Semiconductor Diodes Notes: Bipolar Junction Transistors

Electronics Semiconductor Diode $ Bipolar Junction Transish Physical electronics : The branch of electronics which deals with the flow of electronics in vacuum, gas or solids. Electronic engineering: The branch which deals with design development and deals with design, development and application of electronic devices Energy Level :- are horizontal lines showing the discrete energes possessed by the electrons in an isolated atom. Energy band :- is the wide range of energies Possessed by an electron in a solid. Valence band Conduction band It is the wide range of It is the wide range of energies possessed by the energies possessed by the Valence electrons conduction band electrons. It is the highest energy It is the lowest unfilled band, occupied by the band , for ingulators. Valence electrons. It is completely filled It is partially filled for criert gases, but partially for conductors filled for other materials. current conduction is due to the electrons in this band.

a Forbidden energy gap: The forbidden energy region b/w conduction band and valance band No electron is allowed to Stay in this region. Conductors. insulators and semiconductors. C.BD C.B 1 C.B Eg F.B IEg FB V.B VB V.B.D Insulator Semiconductor C.B conductor conduction band V.B- valance band, F.B forbidden band , Conductors : are substance which allow the Flow of electric current through them. valance band and the conduction band overlap, So Here, no energy required to move the electron from valance band to conduction band. electrons are available in conduction band and thus if Free current flows through it when p.d is applied across eg: Silver, Cu, AL. etc. Insulators : substance which do not allow the flow of electric current through them. Here, large energy gap b/w valance band and conducting band (based on band theory). So electron cannot jump from V.B to CB . eg : dry wood, glass, diamond etc Energy gap of consulator, Eg > 3 cv, For diamond Eg= = Ger

Semiconductors : are substances whose conductivity lies in b/w that of good conductor and an insulator In terms of band theory, Semiconductors have a Small energy gap b/w conduction band and valence band. Since the energy gap Eg is small, the no: of elections jumping to the C.B ig greater compaired to insulator When e jump from V.B to C.B, it leaves some energy level in the Valence band unfilled The electron missing site in the V.B is called a hole. eg: Germanium (cre), Silicon (si) Intringic Semiconductor Semiconductor in the pure form is called intringic Semiconductor eg: Ge and Si In Ge, Si valency is 4. At very low temperature there will be no free elections in C.B. The 4 valence electron are engaged in GB covalent bonding. Thus each atom feels that it has 8 e in outer shell (like nert gas , insulators). These bonds are strong only at very low temperatures. With increase in temperature covalent bond are broken, e come out of bond and eventually overcomes energy gap and excupies the conduction band. Thus semi semiconductors start conducting The no: of e in the C.B

will be equal to no: of holes in the valence band When a covalent band is broken, electron - hole pair is produced Hole is an election deficiency It is the absence of electrons from a filled electronic state. It has no charge. It behaves as if has +ve charge. In a semiconductor there is random motion of electrons and holes; the electrons move faster The electrons and holes act as current carriers. Extrinsic Semiconductor The impurity added Semiconductor is called extrensic Semicons The process of adding impurity is called doping The compurity added is called dopant usually one or two atoms of conpurity is added per 106 atoms of semiconductor Two types ; n type and p-type Semiconductors. P- type semiconductor NP- type semiconductor P- type semiconductor is nl - type semiconductor is Formed formed due to the dopping due to dopping of nitrogen, phospoxes, of (1) and group elements, ue Arsenic, Antimony Bismuth Boron, Aluminium, Thallium These are also known as These are also known as, Trivalent semiconductors Pentavalent Semiconductors Ptype semiconductors is N-type semiconductor is Positive type semiconductors negative type semiconductor ct means it deficiency of it means excess of le is e is present required

In p- type semiconductor In AL type semiconductor majority charge carriers majority charge carriers are holes and menority are eloctions and minority charge carriers are datrons charge carries are holes A hole indicates a messing In N-type Semiconductors election In this, no of the no: of holes is less holes or more than the than the no of Free electron no: of elections Si Si Si Si S Si B S1 Si AS of Si S, Si S Si hole Free electron majority carrir e, minority carrer C.B C.B o e from donar atom Eg acceptor Eg energy level donar energy VB level V.B majority carriers holes minority carros * acceptor compurity added donor impurity added P-N Junction diode PN Junction diode / Semi conductor diode / crystal diode If one side of a semiconductor crystal is doped with acceptor impurity atom and the other side of the same crystal us doped with donar impurity