1. Farzana R ZakiEEE 231: Electronics I1 Semiconductor Diode Instructor: Farzana Rahmat Zaki Senior Lecturer, EEE Eastern University

2. Farzana R ZakiEEE 231: Electronics I2 Physical Operation of Diodes Lecture – 1

3. Farzana R ZakiEEE 231: Electronics I3 Outline Basic Semiconductor Concepts Classification of Silicon Intrinsic Silicon Current flowing mechanism in Crystals: diffusion & drift

4. Farzana R ZakiEEE 231: Electronics I4 Basic Semiconductor Concepts p-n junction: Semiconductor diode is basically a p-n junction. p-n junction consists of p-type semiconductor material (such as Si) & n-type semiconductor material (such as Si).

5. Farzana R ZakiEEE 231: Electronics I5 Classification of Silicon Silicon Intrinsic Si (pure Si) Extrinsic Si p-type (B, Al, etc) n-type (P, N, etc)

6. Farzana R ZakiEEE 231: Electronics I6 Intrinsic Silicon Silicon has 4 valence electrons. Forms covalent bonds with other atoms. At low temperatures, all electrons are in a covalent bond and thus no electrons are free to conduct electricity.

7. Farzana R ZakiEEE 231: Electronics I7 Thermal Ionization At room temperature, some electrons break free from their bonds (free electron) This process leaves a positively charged “hole” at the atom. –Note that the overall charge of the crystal is still neutral

8. Farzana R ZakiEEE 231: Electronics I8 Recombination –As the free electrons move randomly around the silicon, some will fill in some of the holes. –This process is called recombination. –Recombination is proportional to the number of free electrons and holes, which is, in turn, determined by the ionization rate. –The ionization rate is a strong function of temperature. –At thermal equilibrium, the recombination rate is equal to the ionization rate.

9. Farzana R ZakiEEE 231: Electronics I9 Equilibrium At equilibrium, the ionization rate is equal to the recombination rate, and there is a concentration of free electrons and an equal concentration of holes. n = p = n i ; where n i = intrinsic concentration

10. Farzana R ZakiEEE 231: Electronics I10 Summary of Intrinsic Silicon - Intrinsic Si is a crystal with four valence electrons –At room temperature, a number of these break free (ionize). –Some of these recombine with the holes that are left behind –The silicon reaches equilibrium when the recombination rate is the same as the ionization rate.

11. Farzana R ZakiEEE 231: Electronics I11 Current flowing mechanism in Crystals Two mechanisms for motion (current) in crystals –Diffusion Random motion of particles (electrons or holes) due to thermal excitation. Elements move from an area of higher concentration to an area of lower concentration. –Drift Movement of electrons and holes in response to an electric field.

12. Farzana R ZakiEEE 231: Electronics I12 Diffusion mechanism Associated with random motion due to thermal agitation. In a Si crystal with uniform concentrations of free electrons and holes, this random motion does not result in a net flow of current. So, if by some mechanism, the concentration of free electrons is made higher in one part of piece of Si than in other, then electrons will diffuse from the high concentration region to the low concentration region. This process gives rise to a Diffusion current.

13. Farzana R ZakiEEE 231: Electronics I13 Diffusion mechanism-cont. Consider the bar of Si in figure in which the hole concentration profile has been created along the x-axis by some unspecified mechanism. + + + + + + + + + + + x 0 Slope = dP/dx Hole concentration, P

14. Farzana R ZakiEEE 231: Electronics I14 Diffusion mechanism-cont Hole concentration profile results a hole diffusion current in x direction, magnitude of current at any point is proportional to the slope of the curve at that point J P = -q D P dP/dx ( hole diffusion) Where J P = hole current density ( i.e. current per unit area of the plane perpendicular to x axis) in A/cm 2 q = magnitude of electron charge = 1.6×10 -19 C D P = Diffusion constant or diffusivity of holes

15. Farzana R ZakiEEE 231: Electronics I15 Diffusion mechanism-cont For current diffusion J n = q D n dn/dx (electron current diffusion) Where J n = electron current density ( i.e. current per unit area of the plane perpendicular to x axis) in A/cm 2 q = magnitude of electron charge = 1.6×10 -19 C D n = Diffusion constant or diffusivity of electrons For holes and electrons diffusing in intrinsic Si, typical values for diffusion constants D p = 12 cm 2 /s and D n = 34 cm 2 /s

16. Farzana R ZakiEEE 231: Electronics I16 Drift Assume that an electric field is applied across a piece of silicon. Free electrons and holes are accelerated by the electric field and acquire a velocity component called drift velocity. If the field strength is denoted E (in V/cm), the positively charged holes will drift in the direction of E and acquire a velocity given by:

17. Farzana R ZakiEEE 231: Electronics I17 Mobility

18. Farzana R ZakiEEE 231: Electronics I18 Drift Current Density

19. Farzana R ZakiEEE 231: Electronics I19 Summary of Current flowing mechanism Diffusion Current –Based on concentration differences, particles move from higher to lower concentrations Drift Current –In the presence of an electric field, free electrons move opposite the field, holes move in the direction of the field.

20. Farzana R ZakiEEE 231: Electronics I20 Question: Consider a Si crystal having a hole density P and a free electron density n. An electric field E is applied to the crystal. Find the expression of resistivity for that material. Solution: Electric field = E Hole density = p Electron density = n Holes will drift in same direction as E and drift velocity for holes = μ P E Thus, for positive charge density ( qP C/cm 3 ) moving in x direction with velocity (μ P E), in 1s amount of charge flow across a plane A perpendicular to x-axis is I P = qPμ P EA ( current flows for holes) so, J P-drift = I P /A = qPμ P E ---------- (1)

21. Farzana R ZakiEEE 231: Electronics I21 For electrons, negative charge density ( -qn) moving in –x direction with drift velocity (-μ n E), so I n = qnμ n EA and J n-drift = qnμ n E --------------- (2) So, total drift current density, J drift = J p-drift + J n-drift = q(pμ p + nμ n )E Using Ohm’s law, E = IR = I(ρl/A) For unit length (l=1cm), E = I ρ/A = J drift ρ so, ρ =E/J drift ρ = 1 /q(pμ p + nμ n )

22. Farzana R ZakiEEE 231: Electronics I22 Einstein Relationship Relation between carrier diffusivity and mobility is known as Einstein relationship. The relation is as follow— Where V T = thermal voltage = 25 mV at room temperature for intrinsic Si

23. Farzana R ZakiEEE 231: Electronics I23 Lecture 2 Doping of Semiconductors: p-type & n-type Ideal Diode & its characteristics Practical Diode & its characteristics

24. Farzana R ZakiEEE 231: Electronics I24 Doping Semiconductors Doped Semiconductors –By adding an impurity, one kind of carrier predominates –Doped silicon where the majority of charge carriers are the negatively charged electrons is called n-type –Doped silicon where the majority of charge carriers are the positively charged holes is called p- type

25. Farzana R ZakiEEE 231: Electronics I25 N-type Doped with a pentavalent element such as phosphorus. –Five valence electrons four form covalent bonds one becomes a free electron –This is called a donor. No holes are created by this doping, thus there are free electrons without associated holes Electrons – majority charge carrier (concentration independent of T) Holes – minority charge carrier

26. Farzana R ZakiEEE 231: Electronics I26 N type (contd.) If N D = concentration of donor atoms n no = concentration of free electrons in N-type Si in thermal equilibrium then, n n0 = N D So, product of hole & electron concentration remains constant i.e, n no × p n0 = n i 2 Or, p n0 = n i 2 / N D Here, n i is a function of temperature p n0 is a function of temperature n n0 is independent of temperature.

27. Farzana R ZakiEEE 231: Electronics I27 P-type Doped with a trivalent element such as boron. –Three valence electrons three form covalent bonds other bond has a hole. –This is called an acceptor. No electrons are freed by this doping, thus there are holes without associated free electrons. Holes – majority charge carrier (concentration independent of T) Electrons – minority charge carrier

28. Farzana R ZakiEEE 231: Electronics I28 P type (contd.) If N A = concentration of acceptor atoms P p0 = concentration of holes in P-type Si in thermal equilibrium then p po = N A So, product of hole & electron concentration remains constant i.e, p p0 × n p0 = n i 2 Or, n p0 = n i 2 / N A Here, n i is a function of temperature n p0 is a function of temperature p p0 is independent of temperature.

29. Farzana R ZakiEEE 231: Electronics I29 Ideal Diode

30. Farzana R ZakiEEE 231: Electronics I30 Current-Voltage Characteristic of an Ideal diode Forward/Reverse Bias OFF mode ON mode i-v characteristic curve of an ideal diode

31. Farzana R ZakiEEE 231: Electronics I31 Ideal diode Positive terminal of diode is called anode and negative terminal is called cathode. OFF mode: if a negative voltage is applied to diode, no current flows and the diode behaves as Open Circuit. (v<0 and i=0). This mode is called Reverse-biased mode of operation and is said to be Cut Off. V anode Reverse bias ON mode: If a positive voltage is applied to diode, zero voltage drop appears across the diode and the diode behaves as Short circuit. (i>0 and v=0). This mode is called Forward and a forward-conducting diode is said to be Turned on or simply on. V anode > V cathode => Forward bias

32. Farzana R ZakiEEE 231: Electronics I32 Problem 1 What is the current through the diode and the voltage across the diode for the following two circuits?

33. Farzana R ZakiEEE 231: Electronics I33 Problem 2 What is the output voltage for the following circuit? (a Rectifier)

34. Farzana R ZakiEEE 231: Electronics I34 Problem 3 For the following circuit, if is a sinusoid with 24-V peak amplitude, find the fraction of each cycle during which the diode conducts. Find the peak value of the diode current and the maximum reverse-bias voltage that appears across the diode. Conduction angle: Peak value of diode current:

35. Farzana R ZakiEEE 231: Electronics I35 Problem 4 Find I and V in the following circuits.

36. Farzana R ZakiEEE 231: Electronics I36 Find the values of I and V in the following circuits.

37. Farzana R ZakiEEE 231: Electronics I37 Practical Diode

38. Farzana R ZakiEEE 231: Electronics I38 Areas of Operation There are 3 areas of operation –The forward-bias region v > 0 –The reverse-bias region v < 0 –The breakdown region v < -V zk

39. Farzana R ZakiEEE 231: Electronics I39 Areas Expanded: I-V characteristic curve for practical diode

40. Farzana R ZakiEEE 231: Electronics I40 Forward-Bias Region I is the forward-bias current Occurs when v on the diode is positive. the “cut-in” voltage is the voltage beneath which the current is negligible small (generally around 0.5V) The current exponentially increases, and the voltage drop typically lies in a narrow range from.6V to.8V

41. Farzana R ZakiEEE 231: Electronics I41 Saturation Current The saturation current is directly proportional to the cross-sectional area of the diode. For “small-signal” diodes, the saturation current is on the order of 10e-15A. Strongly correlated to temperature –doubles for every 5˚C rise in temperature.

42. Farzana R ZakiEEE 231: Electronics I42 Thermal Voltage

43. Farzana R ZakiEEE 231: Electronics I43 Fudge Factor n is a constant between 1 and 2 that represents variances in the material and physical structure of the diode. Diodes made using standard integrated circuit techniques exhibit an n close to 1. Diodes available as two-terminal devices generally exhibit an n closer to 2. We will use n=1 unless specified.

44. Farzana R ZakiEEE 231: Electronics I44 Voltage as a function of Current This exponential relationship for v holds over many decades of current (as many as seven decades).

45. Farzana R ZakiEEE 231: Electronics I45 Current effect on voltage

46. Farzana R ZakiEEE 231: Electronics I46 Problem 1 Consider a silicon diode with n=1.5. Find the change in voltage if the current changes from 0.1mA to 10mA.

47. Farzana R ZakiEEE 231: Electronics I47 Problem 2 A silicon junction diode with n=1 has v=0.7V at i=1mA. Find the voltage drop at i=.1mA and i=10mA.

48. Farzana R ZakiEEE 231: Electronics I48 Reverse-Bias Region In the reverse-bias region, the current is theoretically Real diodes often exhibit a much larger current due to leakages. However, the current is still quite small (nA range). There is also a slight increase with voltage for reverse-bias current.

49. Farzana R ZakiEEE 231: Electronics I49 Breakdown Region When the voltage reaches a certain negative potential, the diode will begin conducting current. This “knee” is known as the breakdown voltage, V zk. The Z stands for Zener and the K for knee. diodes that make use of the breakdown voltage and it’s near constant voltage/current relationship to be used in voltage regulation.

50. Farzana R ZakiEEE 231: Electronics I50 Thank You