2. Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 COMSATS Institute of Information Technology Virtual campus Islamabad

3. Semiconductor device lab.KwangwoonUniversity Semiconductor Devices. I-V Characteristics of PN Junctions Lecture No: 7

4. PN Junction

5. Ideal I-V Characteristics: Assumptions 1)The space-charge region boundaries represent an a step junction. 2)The abrupt depletion layer approximation applies. - abrupt boundaries & neutral outside of the depletion region 3) No carriers exist in the space-charge region. 4)In the bulk of the diode outside the depletion region, the semiconductor is neutral. 5)Diode operation is considered at a temperature at which all impurity atoms are ionized. 6)Perfect ohmic contacts are made to the ends of the p and n regions. 7)The Maxwell-Boltzmann approximation applies to carrier statistics. 8) The Concept of low injection applies.

6. Qualitative Description of Current Flow Equilibrium Reverse biasForward bias

7. Semiconductor device lab.KwangwoonUniversity Semiconductor Devices.  Current-Voltage Relationship Quantitative Approach

8. Current-Voltage Characteristics THE IDEAL DIODE Positive voltage yields finite current Negative voltage yields zero current REAL DIODE

9. Voltage-Current Characteristics of a P-N Junction

10. Built-in-Potential

11. Boundary Conditions: If forward bias is applied to the PN junction

12. The Steady state : Under the idealized assumptions, no current is generated within the depletion region; all currents come from the neutral regions. In the neutral n region, there is no electric field, thus in the steady-state the solution of the continuity equation, with the boundary conditions gives:

13. Semiconductor Devices Minority Carrier Distribution Steady state condition :

14. Semiconductor Devices Ideal PN Junction Current

15. Effect of Temperature on diode Curves: Doping Levels Junction Area The Junction Temperature. All other factors may be regarded as being constant. However, temperature dependence is very strong.

16. Semiconductor Devices Total PN Junction Current

17. Semiconductor Devices Temperature Effect Js : strong function of temperature

18. Semiconductor Devices Reverse Bias-Generation Current Recombination rate of excess carriers (Shockley-Read-Hall model) In depletion region, Total reverse bias current density, J R n=p=0

19. Semiconductor Devices Forward Bias Recombination Current Recombination rate of excess carriers (Shockley-Read-Hall model) R = R max at x=o

20. Semiconductor Devices Total Forward Bias Current Total forward bias current density, J In general, (n : ideality factor)

21. SUMMARY

22. Semiconductor device lab.KwangwoonUniversity Semiconductor Devices. BreakDown Junction Break Down  Breakdown Characteristics * * Zener Breakdown * Avalanche Breakdown

23. Semiconductor Devices Zener Breakdown Zener effect Doping level > 10 18 /Cm 3  Highly doped junction ( narrow W)  Mechanism is termed tunneling or Zener breakdown

24. Zener Effect Zener Break Down: V D <= V Z : V D = V Z, I D is determined by the circuit. In case of standard diode the typical values of the break down voltage V Z of the Zener effect -20... -100 V Zener Diode – Utilization of the Zener effect – Typical break down values of V Z :-4.5... -15 V

25. Avalanche Breakdown  Impact Ionization Mechanism Mechanism Total current during avalanche multiplication I n (w) = M * I no

26. Semiconductor Devices Critical Electric Field & Voltage at Breakdown Critical electric field at breakdown in a one-sided junction Total current during avalanche multiplication  The breakdown voltage will decrease for a linearly graded junction

27. 26 Fig 2.28-30 Zener characteristics.

28. 27 Fig 2.31 Determining Zener impedance.

29. 28 Fig 2.32 Zener equivalent circuits. Ideal: Z Z = 0 Prac.: Z Z > 0

30. 29 Example 2.13 Zener diode. A 1N754A Zener diode has a dc power dissipation rating of 500 mW and a nominal Zener voltage of 6.8 V. What is the value of I ZM for the device?

31. Semiconductor Devices Metal Contacts No rectifying action. The current can flow in both direction The difference of carrier concentrations of the two materials at the contact. A barrier potential exists. rectifying action occurs. Mostly used in switching circuits. (turn on/off switches)

32. Semiconductor Devices Metal Contacts I-V Characteristics

33. LED Light emitting diode, made from GaAs – V F =1.6 V – I F >= 6 mA

34. 33 Fig 2.35-37 Light emitting diodes. LED symbol

35. 34 Table 2.4 Common LEDs. ElementsForward voltage (V F )Color Emitted GaAs1.5 V @ I F = 20 mAInfrared (invisible) AlGaAs1.8 V @ I F = 20 mARed GaP2.4 V @ I F = 20 mAGreen AlGaInP2.0 V @ I F = 20 mAAmber (yellow) AlGaInN3.6 V @ I F = 20 mABlue

36. 35 Fig 2.38 A LED needs a current- limiting resistor.

37. 36 Fig 2.39 Multicolor LED.

38. 37 Fig 2.43 Common diodes. RectifierZenerLED Schematic symbol Bias for normal operation Switched back and forth between forward and reverse. ReverseForward Normal V F Si: V F = 0.7 V Ge: V F = 0.3 V V F = 0.7 V (not normally operated) Normal V R Equal to applied voltage. Equal to V Z.Equal to applied voltage. Primary factors to consider for device substitution I 0 and V RRM ratings. P D(max) and V Z ratings. V F(min), I F(max), and V BR