Transistor for std to 10 C B S E Course

3.5  TRANSISTORS

The term ‘transistor’ is a shortened form for transfer resistor. Transistor is a specially constructed three terminal semiconductor device. It is a device which consists of either one p-type region sandwiched between two n-type regions (npn transistor) or one n-type region sandwiched between two p-type regions (pnp transistor). The middle region is called the base and the two end regions are called the emitter and the collector. Figure shows npn transistor with its circuit symbol.

Transistor

Transistor was first fabricated in 1947 at the Bell Telephone Laboratories, by three American physicists John Bardeen, Walter H. Brattain and William B. Shockley. They were awarded Nobel prize for physics in 1956.

npn Transistor

A pnp transistor with its circuit symbol is shown in Fig. The   base is very thin compared to the emitter and the collector. In a transistor, the emitter is more doped then the collector region and the base is lightly doped. The function of the emitter is to emit electrons (in the case of npn transistor) or holes (in the case of pnp transistor) into the base. The base controls the flow of charge carriers into the collector which collects them from the base. Emitter of a transistor is always forward biased and the collector is always reverse biased.

pnp Transistor

Action : in the circuit shown in fig. using a n-p-n transistor the voltage from a source is applied to the base of the transistor. The emitter n layer has a small negative voltage with respect to the base p layer to permit the forward flow of current across the n-o junction. The collector n layer has a large positive voltage with respect to base p layer to prevent the reverse flow of current. Electrons from the emitter are attracted to the collector and a current flows through the outer circuit.

Circuits of npn transistor

To understand the practical application of the circuit, in Fig  3.6, circuit shown in fig 3.7 is considered. A variable weak voltage is applied across a resistor in the emitter base circuit(This is called input voltage) . The output voltage is taken across a resistor (called load) in the collector emitter circuit. Small changes in the input voltage result in large changes in output voltage; in other words, there is amplification.

Amplication of npn transistor

Applications of transistors :

            Transistor is used in

                     (i)        Amplifier

                    (ii)        Oscillator

                   (iii)        Switching circuits.

            Oscillator is a device for producing electric oscillations of a desired frequency.

Activity

Draw an appropriate circuit diagram, using p-n-p transistor.

For more see on LABLE list on right side.

Types of Semiconductors Std 8 to 10

3.3  TYPES OF S SEMICONDUCTORS

In a pure semiconductor, holes and electrons are always present in equal numbers and the resulting conductivity is called intrinsic conductivity. Pure semiconductors are called intrinsic semiconductors.

            The conductivity of semiconductors can be enhanced by the addition of minute traces of impurities (other elements) called dopants. The process of adding dopants is called doping and the resulting semiconductor is called doped semiconductor or extrinsic semiconductor. The conductivity in this case, is called extrinsic conductivity.

            There are two types of extrinsic semiconductors depending upon the types of impurities. They are classified as

(A) n-type semiconductors and

(B) p-type semiconductors.

These are used in the manufacture of diodes and transistors.

(A)   n-type semiconductors : if a very small amount of pentavalent impurity for example antimony (or arsenic or phosphorous), is introduced into the crystal of germanium, four out of five electrons of each impurity atom enters into bonds with the nearest germanium atoms, to form covalent bonds. The fifth electron is set free. Such free electrons act as current carries.

The added pentavalent impurity donates electrons to the semiconductor and the semiconductor becomes n-type semiconductor. The name n-type indicates that the majority charge carries are electrons. There will be free-electrons and holes in equal numbers due to breaking of some covalent bonds.

n-type Semiconductors

(B)   p-type semiconductors : When a trivalent impurity like boron, indium, gallium or aluminum is introduced into a germanium crystal, each impurity atom takes away one electron from the neighboring germanium atoms, to form covalent bonds. This results in the creation of  holes. Electrons from neighboring atoms can fill up these holes and there will be apparent motion of holes. Thus holes act as current carries. The added trivalent impurity accepts electrons from the semiconductor and the semiconductor becomes a p-type indicates that the majority charge carries are holes equivalent to positive charge. There will be free electrons and holes in equal numbers due to breaking of some covalent bonds.

p-type semiconductors

Semiconductors For std 10 CBSE Course

Semiconductors

You know that metals are good conductors of electricity. Substances like glass, plastic, porcelain are insulators. Some substances like germanium and silicon are neither good conductors nor insulators. Their conductivity lies in between that of conductors and insulators. Such substances are called semiconductors.

Conductivity of semiconductors : you know that the atoms of silicon and germanium have four electrons each in their outermost shell. All these electrons are involved in covalent bonding and are not free. Hence these elements should behave like insulators.  

However in these cases, usually a small amount of energy (in the form of heat or light) is sufficient to break a covalent bond and  set and electron free to move. Thus even at room temperatures, a substantial number of electrons will be dissociated from their atoms and this number increases with temperature. This is the reason for these elements exhibiting conductivity.

  Further, when an electron is removed from a covalent bond, it leaves a vacancy, equivalent to positive charge. An electron from a neighbouring atom, can drop into this vacancy, leaving the neighbour with a vacancy. In this way, the vacancy usually called a ‘hole’ can move and serves as an additional ‘charge carrier’ 

Electronics std 10 for CBSE board

ØElectron, a negatively charged particle (charge= 1.6 X 10-19C and mass 9.1 X 10-31 kg) is a common constituent of all matter. Electrons resolve round the nucleus of an atom, in different orbits.

Ø Flow of electrons constitutes electric current.

Ø Substances which allow electric current to pass through them, are called conductors, substances which do not allow electric current to pass through them are called insulators.

ØRadio waves are electromagnetic waves. They are used for communication such as radio broadcasting, television broadcasting, satellite communication etc.

3.1 INTRODUCTION

You have seen radio, T.V., tape recorder, computer and other electronic gadgets. Have you at any time wondered.

why they are called electronic gadgets?

Or

what is meant by electronics?  

The term ‘electronics’ owes its origin to the word ‘electron’. The term originally applied to the study of electron behavior and movement. Later with the advances in the knowledge about the fundamental nature of electron and about the way in which the motion of these particles could be controlled and utilized, the term came to be used in its existing broader sense.

Toady, electronics is a field of physics and engineering (both pure and applied) dealing with the design and application of devices, usually electronic devices. The operation of these depends upon the emission, behavior and effects of electrons. The development in these areas of knowledge has made it possible to manufacture a wide range of consumer, industrial and military products.

The changes that are taking place in the field of electronics, are so rapid that today’s technology may be obsolete tomorrow. Electronics finds applications in almost every walk of life. It has influenced areas such as communication, entertainment, defense, industry and medical science. The most recent advances in electronics include digital circuits, micro processors, digital computers, opto-electronics, laser technology and so on.

Wilmot’s bubbler Light Intensity, Light Quality and Concentration of Carbon dioxide on the Rate of photosynthesis

Study of the Effect of Light Intensity, Light Quality and Concentration of Carbon dioxide on the Rate of photosynthesis using Wilmott’s bubbler

AIM :-

To study the effect of light intensity, light quality and concentration of carbon dioxide on the rate of photosynthesis using Wilmott’s bubbler

INSTRUMENTS AND MATERIALS

Wilmott’s bubbler, Cellophane papers of red, blue and green colours,

Hydrilla (fresh plants), Vaseline, Sodium bicarbonate, water

Experiment measuring the rate of Photosynthesis by Wilmott’s Bubbler

Observation table fot Wilmot;s bubbler Experiment

AIM 2 :-

To study the effect of light quality on the rate of photosynthesis.

INSTRUMENTS AND MATERIALS

Wilmott’s bubbler, Cellophane papers of red, blue and green colours,

Hydrilla (fresh plants), Vaseline, Sodium bicarbonate, water

Red Cellophane paper Experiments

Blue Cellophane paper Experiments

Green Cellophane paper Experiments

Observation table with cellophane paper

Wilmot's Bubbler Video

Study of the Effect of Light Intensity, Light Quality and Concentration of Carbon dioxide on the Rate of photosynthesis

Study the Influence of Heat, Chemicals on the Release of Pigments from Root and other Tissues of Plant.

Study the Influence of Heat (temperature) and Chemicals on the Release of Pigments from Root and other Tissues of Plant.

AIM :-       

To study the influence of heat (temperature) and chemicals on the release of pigments from root and other tissues of plant.

INSTRUMENTS AND MATERIALS :-

Test tubes, cork borer, blade,  beakers, measuring cylinder, thermometer, burner,

Fresh beat roots, alcohol, formalin, benzene, distilled water, ice cubes.

Beat Root experiment shows influence of heat and chemicals

Influence of heat and chemicals on Beat root experiment Observation table

Influence of heat and chemicals on releases of Pigments from Beat root and other tissues of Plants

Video Presentation

Blood cell counting biology experiment for medical science

Counting of different types of Blood Cells under Microscope

AIM :- To count different types of blood cells under microscope.

INSTRUMENTS AND MATERIALS :- Slide, Pricking needle, Microscope, Cotton swab, Spirit, Leishman blue stain. 

Apparatus for Blood cell counting 

Blood cell counting with microscope

Blood cell counting Observation table

Blood cell counting experiment Video