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Chapter-2 Semiconcuctors Diodes & Transistors

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Chapter-2 Semiconcuctors Diodes & Transistors Semiconcuctors and Diodes & Transistors 1 Chapter-2 Semiconcuctors Diodes & Transistors Contents Semiconcuctors and Diodes: Energy Band of Insulators, Conductors and Semi Conductors Intrinsic and extrinsic semiconductors. PN junction diode, barrier potential, V-I characteristics. Special Purpose Diodes: Zener diode, Varactor diodes ,Light Emitting Diodes (LEDs),photo diodes, Solar cell. Specification parameters of diodes and numbering. Bipolar Junction Transistors: Structure, typical doping, Principle of operation,Detailed study of input and output characteristics of common emitter configuration,Transistor specifications. 2.1 Introduction A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors. Examples are: germanium and silicon. In terms of energy bands, semiconductors can be defined as those materials which have almost an empty conduction band and almost filled valence band with a very narrow energy gap (of the order of 1 eV) separating the two. Some materials are intrinsic semiconductors. An intrinsic semiconductor is one which is made of the semiconductor material in its extremely pure form. The semiconducting properties occur in these materials naturally. However, most of the semiconducting materials used in electronics are extrinsic. Those intrinsic semiconductors to which some suitable impurity or doping agent has been added in extremely small amounts are called extrinsic or impurity semiconductors. Depending on the type of doping material used, extrinsic semiconductors can be sub-divided in to N-type and P-type semiconductors. The p-n junction is a homojunction between a p-type and an n-type semiconductor. It acts as a diode, which can serve in electronics as a rectifier, logic gate, voltage regulator (Zener diode), switching or tuner (varactor diode); and in optoelectronics as a light-emitting diode (LED), photodiode or solar cell. In a relatively simplified view of semiconductor materials, we can envision a semiconductor as having two types of charge carriers- holes and free electrons which travel in opposite directions when the semiconductor is subject to an external electric field, giving rise to a net flow of current in the direction of the electric field. Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 2 2.2 Semi Conductors and Junction Diodes The electrical properties of a material depend largely upon how tightly outer electrons with in the atoms of that material are bound to the central nucleus. On the basis of this, materials can be classified in to the following three groups. Conductors Insulators Semi conductors Material in which the electrons are loosely bound to the central nucleus is called conductor.In the conductor electrons are free to drift around the material at random from one atom to another. Examples: Copper, Aluminium, Silver etc. Material in which the outer electrons are tightly bound to the nucleus is called insulator. There are no free electrons in insulator to move around the material. Examples: PVC, Rubber, Wood etc. Semi conductors are those materials their conductivity lies in between the conductivity of conductors and insulators and are called Semi conductors. Examples: Germanium, Silicon, Carbon etc. As per the rule of octate, the electrical properties of materials can again be defined on the basis of valance electrons (the electrons in the outer most orbits) numbers. If the number of valance electrons is less than 4, the material is generally called conductor. Instead of accepting electrons, it is easier to donate electrons to fill the outer sub shell as 8. If the number of valance electrons is more than 4, the material is generally called insulator. Instead of donating electrons, it is easier to accept lesser electrons to fill the outer sub shell. If the number of valance electrons is equal to 4, the material is generally called semi conductor. Here the probability of donating and accepting electrons is equal. 2.2.1 Energy Bands: In a single isolated atom, the electrons in the any orbit possess a definite energy. However an atom in solid is greatly influenced by the closely packed neighboring atoms. The electrons of the outer sub shell are shared by more than one atom in solid, the energy levels of outer shell electrons are changed considerably. Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 3 Because of this, electrons in the same shell have a range of energies rather than a single energy. This range of energy is known as energy bands. The figure shows the basic concept of energy bands in solid. The electrons in the first orbit have range of energy and form the 1st energy band. In the same way second orbit electrons form the 2nd energy band; third orbit electrons form the 3rd energy band and so on. The following are the more important energy bands. Valance Band Valance band in a solid is the energy band possessed by the valance electrons. Under normal condition valance band has the electrons of highest energy. Depending on materials this band may be filled completely or partially. Conduction Band The energy band which possesses the conduction electrons in a solid is called Conduction Band. In metals, the valance electrons are loosely attached to the nucleus and they can be easily detached. These electrons are called free electrons or conduction electrons. They are responsible for the conduction of current through the material. The current conduction is not possible, if there are no free electrons in the conduction band. The gap between the valance band and conduction band is called forbidden energy gap. The width of energy gap represent, how stronger the valance electrons are bonded to the nucleus. The greater the gap more tightly the valance electrons are bonded to the nucleus. To make valance electron free, an external energy equal to the forbidden energy gap must be supplied to lift the electrons from the valance band to the conduction band. The forbidden energy gap is usually expressed in terms of electron volt (eV). Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 4 2.2.1.2 Energy Band of Insulators, Conductors and Semi Conductors: The electrical behavior of solid can be explained with the help of energy bands. Insulators Fig (a) shows the energy band diagram of insulators. Here the valance band is full while the conduction band is empty. More over the energy gap between valance band and conduction band is very large (15 eV).Therefore a very high electric field is required to lift the valance electrons to the conduction band. Due to this reason the electrical conductivity of insulator is extremely small and can be regarded as zero under normal condition. Conductors In the energy band diagram of conductors, there is no forbidden energy gap between the valance band and the conduction band .The two bands actually overlap as shown in fig(b).It indicates that, the valance band energies are the same as the conduction band energies and it is very easy for a valance electron to become a conduction electron. Therefore without supplying additional energy these materials can have a large number of free electrons and act as good conductors. Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 5 . Semi Conductors In the case of semi conductors, the valance band is almost filled and conduction band is empty. But the forbidden energy gap is very small (1 eV) as shown in fig(c).There fore comparatively a smaller electric field (smaller than required in the case of insulator but greater than conductor) is required to lift the valance electrons to the conduction band. Thus the conductivity of semiconductor lies between a conductor and insulator. 2.2.2 Intrinsic Semi Conductor: A semi conductor in its purest form is known as intrinsic semi conductor. To form molecules of matters, the atoms in every element are held together by the bonding action of valance electrons. Each atom has the tendency to fill its outer most shell by acquiring eight electrons in it. In the case of an intrinsic semiconductor such as Ge or Si, it has only four electrons in its outer shell of its atom. To fill the shell as eight it requires four electrons more. This is acquired by forming bond through sharing one valance electron from each of the neighboring atoms. Such bonds are called Co-valent bond. Thus in semi-conductors the atoms are Muhammed Riyas A.M,Assistant Professor,Dept. of ECE,MCET Pathanamthitta Semiconcuctors and Diodes & Transistors 6 arranged themselves in a uniform three dimensional pattern, so that each atom is surrounded by four atoms. This orderly pattern is known as crystal. Figure shows a two dimensional symbolic representation of silicon crystal. Here each of the valance electrons of silicon atom is shared by one of its four nearest neighbors to form covalent bond. At this state all the valance electron within the crystal are tightly bond to the parent atoms and no free electrons are available to cause electrical conduction. Therefore at absolute zero temperature, intrinsic semi conductor act as a perfect insulator. Due to temperature, covalent bond with in an intrinsic semi conductor will break and free electrons and holes are produced. This process is called electron hole pair generation. The number of free electrons is equal to the number of holes. These free electrons and holes moves in the crystal in a random manner. If an electron meeting a hole in a broken covalent bond and covalent bond is re-established. This process is called electron hole recombination. 2.2.3 Extrinsic Semi Conductor: The conductivity of the intrinsic semiconductor can be increased by adding small amount of impurities. The process of adding impurities to the intrinsic (pure) semiconductor is called doping.
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