Note for Basic Electronics - BE by Swapnil Kaware

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Chapter No. 1. Semiconductor Diode. Points to remember….. (i). Valence bond:- Outermost (highest) band filled with electrons & represents the valency of an atom. (ii). Conduction bond:- Next highest band to valence band contains free electrons for conduction. (iii). Covalent bond:-These bonds are formed due to sharing of electrons. (iv). Doping:-Adding impurities to semiconductor& increases the conductivity of semiconductor. (v). Forbidden gap:- It separates conduction & valence band. No electrons can normally exist in this gap. Q(1). Explain or Define Conductor?. (i). Conductors are the substances or materials which allows the electric current to pass through them. (ii). There is no ‘forbidden gap ’is present between valence band &conduction band. (iii). Hence it contains larger number of free electrons in conduction band.(iv). In this conductivity is very high & resistivity is very low.(v). In case of conductors, conductivity is directly proportional to the temperature. e.g. copper, gold, silver, iron, aluminum, water, graphite etc. Q(2). Explain or Define Insulator?. (i). Insulators are the substances or materials which does not allows the electric current to pass through them. (ii). There is large ‘forbidden gap’ is present between valence band & conduction band. (iii). Hence it contains small or even zero number of free electrons.(iv). In this conductivity is very low & resistivity is very high. e.g. rubber, plastic, wood, glass, paper etc. Q(3). Explain or Define Semiconductor?. Page 1 (i). These are the substances or materials which can acts as a conductor as well as an insulator. (ii). A semiconductor material is one whose electrical properties lies in between those of insulators and conductors. (iii). In semiconductors ‘forbidden gap’ is very small.(iv). The conductivity of the semiconductor increases as temperature increases.(v). Hence as temperature increases electrical resistance of the semiconductor decreases. e.g. Germanium (Ge), Silicon (Si), Selenium (Se), Carbon (C) etc. Classification of semiconductors:- * Basic Electronics Notes By Er. Swapnil V. Kaware (svkaware@yahoo.co.in)*

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Q(4). Explain or Define Impurities or Dopant?. Impurities/dopants are added to semiconductor for increasing the conductivity of semiconductor. There are 2 types of impurities/dopants: (i). Donors (N type): These are basically pentavalent atoms (i.e. having valency=5) &they increases number of free electrons(i.e. negatively charged ions). e.g. Arsenic (As), Phosphorus(P) or Antimony(Sb) etc. (ii). Acceptors (P type ): These are basically trivalent atoms (i.e. having valency=3) &they increases number of holes (i.e. positively charged ions). e.g. Boron (B), Aluminium (Al) or Gallium(Ga) etc. Q(5). Explain or Define Intrinsic or Pure Semiconductor?. (i). An intrinsic semiconductor is made from the semiconductor material in its extremely pure form. (ii). It is also called as ‘undoped semiconductor’ or ‘i-type semiconductor’. (iii). The ratio of impurity atoms (other atoms) to the semiconductor part is <1:100 million. (iv). There is no concept of doping. (v). The amount of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities. (vi). The number of electrons in the conduction band is equal to the number of holes in the valence band i.e. ‘n=p’. (vii). No free electrons are available at zero temperature hence these type of semiconductors acts as a perfect insulator at zero temperature. (viii). Hence intrinsic semiconductors are not practically used for manufacturing of devices. (ix).Such type of semiconductors have total 4 electrons in its valence orbit section. (x). Hence to complete the valence shell section, each silicon (Si) atom acquires four more electrons by sharing one electron from each of the four neighboring atoms. * Basic Electronics Notes By Er. Swapnil V. Kaware (svkaware@yahoo.co.in)* Page (i). An extrinsic semiconductor is made from the semiconductor material in its extremely impure form. (ii). It is also called as ‘doped semiconductor’ or ‘impure type semiconductor’. (iii). The ratio of impurity atoms (other atoms) to the semiconductor part is >1:100 million. (iv). There is a concept of doping. (v). The amount of charge carriers is therefore determined by the amount of impurities instead of properties of the material itself. (vi). The number of electrons in the conduction band is equal to the number of holes in the valence band i.e. ‘n≠p’. (vii). Free electrons are available at zero temperature hence these type of semiconductors acts as a perfect conductor at zero temperature. (viii). Hence extrinsic semiconductors are practically used for manufacturing of devices. (ix). Such type of semiconductors have total 3 electrons in its valence 2 Q(6). Explain Extrinsic or Impure Semiconductor?.

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orbit section. (x). Hence to complete the valence shell section, each silicon (Si) atom acquires four more electrons by sharing one electron from each of the four neighboring atoms. (viii).Extrinsic semiconductors can be sub-divided into two types: (i). N-type semiconductors &(ii). P-type semiconductors. Q(7). Explain N-type semiconductors?. (i). Such type of semiconductors are obtained when a pentavalent impurity/material like Arsenic (As) or Phosphorus(P) or Antimony (Sb) is added to pure germanium or silicon crystal. (ii). As shown in Fig. below, each Arsenic (As) atom forms covalent bonds with the surrounding four silicon atoms with the help of four of its five electrons. (iii). The fifth electron is loosely bound to the Arsenic (As) atom. (iv). Hence, it can be easily excited/travel from the valence band to the conduction band by the application of external electric field or temperature. (v). This extra electron becomes available for conduction. (vi). Hence electrons are the majority carriers while holes constitute the minority carriers. Hence such type of semiconductors are called as N-type semiconductors. Q(8). Explain P-type semiconductors?. Page 3 (i). Such type of semiconductors are obtained when a Trivalent impurity/material like Boron (B), Aluminium (Al) or Gallium(Ga)is added to pure germanium or silicon crystal. (ii).As shown in Fig. below, each Gallium (Ga) atom forms covalent bonds with the surrounding four silicon atoms with the help of its three electrons. (iii).Hence fourth covalent bond becomes incomplete because Gallium (Ga) atom has only three valence electrons. (iv). Hence there is no electron around the Gallium atom. (v). Thus the resulting charge carrier is known as ‘hole’ which is positively charged carrier. (vi). Hence holes are the majority carriers while electrons constitute the minority carriers. So hence such type of semiconductors are called as P-type semiconductors. * Basic Electronics Notes By Er. Swapnil V. Kaware (svkaware@yahoo.co.in)*

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Q(9). Compare/write differences between P type & N type semiconductor?. Please refer the question numbers 7 & 8. Q(10). Compare/write differences between Intrinsic type & Extrinsic type semiconductor?. Please refer the question numbers 5 & 6. Q(11). What is Diode (or P-N junction diode), draw its symbol & also give its construction details?. The symbol of a diode is as shown in above figure. (i). The diodes are unidirectional & allows current to flow in one direction only and restrict such current to flow in another direction. (ii). It has two terminals namely P side (as Anode) & N side (as cathode). (iii). P side contains large number of free holes & N side contains large number of free electrons. (iv). Diodes are used in rectifier circuits, power supply circuits& inverter circuits etc. Page (i). In forward bias mode, positive terminal of battery is connected to P region while negative terminal of battery is connected to N region of P-N junction diode. (ii). In forward bias mode, negative terminal of battery repels/opposes the electrons present in N region towards P region. (iii). Similarly, positive terminal of battery repels/opposes the holes present in P region towards N region. (iv). As soon as, we provides increasing supply voltage, then the holes combine with negative ions & electrons combine with positive ions and finally both ions get converted into neutral atoms (i.e. immobile ions). (v). Such recombination of holes & electrons creates depletion region. (vi). Due to neutral ions width of the depletion region decreases. (vii). As barrier potential is directly proportional to depletion region width so hence barrier potential also reduces. (viii). Finally, there is no opposition to the flow of charge carriers (i.e. electrons & holes), which in turn helps to flow electric current also called as forward current (If). (ix). This forward current (If) flows due to majority carriers & having order of few mA. (x). This current flows from anode to cathode. 4 Q(12). Explain the formation of depletion region in P-N junction diode?. OR Q(13). Explain working/operation of P-N junction Diode in forward bias mode?. * Basic Electronics Notes By Er. Swapnil V. Kaware (svkaware@yahoo.co.in)*