Semiconductors لللللللللللللل Summary.pptx

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1. Semiconductors summary Prepared by Dr . Abdelhamid Khattab Lecture 4 in Electronics ( BS1102)

2. Material Used in Electronics

3. Material Used in Electronics

4. Band Gap When an electron acquires enough additional energy, it can leave the valence shell , become a free electron, and exist in what is known as the conduction band The difference in energy between the valence band and the conduction band is called an energy gap or band gap. Once in the conduction band, the electron is free to move throughout the material and is not tied to any given atom

5. Band Gap

6. Semiconductors Within the crystalline structure, there are two types of charge movement (current): 1) The conduction band electrons are free to move under the influence of an electric field. (electron Current ) Current in Semiconductors

7. Doping Process Doping process , increases the number of current carriers (electrons or holes). The two categories of impurities are n- type and p- type.

8. N- Type Semiconductors Since most of the current carriers are electrons, silicon (or germanium) doped with pentavalent atoms is an n- type semiconductor ( the n stands for the negative charge on an electron )

9. P- Type Semiconductors The p stands for the positive charge on an holes )

10. The pn junction Take a block of silicon and dope part of it with a trivalent impurity and the other part with a pentavalent impurity, a boundary called the pn junction is formed. The pn junction is the basis for diodes, certain transistors, solar cells, and other devices

11. The pn junction

12. Semiconductor Diodes and Applications

13. Semiconductor Diodes and Applications What is a Diode ? Basic, Types, Characteristics, and Applications 15

14. The Diode A diode is a semiconductor device, made from a small piece of semiconductor material, such as silicon, in which half is doped as a P region and half is doped as an N region with a PN junction and depletion region in between. The P region is called the anode and N region is called the cathode . It conducts current in one direction and offers high resistance in other direction. The basic diode structure and symbol are shown in Figure below. A diode is a device which allows current to flow only in one direction . It is made from P type and N type semiconductors joined together. It has a depletion layer PN junction / potential barrier 16 Semiconductor Diodes and Applications

15. p-n Junction Diodes In an n-type semiconductor, majority carriers are electrons and minority carriers are holes. Similarly, in a p-type semiconductor, majority carriers are holes and minority carriers are electrons. When we join n-type and p-type semiconductors (Silicon or Germanium) together, we obtain a p-n junction as shown in Figure below. Current formed due to the movement of majority carriers across the junction is called the majority carrier current, . Similarly, current formed due to the movement of minority carriers across the junction is called the minority carrier current, . Note that, minority carrier and majority carrier currents flow in opposite directions .   17 Semiconductor Diodes and Applications

16. p-n Junction Diodes Electrons in the n-type material migrate across the junction to the p-type material (electron flow). Or, you could also say that holes in the p-type material migrate across the junction to the n-type material (conventional current flow). The result is the formation of a depletion layer around the junction intersection, as shown in Figure below. Normally, depletion layer is not symmetric around the intersection as the doping levels of n-side and p-side are usually not the same. 18 Semiconductor Diodes and Applications

17. Operating Conditions No Bias: No voltage is applied and no current is owing. Reverse Bias: Negative voltage ( i.e., opposite polarity with the p-n junction ) is applied. Forward Bias: Positive voltage ( i.e., same polarity with the p-n junction ) is applied. 19 Semiconductor Diodes and Applications

18. No Bias Condition No external voltage is applied to the p-n junction as shown in Figure below. So, and no current is owing . Under no bias, only a modest depletion layer exists as seen in Figure below.   Operating Conditions : No Bias Condition 20 Semiconductor Diodes and Applications

19. External voltage is applied across the p-n junction in the opposite polarity of the p - and n -type materials. Reverse Bias

20. Operating Conditions : Reverse Bias Condition This causes the depletion layer to widen as shown below, as electrons in the n-type material are attracted towards the positive terminal and holes in the p-type material are attracted towards the negative terminal. Thus, the majority carrier current is zero, i.e., . However, minority carriers move along the electric field across the junction forming the minority carrier current, . Sometimes, this current is also called as the reverse saturation current   22 Semiconductor Diodes and Applications

21. 23 Operating Conditions : Reverse Bias Condition Thus, diode current under reverse bias is given by :   Semiconductor Diodes and Applications

22. External voltage is applied across the p-n junction in the same polarity of the p- and n-type materials , as shown in Figure below. The depletion layer is narrow. So, electrons from the n-type material and holes from the p-type material have sufficient energy to cross the junction forming the majority carrier current, Minority carrier current is still present in the opposite direction   Operating Conditions : Forward Bias Condition 24 Semiconductor Diodes and Applications

23. Thus, diode current under forward bias is given by : Normally , so diode current under forward bias is approximately equal to the majority carrier current, i.e.,   Operating Conditions : Forward Bias Condition 25 Semiconductor Diodes and Applications

24. Voltage-Current Characteristic of a Diode 26 Semiconductor Diodes and Applications

25. Voltage-Current Characteristic of a Diode 27 Semiconductor Diodes and Applications

26. Voltage-Current Characteristic of a Diode 28 Semiconductor Diodes and Applications

27. Voltage-Current Characteristic of a Diode 29 Semiconductor Diodes and Applications

28. V-I Characteristic for Reverse Bias Voltage-Current Characteristic of a Diode The Complete V-I Characteristic Curve 30 Semiconductor Diodes and Applications

29. 31 Forward and Reverse Bias Bias voltage connections: Positive to Anode (A); Negative to Cathode (K). Bias voltage connections: Positive to Cathode (K); Negative to Anode (A). The bias voltage must be greater than the barrier potential. The bias voltage must be less than the breakdown voltage. Barrier potential: 0.7 V for silicon. There is no majority carrier current after transition time. Majority carriers provide the forward current. Minority carriers provide a negligibly small reverse current. The depletion region narrows. The depletion region widens. Semiconductor Diodes and Applications

30. Forward Bias Turn-On Voltage The point at which the diode changes from no bias condition to forward bias condition happens when the electron and holes are given sufficient energy to cross the p-n junction. This energy comes from the external voltage applied across the diode. This voltage (can be deduced from the diode characteristics curve) is called the turn-on voltage or the threshold voltage, and denoted by . The forward bias voltage required to turn on the diode for a Silicon diode: & Germanium diode:   Temperature Effects 32 Semiconductor Diodes and Applications

31. Load Line and Operating Point (Q-point) 33 From Figure above, we obtain We can rearrange the circuit equation above to get on the left-hand side of the equation, i.e., This equation obtained from the diode circuit is called the load line equation. The load line equation gives us all the possible current ( ) values for all the possible voltage ( ) values obtained across the diode in a given circuit.   Semiconductor Diodes and Applications

32. Load Line and Operating Point (Q-point) 34 Once we draw the load line over the diode characteristics curve given in Figure , and the intersection point will give us the solution ( ; ) of the diode current and diode voltages and for the given circuit, respectively. The result is shown in Figure below.   Drawing the load line and finding the point of operation This plot is called the load line plot . Also, the intersection point of the load line and the diode characteristics curve is called the operating point or the Q-point specified by the ( ; ) pair.     Semiconductor Diodes and Applications

33. Load Line and Operating Point (Q-point) 35 Once we draw the load line over the diode characteristics curve given in Figure , and the intersection point will give us the solution ( ; ) of the diode current and diode voltages and for the given circuit, respectively. The result is shown in Figure below.   Drawing the load line and finding the point of operation This plot is called the load line plot . Also, the intersection point of the load line and the diode characteristics curve is called the operating point or the Q-point specified by the ( ; ) pair.     Semiconductor Diodes and Applications

34. 36 DC Resistance (Static Resistance) Figure : Determining the DC resistance of a diode at a particular operating point For a specific applied DC voltage V , the diode will have a specific current and consequently a specific resistance . The amount of resistance , depends on the applied DC voltage and current. For a given operating point as shown in Figure above, we can determine the DC resistance as follows   Semiconductor Diodes and Applications

35. Diode Models 37 Diode iv characteristics equation Semiconductor Diodes and Applications

36. 38 Ideal Diode Characteristic Curve Ideal diode is short circuit (i.e., ON) when it is forward biased. Diode Models Forward bias : , . Reverse bias :   Semiconductor Diodes and Applications

37. 39 Practical Diode Characteristic Curve In most cases we consider only the forward bias voltage drop of a diode. Once this voltage is overcome the current increases proportionally with voltage . This drop is particularly important to consider in low voltage applications. Diode Models Forward bias : , Reverse bias :   Semiconductor Diodes and Applications

38. 40 Complete Characteristic Curve of a Diode The voltage drop is not the only loss of a diode. In some cases we must take into account other factors such as the resistive effects as well as reverse breakdown. Diode Models Forward bias : , Reverse bias :   Semiconductor Diodes and Applications

39. Example: Determine the forward voltage and forward current for the diode in Figure for each of the diode models. Also find the voltage across the limiting resistor in each case. Assume at the determined value of forward current. 41 Diode Models Semiconductor Diodes and Applications

40. 42 Diode Models Semiconductor Diodes and Applications

41. Example: Determine the reverse voltage and reverse current for the diode in Figure for each of the diode models. Also find the voltage across the limiting resistor in each case. Assume   43 Diode Models Example: For the series diode configuration of Fig. below , determine V D , V R , and I D . Semiconductor Diodes and Applications

42. Determine the current I in the circuit shown in Fig. below (i). Assume the diodes to be of silicon and forward resistance of diodes to be zero. Solution : The conditions of the problem suggest that diode D1 is forward biased and diode D2 is reverse biased. We can, therefore, consider the branch containing diode D2 as open as shown in Fig. (ii). Further, diode D1 can be replaced by its simplified equivalent circuit. Diode Models 44 Semiconductor Diodes and Applications

43. Find the voltage V A  in the circuit shown in Fig. 1. Use simplified model. Determine current through each diode in the circuit shown in Fig. 2. Use simplified model. Assume diodes to be similar. Diode Models 45 Fig. 1 Fig. 2 Determine I , V 1 , V 2 , and V o for the series dc configuration of Fig. below . Semiconductor Diodes and Applications

44. Diode Specification Sheets Data about a diode is presented uniformly for many different diodes. This makes cross-matching of diodes for replacement or design easier. Some of the key elements is listed below: : forward voltage at a specific current and temperature : maximum forward current at a specific temperature : maximum reverse current at a specific temperature PIV or PRV or VBR: maximum reverse voltage at a specific temperature Power Dissipation: maximum power dissipated at a specific temperature C: Capacitance levels in reverse bias : reverse recovery time Temperatures: operating and storage temperature ranges   46 Semiconductor Diodes and Applications