Unit 2 semiconductors
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This document provides an introduction to semiconductors. It discusses how semiconductors can behave as either conductors or insulators depending on doping, and describes the crystal lattice structure of semiconductors. It also explains intrinsic and extrinsic semiconductors, detailing how doping with trivalent or pentavalent impurities creates N-type or P-type materials respectively. The document concludes by discussing diode operation under forward and reverse bias conditions.
Unit 2 semiconductors
- 2. Electronic Materials • The goal of electronic materials is to generate and control the flow of an electrical current. • Electronic materials include: 1. Conductors: have low resistance which allows electrical current flow 2. Insulators: have high resistance which suppresses electrical current flow 3. Semiconductors: can allow or suppress electrical current flow
- 3. Conductors • Good conductors have low resistance so electrons flow through them with ease. • Best element conductors include: – Copper, silver, gold, aluminum, & nickel • Alloys are also good conductors: – Brass & steel • Good conductors can also be liquid: – Salt water
- 4. Semiconductor Valence Orbit • The main characteristic of a semiconductor element is that it has four electrons in its outer or valence orbit.
- 5. Crystal Lattice Structure • The unique capability of semiconductor atoms is their ability to link together to form a physical structure called a crystal lattice. • The atoms link together with one another sharing their outer electrons. • These links are called covalent bonds. 2D Crystal Lattice Structure
- 6. Semiconductors can be Insulators • If the material is pure semiconductor material like silicon, the crystal lattice structure forms an excellent insulator since all the atoms are bound to one another and are not free for current flow. • Good insulating semiconductor material is referred to as intrinsic. • Since the outer valence electrons of each atom are tightly bound together with one another, the electrons are difficult to dislodge for current flow. • Silicon in this form is a great insulator. • Semiconductor material is often used as an insulator.
- 7. Semiconductors are mainly two types 1. Intrinsic (Pure) Semiconductors 2. Extrinsic (Impure) Semiconductors
- 8. Intrinsic Semiconductor • A Semiconductor which does not have any kind of impurities, behaves as an Insulator at 0k and behaves as a Conductor at higher temperature is known as Intrinsic Semiconductor or Pure Semiconductors. • Germanium and Silicon (4 th group elements) are the best examples of intrinsic semiconductors and they possess diamond
- 9. Doping • To make the semiconductor conduct electricity, other atoms called impurities must be added. • “Impurities” are different elements. • This process is called doping.
- 10. The Intrinsic Semiconductors doped with pentavalent impurities are called N-type Semiconductors. The energy level of fifth electron is called donor level. The donor level is close to the bottom of the conduction band most of the donor level electrons are excited in to the conduction band at room temperature and become the Majority charge carriers. Hence in N-type Semiconductors electrons are
- 11. N-type Semiconductor Si Si P Si Si Free electron Impure atom (Donor)
- 12. Conduction band E Ed Ev Donor levels Eg Valence band Ec Ec Electron energy Distance
- 13. Electronic Properties of Si · Silicon is a semiconductor material. – Pure Si has a relatively high electrical resistivity at room temperature. · There are 2 types of mobile charge-carriers in Si: – Conduction electrons are negatively charged; – Holes are positively charged. · The concentration (#/cm3) of conduction electrons & holes in a semiconductor can be modulated in several ways: 1. by adding special impurity atoms ( dopants ) 2. by applying an electric field 3. by changing the temperature 4. by irradiation
- 14. Another Way to Dope • You can also dope a semiconductor material with an atom such as boron that has only 3 valence electrons. • The 3 electrons in the outer orbit do form covalent bonds with its neighboring semiconductor atoms as before. But one electron is missing from the bond. • This place where a fourth electron should be is referred to as a hole. • The hole assumes a positive charge so it can attract electrons from some other source. • Holes become a type of current carrier like the electron to support current flow.
- 15. Si Si In Si Si Hole Co-Valent bonds Impure atom (acceptor)
- 16. E Ea Ev Acceptor levels Valence band Ec Conduction band Ec Electron energy temperature Eg
- 17. • The dominant charge carriers in a doped semiconductor (e.g. electrons in n-type material) are called majority charge carriers. Other type are minority charge carriers • The overall doped material is electrically neutral
- 18. Current Flow in N-type Semiconductors • The DC voltage source has a positive terminal that attracts the free electrons in the semiconductor and pulls them away from their atoms leaving the atoms charged positively. • Electrons from the negative terminal of the supply enter the semiconductor material and are attracted by the positive charge of the atoms missing one of their electrons. • Current (electrons) flows from the positive terminal to the negative terminal.
- 19. Current Flow in P-type Semiconductors • Electrons from the negative supply terminal are attracted to the positive holes and fill them. • The positive terminal of the supply pulls the electrons from the holes leaving the holes to attract more electrons. • Current (electrons) flows from the negative terminal to the positive terminal. • Inside the semiconductor current flow is actually by the movement of the holes from positive to negative.
- 20. p- n junction formation What happens if n- and p-type materials are in close contact? Being free particles, electrons start diffusing from n-type material into p-material Being free particles, holes, too, start diffusing from p-type material into n-material Have they been NEUTRAL particles, eventually all the free electrons and holes had uniformly distributed over the entire compound crystal. However, every electrons transfers a negative charge (-q) onto the p-side and also leaves an uncompensated (+q) charge of the donor on the n-side. Every hole creates one positive charge (q) on the n-side and (-q) on the p-side
- 21. Diode concepts
- 22. pn Junction Under Open-Circuit Condition • The diffusion of positive charge in one direction and negative charge in the other produces a charge imbalance – this results in a potential barrier across the junction
- 23. Contd… Fig (a) shows the pn junction with no applied voltage (open-circuited terminals). Fig.(b) shows the potential distribution along an axis perpendicular to the junction.
- 24. Forward bias if the p-type side is made positive with respect to the n-type side the height of the barrier is reduced more majority charge carriers have sufficient energy to surmount it the diffusion current therefore increases while the drift current remains the same there is thus a net current flow across the junction which increases with the applied voltage
- 25. 10.1.4 Biasing the PN-Junction *Forward Bias:
- 26. Reverse bias • dc voltage negative terminal connected to the p region and positive to the n region. Depletion region widens until its potential difference equals the bias voltage, majority-carrier current ceases. if the p-type side is made negative with respect to the n-type side the height of the barrier is increased the number of majority charge carriers that have sufficient energy to surmount it rapidly decreases . The diffusion current therefore vanishes while the drift current remains the same thus the only current is a small leakage current caused by the (approximately constant) drift current the leakage current is usually negligible (a few nA)
- 27. *Reverse Bias: majority-carrier current ceases. * However, there is still a very small current produced by minority carriers.
- 28. Biasing the PN-Junction * Reverse Breakdown: As reverse voltage reach certain value, avalanche occurs and generates large current.
- 29. • Currents in a pn junction
- 30. Diode Characteristic I-V Curve
- 31. Shockley Equation * The Shockley equation is a theoretical result under certain simplification: ù 1 é æ i I exp v D nV ö where I 10 A at 300K is the (reverse) saturation current, n 1 to 2 is the emission coefficient, 0.026V at 300K is the thermal voltage V k T q k is the Boltzman' s constant, q 1.60 10 C æ when v 0.1V, i I exp v D nV ö This equation is not applicable when v 0 D T D D s -19 T -14 s T D s < ÷ ÷ø ç çè ³» @ = ´ = @ @ @ úû êë - ÷ ÷ø ç çè =
- 32. Symbol and Characteristic for the Ideal Diode (a) diode circuit symbol; (b) i–v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction.
- 33. Diode currents Switching times of diode Diode equivalent models Avalanche and zener breakdown
- 34. Diode currents
- 35. Switching times of diode • The switching time of a diode is defined as the time which a diode takes to change its state from forward biased state to reverse biased state or in other words the forward current through diode doesn’t reduce to reverse saturation current immediately as the reverse voltage is applied. • In fact it takes time for the current to reduce from forward current to reverse saturation current. This time is also called reverse recovery time.
- 36. Charge distribution of diode in Forward Biased state The red curve shows the level of concentration of minority carriers at different distances on the both sides of junction and the shaded blue part shows the increase in the concentration of minority carriers after forward biasing the diode. There is a difference in the peak level of minority carriers as we have the difference in the doping level of both sides.
- 37. Charge distribution of diode in Reverse Biased state • When we reverse biase any diode, the minority carriers from both sides cross the junction and then recombine after reaching the other side. Hence the holes from n-side move towards p-side and after reaching p-type material become majority carriers
- 38. Diode switching times • Now let’s analyze that what would happen when we change diode state from forward biase from reverse bias. • Firstly we are in forward biase state when voltage applied is +V. diode is now forward biased • Now we change the applied voltage to –V at time t=t1. i.e. diode is now reverse biased. • As minority carrier concentration in both sides was large near junction in the forward bias, when we have instantly changed the state to reverse biased, those minority carriers start moving in the opposite direction. And due to large concentration of such minority carriers, the amount of current flowing remains the same, only direction changes as shown below:
- 39. Contd.. • But the high reverse current continues for small time because the concentration of the stored minority carriers start decreasing and the current also starts decreasing exponentially as shown • The time gap t2 - t1 in which the reverse current is high (i.e. equal to I) is known as storage time and the time gap from t2 to t3 i.e. the time reverse current becomes equal to reverse saturation current is known as transient time. The total time from t1 to t3 is known as reverse recovery time.
- 40. Effect of doping on reverse recovery time
- 41. IDEAL DIODE • about the ideal diode, the diode is a device which acts as a short circuit when forward biased and acts as open circuit when reverse biased. • Hence the behavior of ideal diode can be shown in the following graph:
- 42. Simplified Diode model /Equivalent model of diode when forward biased.
- 43. Constant voltage drop model
- 44. Diode Voltages To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE. A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium.
- 45. Types of diodes • Varactor diodes – a reversed-biased diode has two conducting regions separated by an insulating depletion region – this structure resembles a capacitor – variations in the reverse-bias voltage change the width of the depletion layer and hence the capacitance – this produces a voltage-dependent capacitor – these are used in applications such as automatic tuning circuits
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- 48. Diode Applications Rectifier Simple Half-Wave RectifieCr ircuits What would the waveform look like if not an ideal diode?
- 49. Rectifier Circuits (contd) Bridge Rectifier Looks like a Wheatstone bridge. Does not require a enter tapped transformer. Requires 2 additional diodes and voltage drop is double.
- 50. Rectifier Circuits Peak Rectifier To smooth out the peaks and obtain a DC voltage, add a capacitor across the output.
- 51. • Transition and diffusion capacitances also refer to notes or text book
- 53. PN Junction Capacitance • A reverse-biased PN junction can be viewed as a capacitor. The depletion width (Wdep) and hence the junction capacitance (Cj) varies with VR. C = e si j W dep