Microelectronics 2

1. Electrical Engineering 2 Lecture 11 Microelectronics 2 Dr. Peter Ewen (Room G08, SMC; email - pjse)

2. Diode resistance Static resistance: Dynamic resistance: I / mA 5 4 3 2 1 V ΔI I ΔV Is V / volts 0.8 0.4 -1 -2 -3 -4 -5 For V >> 26 mV, I in mA and assuming T = 300 K and  = 1 I / mA

3. Reverse Breakdown I / mA Fig. 58 VBD V / volts Diode equation prediction In a real device BREAKDOWN occurs: VBD – the BREAKDOWN VOLTAGE

4. Avalanche Breakdown Si Si Si Si Si Si Si Fig. 59(a) Si Si Si Si Si VREV EEnergy of drifting carriers Strong E field in depletion region

5. Zener Breakdown • Electrons torn directly from • bonds by the high E field Si Si Si Si Si Si Si Fig. 59 Si Si Si Si Si Strong E field in depletion region

6. Fig. 63 I / mA Characteristic for real device ______ 100 50 Characteristic predicted by the diode equation - - - • Characteristic becomes • more linear at large forward voltages due to resistance of p and n regions (R1 and R2). VBD -5 -10 Is 0.4 0.8 V / volts 2. Slope due to surface leakage. -50 -100 3. Breakdown occurs at VBD I / nA

7. Diode Capacitance depletion region parallel plate capacitor Fig. 65(a) d W - + - - + + - - + + - - + + - - + + - - + + p n junction A = plate area; ε = permittivity of medium between the plates CJ is the “junction capacitance” K = a constant; VB = barrier potential V = applied bias (-ve for reverse bias) n = ½ for an abrupt junction

8. LECTURE 11 • Diode equivalent circuit • Photodiode • photoconductive mode • photovoltaic mode  Light-emitting diode (LED) • The semiconductor or diode laser

9. The Diode Equivalent Circuit The forward characteristic of a diode can be approximated by I dI/dV = RF-1 VBD V V0 The reverse characteristic can be approximated by Forward bias Reverse bias I o RB D2 RZ VBD D1 RF Vo ideal diode CJ RL V Fig. 66 o

10. Diodes in Optoelectronics

11. Depletion Region p n Electrons Diffusion Flows (current p→n) Majority carriers Holes Drift Flows (current n→p) Electrons Minority carriers Holes Junction Diode Equation Electrons p n Diffusion Current Holes p n Electrons Drift Current Holes

12. electron hole Illumination h > Eg E p n - + Fig. 67: Schematic structure of the photodiode electron drift flow hole drift flow W Depletion Region Ip is the optically generated current R V Ip Light Intensity, P; Ip~ mA for P ~ mWcm-2

13. 0 mWcm-2 0.075 mWcm-2 0.150 mWcm-2 0.225 mWcm-2 0.300 mWcm-2 IF / A 50 40 30 20 10 • Good linearity (Ip P) • Fast response time (~10-9s) 0 0.1 0.7 Light Intensity, P mWcm-2 0.2 0.6 0.3 0.5 0.4 0 mWcm-2 0.1 0.2 0.3 0.4 0.5 -3 -2 -1 0 V / volts -30 -60 -90 -120 -150 Fig. 68: Typical photodiode characteristics IR / A

14. IL  Rload VL Fig. 70: Photodiode used in photovoltaic mode to convert optical to electrical power. A V  Fig. 69: Photodiode used in photoconductive mode to detect light. Alternative symbol

15. IF / A 0 mWcm-2 0.075 mWcm-2 0.150 mWcm-2 0.225 mWcm-2 0.300 mWcm-2 50 40 30 20 10 Fig. 68: Typical photodiode characteristics IL = 0 = Isc  Rload VL Voc = =0 0.1 0.2 0.3 0.4 0.5 -3 -2 -1 0 V / volts Voc -30 -60 -90 -120 -150 load line Slope of load line is -1/Rload Isc Photovoltaic region Photoconductive region IR / A

16. 6. Solar cell open circuit voltage, Voc (Bogart 4th Ed., p. 873) A solar cell has Voc = 0.4V for a certain illumination intensity, P. Find Voc if P is doubled. (Take  = 1)

17. 6. Solar cell open circuit voltage, Voc Voc for P = 0.075 mWcm-2 Voc for P = 0.150 mWcm-2 P= 2P= For a photodiode:

18. For an open-circuit, I = 0, hence initially: where Voc = 0.4V If the light intensity is doubled then Ip→ 2Ip and Voc will change to a new open-circuit value, say . Hence: Dividing (2) by (1):

19. 7. Solar cell array (Bogart 4th Ed., p.878) A solar cell array is required to deliver 4 A at 6 V at a certain light intensity. If each cell delivers 0.8 A at 0.6 V at this intensity, how many cells are required and how should they be connected?

20. 7. Solar cell array IL=0.8A P  Rload VL=0.6V 4A requires 4/0.8 = 5 cells in parallel 6V requires 6/0.6 = 10 cells in series 0.6V   4A            

21. 7. Solar cell array                                                   IL=0.8A P  Rload VL=0.6V 4A requires 4/0.8 = 5 cells in parallel 6V requires 6/0.6 = 10 cells in series Hence 50 cells are required in a 5  10 array. 4A

22. Light Emitting Diodes (LED’s) C.B. C.B. photon photon Eg Eg  = hc/Eg V.B. V.B. • The basic process • in a photodiode. (b) The basic process in an LED - radiative recombination. Fig. 71: Photon absorption (a) and photon emission (b) in a semiconductor.

23. C.B. defect levels phonons V.B. Fig. 72: Non-radiative recombination.  = hc/Eg For visible emission 1.4 < Eg < 3 eV GaAs – red light GaP – green light GaAsP – yellow light InGaN – blue light

24. The Semiconductor Laser • Compact • Reliable • Light output can be easily modulated by applied voltage signal Fig. 73: World laser diode sales ($ billions)

25. Laser light – monochromatic, i.e. single frequency – coherent, i.e. the photons are in phase C.B. C.B. photon photon  = hc/Eg V.B. V.B. (a) Spontaneous emission (b) Stimulated emission C.B. C.B. V.B. V.B. Population inversion Fig.75

26. THE SEMICONDUCTOR LASER p-type n-type Heavily doped pn junction EC Eg electrons EC EV holes forward bias electrons EV Fig.75: Semiconductor laser EC Eg EV holes EF EF electrons EC electrons zero bias EV electrons EC EC Eg strong forward bias EV holes EV

27. + Ohmic contact p-GaAs Flat, polished end Laser light output n-GaAs Ohmic contact Flat, polished end − Fig. 76: Schematic structure of a laser diode.

28. Summary electron hole THE PHOTODIODE • The photodiode is basically a diode constructed in such a way that it can be illuminated. • Illumination increases the minority carrier concentrations throughout the device and hence increases the drift current across the junction. Illumination h > Eg E p n - + electron drift flow hole drift flow W Depletion Region

29. Illumination gives rise to an additional current –Ip which is added to the normal diode current. • Ip P ; IP~ mA for P ~ mWcm-2 • PHOTOCONDUCTIVE MODE – photodiode is reverse-biased and acts as a light detector. • PHOTOVOLTAIC MODE – photodiode connected to load converts optical to electrical power. • For maximum power output the operating point should be at the “knee” of the characteristic.

30. In non-radiative recombination electrons hop down from the CB to the VB via defect levels in the gap. At each hop they give up energy as a lattice vibration (phonon). • LED’s must be made from materials for which radiative recombination is the main recombination mechanism and must have suitable Eg for operation in the visible. C.B. defect levels phonons V.B. GaAs – red light GaP – green light GaAsP – yellow light InGaN – blue light

31. Laser diodes are important because they are compact, reliable and can be used to convert electrical to optical signals. • In a laser diode population inversion is achieved by strongly forward biasing a pn junction which is heavily doped on both sides. electrons EC EC Eg strong forward bias EV holes EV