Semiconductor fundamentals.pptx
- 2. Conductors Insulators Semiconductors Materials have low resistance which allows electrical current flow Ex.:Copper, silver, gold, aluminum, & nickel have high resistance which suppresses electrical current flow Ex:Glass, ceramic, plastics, & wood can allow or suppress electrical current flow Ex: carbon, silicon, and germanium 2
- 3. 1. Insulators: suppress flow of electric current (high resistance). 2. Conductors: allow electrical current flow. Low resistance, high conductivity. 3. Semiconductors: can allow or suppress electrical current flow . Difference in conductivity
- 4. Energy Band models for solids Band Theory An energy band is a range of allowed electron energies. Useful way to visualize the difference between conductors, insulators and semiconductors is to plot the available energies for electrons in the materials. The last completely filled (at least at T = 0 K) band is called the Valence Band • The next band with higher energy is the Conduction Band • The Conduction Band can be empty or partially filed • The energy difference between the bottom of the CB and the top of the VB is called the Band Gap (or Forbidden Gap)
- 5. Energy Bands for Solids
- 6. Insulators material does not conduct electric current. Example: glass, ceramics, plastics valence electron are tightly bound to the atom – less free electron there is a large forbidden gap between the energies of the valence electrons and the energy at which the electrons can move freely through the material (the conduction band). Semiconductors, Conductors and Insulators
- 7. Conductors material that easily conducts electrical current. Metals (copper, silver, gold, aluminum) are good conductors of electricity. valence electrons are very loosely bound to the atom There is an overlap of the valence band and the conduction band so that at least a fraction of the valence electrons can move through the material. Semiconductors, Conductors and Insulators
- 8. Semiconductor 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.
- 9. • Conductors have free electrons and partially filled valence bands, therefore they are highly conductive . • Insulators have filled valence bands and empty conduction bands, separated by a large band gap E g(typically >4eV), they have high resistivity (c ). • Semiconductors have smaller band gap. Some electrons can jump to the empty conduction band by thermal or optical excitation E g=1.1 eV for Si, 0.67 eV for Ge and 1.43 eV for GaAs 9 Band gap = the minimum photon energy required to excite an electron up to the conduction band from the valence band.
- 10. Semiconductor Materials (cont.) Semiconductor Bandgap Energy EG (eV) Carbon (diamond) 5.47 Silicon 1.12 Germanium 0.66 Tin 0.082 Gallium arsenide 1.42 Gallium nitride 3.49 Indium phosphide 1.35 Boron nitride 7.50 Silicon carbide 3.26 Cadmium selenide 1.70
- 12. Intrinsic semiconductors • A Semiconductor in its extremely pure form is said to be an intrinsic semiconductor. • Germanium and Silicon (4th group elements) are the best examples of intrinsic semiconductors and they possess diamond cubic crystalline structure.
- 13. Si Si Si Si Si Valence Cell Covalent bonds Intrinsic Semiconductor
- 14. Extrinsic semiconductors • Extrinsic semiconductors are the impure semiconductor that contains added impurities. • The conductivity of an extrinsic semiconductor is very large compared to the intrinsic material, due to the presence of impurity atoms. • An impure semiconductor, which is formed by doping a pure semiconductor is called as an extrinsic semiconductor Doping The process of adding impurities to the semiconductor materials is termed as doping. • There are two types of extrinsic semiconductors depending upon the type of impurity added .They are N- type extrinsic semiconductor and P-Type extrinsic semiconductor.
- 15. N-type semiconductor: - Pentavalent impurities are added to Si or Ge, the result is an increase the free electrons - Extra electrons becomes a conduction electrons because it is not attached to any atom - No. of conduction electrons can be controlled by the no. of impurity atoms - Pentavalent atom gives up an electron -call a donor atom - Current carries in n-type are electrons – majority carries - Holes – minority carries Pentavalent impurity atom in a Si crystal Sb impurity atom N-types and P-types Semiconductors
- 16. N-Type A small amount of pentavalent impurity is added to a pure semiconductor to result in N type extrinsic semiconductor. The added impurity has 5 valence electrons.
- 17. - Trivalent impurities are added to Si or Ge to create a deficiency of electrons or hole charges - The holes are created by doping process - The no. of holes can be controlled by the no. of trivalent impurity atoms - The trivalent atom can take an electron- acceptor atom - Current carries in p-type are holes – majority carries - electrons – minority carries B impurity atom P-types Semiconductors
- 18. P-Type A small amount of trivalent impurity is added to a pure semiconductor to result in P- type extrinsic semiconductor. The added impurity has 3 valence electrons.
- 19. P-Type Atoms from column III (B, Al, Ga, and In) introduce impurity levels in Ge or Si near the valence band. These levels are empty of electrons at 0 K. Since this type of impurity level "accepts" electrons from the valence band, it is called an acceptor level, and the column III impurities are acceptor impurities in Ge and Si. Doping with acceptor impurities can create a semiconductor with a hole concentration p0 much greater than the conduction band electron concentration n0 (this is p-type material). Thus holes are majority carriers and electrons are minority carriers.
- 20. PN Junction Diode • A p-n junction diode is two-terminal or two-electrode semiconductor device, which allows the electric current in only one direction while blocks the electric current in opposite or reverse direction. • A PN junction behaves as ideally short circuit when it is in forward biased and behaves as ideally open circuit when it is in the reverse biased
- 21. • When the junction is first formed, mobile carriers diffuse across the junction (due to the concentration gradients) – Holes diffuse from the p side to the n side, leaving behind negatively charged immobile acceptor ions – Electrons diffuse from the n side to the p side, leaving behind positively charged immobile donor ions A region depleted of mobile carriers is formed at the junction. • The space charge due to immobile ions in the depletion region establishes an electric field that opposes carrier diffusion. Equilibrium pn Junction + + + + + – – – – – p n acceptor ions donor ions
- 22. Unbiased pn junction Potential barrier • the barrier opposes the flow of majority charge carriers • for an isolated junction net current is zero
- 23. Diode Forward biased VD The process of applying the external voltage to a p-n junction semiconductor diode is called biasing.
- 24. • In Forward bias, negative terminal is connected to the n-type semiconductor and positive terminal is connected to the p-type semiconductor. • Holes from the p-side are attracted towards the negative terminal whereas free electrons from the n-side are attracted towards the positive terminal. • The electrons travel from the n-side to the p-side and go to the positive terminal of the battery. The holes that travel from the p-side to the n-side combine with the electrons injected into the n-region from the negative terminal of the battery. This way the diode conducts when forward-biased. • This decreases the width of depletion region. • Hence, the width of the depletion region decreases with increase in voltage.
- 25. The p-side or the positive side of the semiconductor has an excess of holes and the n-side or the negative side has an excess of electrons. In a semiconductor, the p-n junction is created by the method of doping.
- 27. In Reverse bias, negative terminal is connected to the p-type semiconductor and positive terminal is connected to the n-type semiconductor, holes from the p-side are attracted towards the negative terminal whereas free electrons from the n-side are attracted towards the positive terminal. If the reverse biased voltage applied on the p-n junction diode is further increased, then even more number of free electrons and holes are pulled away from the p-n junction. This increases the width of depletion region. Hence, the width of the depletion region increases with increase in voltage. The wide depletion region of the p-n junction diode completely blocks the majority charge carriers. Hence, majority charge carriers cannot carry the electric current.
- 29. Junction Diode Static I-V Characteristics • The forward voltage at which the flow of current through the PN Junction starts increasing quickly is known as knee voltage. This voltage is also known as cut-in voltage • The maximum reverse bias voltage that can be applied to a p-n diode is limited by breakdown. Breakdown is characterized by the rapid increase of the current under reverse bias. The corresponding applied voltage is referred to as the breakdown voltage.
- 30. Ideal Diode Equation 1 nkT qV S D D e I I
- 31. Diode Applications • Rectifying a voltage, such as turning the AC into DC voltages. • Isolating signals from a supply. • Voltage Reference. • Wave shaping • Detection signals. • Lighting systems (LED) • LASER diodes. 31
- 32. Diode Breakdown Voltage • When a reverse bias is applied no conduction should take place. • But due to the presence of minority charge carriers, a small reverse current flows through the diode known as leakage current. • Due to the flow of reverse current the width of the junction barrier increases. • When this applied reverse bias voltage is increased gradually at a certain point a rapid increase in the reverse current can be observed. This is known as Junction breakdown. • The corresponding applied reverse voltage at this point is known as Breakdown Voltage of the PN junction diode. This is also known as Reverse Breakdown Voltage. • Exceeding this voltage level causes the exponential increase in the leakage current of the diode. When a diode breakdown’s, overheating can be observed.
- 33. Zener vs. Avalanche Breakdown • Zener breakdown is a result of the large electric field inside the depletion region that breaks electrons or holes off their covalent bonds. • Avalanche breakdown is a result of electrons or holes colliding with the fixed ions inside the depletion region.