether, which had the property of both elasticity and inertia. From the source of light, periodic disturbances are produced, which travel in the form of waves and thus energy is equally distributed in all directions. Huygens assumed that light waves are longitudinal. But Fresnel, using the phenomenon of polarization, proved that light waves are transverse in nature. Using wave theory, Huygens could explain reflection and refraction. Young explained interference and Fresnel explained diffraction and polarization on the basis of the wave theory.
C.      Electromagnetic theory: Maxwell in 1905 proved theoretically that light gets propagated in the form of electromagnetic waves, consisting of electric and magnetic fields mutually perpendicular as well as transverse to the direction of propagation of light. The electromagnetic waves propagate in free space with the velocity of light. Hertz demonstrated experimentally that electromagnetic propagate waves with the velocity equal to that of light.

D.      Quantum theory: Einstein in 1905 proposed a new theory of light, called a quantum theory. In order to explain the photoelectric effect. According to this theory, light is transmitted as tiny packets of energy called photons and energy of each photon is given by, E=hf, where f is the frequency of light and h is the plank’s constant.

E.       Duel nature of light: From the discussion of the various theories of light, we conclude that light can exist in particle form as well as wave form. Using quantum theory,we can explain photoelectric effect.But this theory could not explain interference, diffraction and polarization. These phenomena can be explained using wave theory. but wave theory could not explain photoelectric effect. so we have to accept the dual form of light viz. particle form and wave form.

Sunday, April 8, 2012 by Pooja · 30

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