1. EE19301 ANALOG ELECTRONIC CIRCUITS Presented By

2. UNIT I ELECTRONIC DEVICES AND THEIR CHARACTERISTICS  PN junction diodes – Structure, Operation and VI characteristics;  Diffusion Capacitance and Transient Capacitance;  Zener Diode – BJT, JFET - Structure, Operation and Characteristics  UJT based relaxation oscillator.

3. PN Junction Diodes

4. Technical Terms  Semiconductor  Intrinsic  Extrinsic  Impurities  Anode  Cathode  Electrons  Holes  P-N junction  Bias  Forward bias  Reverse bias  Depletion Layer (or) Forbidden Energy Gap  Barrier potential

5. Electrons in Isolated Atoms  Isolated atoms have energy levels  The electrons can only be found in these energy states

6. Atoms in Solids  Atoms form a lattice structure The lattice affects the structure of the energy levels of each atom – we now have joint levels for the entire structure

7. Band Theory  Three bands of energy levels form  Valence Band – most of the electrons are here  Conduction Band – electrons here give the material electrical conductivity  Forbidden Band – electrons must jump this band to get from the valence to the conduction band

8. Lattice Bands Insulator: > 6eV Semiconductor: 1.1 eV

9. PN Junction Diode

10. PN Junction Formation

11. P-Type semiconductor having excessive holes N-Type semiconductor having excessive free electrons Junction Acceptor atoms Donor atoms

12. Formation of depletion layer The excess electrons in the N region cross the junction and combine with the excess holes in the P region. N region loses its electrons ……becomes + vly charged P region accepts the electrons ……becomes –vly charged At one point, the migratory action is stopped. The depletion layer contains no free and mobile charge carriers but only fixed and immobile ions. Its width depends upon the doping level.. Heavy doped……..thin depletion layer lightly doped……..thick depletion layer

13. Conduction  In order for an electron to become free and participate in current flow, it must gain enough energy to jump over the forbidden band  For semiconductors at room temperature, there is not enough energy to conduct.  As temperature increases more electrons have the energy to jump the forbidden band  Resistivity decreases  This is the opposite behavior of conductors

14. Resistivity R T Conductor Semiconductor

15. Semiconductors  When an electron becomes free, it creates a “hole” in the lattice structure A hole is effectively a positive charge

16. Electron and Hole Movement

17. Intrinsic Semiconductor  Elemental or pure semiconductors have equal numbers of holes and electrons  Depends on temperature, type, and size.  Compound Semiconductors can be formed from two (or more) elements (e.g., GaAs)

18. Extrinsic Semiconductors  A pure semiconductors where a small amount of another element is added to replace atoms in the lattice (doping).  The aim is to produce an excess of either electrons (n-type) or holes (p-type)  Typical doping concentrations are one part in ten million  Doping must be uniform throughout the lattice so that charges do not accumulate

19. N-Type and P-Type  One valence electron too many (n-type)  Arsenic, antimony, bismuth, phosphorus  One valence electron too few (p-type)  Aluminum, indium, gallium, boron

20. In a dc circuit

21. The PN Junction Diode Start with a P and N type material. Note that there are excess negatives in the n-type and excess positives in the p-type Merge the two – some of the negatives migrate over to the p-type, filling in the holes. The yellow region is called the depletion zone. More positive than rest of N More negative than rest of P

22. Biasing the Junction Apply a voltage as indicated. The free charge carriers (negative charges in the N material and positive charges in the P material) are attracted to the ends of the crystal. No charge flows across the junction and the depletion zone grows. This is called reverse bias. Switch polarity. Now the negative charges are driven toward the junction in the N material and the positive charges also are driven toward the junction in the P material. The depletion zone shrinks and will disappear if the voltage exceeds a threshold. This is called forward bias.

23. PN junction can basically work in two modes, (A battery is connected to the diode )  Forward bias mode ( positive terminal connected to p-region and negative terminal connected to n region)  Reverse bias mode ( negative terminal connected to p-region and positive terminal connected to n region) Operation of PN Junction Diode

24. Forward biased PN junction It forces the majority charge carriers to move across the junction ….decreasing the width of the depletion layer. Once the junction is crossed, a number of electrons and the holes will recombine. For each hole in the P section that combines with an electron from the N section, a covalent bond breaks and an electron is liberated which enters the positive terminal Thus creating an electron hole pair. Current in the N region is carried by ….electrons Current in the P region is carried by …. Holes.

25. Diode Circuit Symbols Reverse BiasForward Bias N material (cathode) P material (anode)

26. Forward Biased Junction ++++++ ------ E N P + - Opposing field due to battery Depletion region shrinks and drift current begins to flow

27. Forward Biased Junction

28. Reverse biased pn junction If the + of the battery is connected to the n-type and the – terminal to the p-type, the free electrons and free holes are attracted back towards the battery, hence back from the depletion layer, hence the depletion layer grows. Thus a reverse biased pn junction does not conduct current Only the minority carriers cross the junction constituting very low reverse saturation current. This current is of the order of micro ampere.

29. Reverse Biased Junction ++++++ ------ E N P + - Supporting field due to battery Depletion region expands and opposes flow of current

30. Reverse Biased Junction

31. (V-I) CHARACTERISTICS OF PN JUNCTION DIODE The curve drawn between voltage across the junction along X axis and current through the circuits along the Y axis. They describe the d.c behavior of the diode.

32. DIODE Characteristics

33. I-V Curve Recall Ohm’s Law (V=IR) Put it into slope-intercept form to get I = V/R. The slope of the graph is 1/R. Large slopes mean small R.

34. When it is in forward bias, no current flows until the barrier voltage (0.3 v for Ge) is overcome. Then the curve has a linear rise and the current increases, with the increase in forward voltage like an ordinary conductor. Above 0.3 v, the majority carriers passing the junction gain sufficient energy to knock out the valence electrons and raise them to the conduction band. Therefore, the forward current increases sharply. When it is in forward bias, no current flows until the barrier voltage (0.3 v for Ge) is overcome. Then the curve has a linear rise and the current increases, with the increase in forward voltage like an ordinary conductor. Above 0.3 v, the majority carriers passing the junction gain sufficient energy to knock out the valence electrons and raise them to the conduction band. Therefore, the forward current increases sharply.

35. With reverse bias,  potential barrier at the junction increased.… junction resistance increase…and prevents current flow.  However, the minority carriers are accelarated by the reverse voltage resulting a very small current (REVERSE CURRENT)….in the order of micro amperes.  When reverse voltage is increased beyond a value, called breakdown voltage, the reverse current increases sharply and the diode shows almost zero resistance. It is known as avalanche breakdown.  Reverse voltage above 25 v destroys the junction permanently.

36. To Remember  Diode cannot conduct in reverse bias condition.  Diode can conduct in forward bias condition.  Silicon diode has voltage drop of 0.7V  Germanium has voltage drop of 0.3V

37. RECTIFIER  Thus the P N junction diode allows the electrons flow only when P is positive.  This property is used for the conversion of AC into DC,Which is called rectification

38. Automatic switch When the diode is forward bias,the switch is CLOSED. When it is reverse biased, it is OPEN  When the diode is forward bias,the switch is CLOSED.  When it is reverse biased, it is OPEN

39. Advantages  No filament is necessary  Occupies lesser space  Long life.

40. APPLICATIONS  …. as rectifiers to convert AC into DC.  As an switch in computer circuits.  As detectors in radios to detect audio signals  As LED to emit different colours.

41.  What is PN Junction Diode?  How it works?  What are all the characteristics of PN Junction Diode?  Where it is used?