3. Summary: Intrinsic vs. Extrinsic (n or p) • Intrinsic : # electrons = # holes (n = p) --case for pure Si • Extrinsic : --n ≠ p --occurs when DOPANTS are added with a different # valence electrons than the host (e.g., Si atoms) • N-type Extrinsic: (n >> p) • P-type Extrinsic: (p >> n)

4. Conductivity Summary • Conductors, semiconductors, and insulators... --difference is whether there are accessible energy states for conductance electrons. • For metals, conductivity increases with less scattering: --reducing deformation. --reducing imperfections. --decreasing temperature. • For intrinsic semiconductors, conductivity increased by --increasing temperature. • For doped semiconductors --increasing temperature doesn’t make a difference until T increases sufficiently for intrinsic carriers to dominate. --with increased dopants, mobility decreases (more scattering, same as metals).

5. Semiconductor Properties Intrinsic SCs: Conductivity only via thermally induced jumps from the VB to the CB across the band gap. Extrinsic SCs: Conductivity due to thermally induced much smaller jumps from the donor level E D to the CB (n-SC) or from the VB to the acceptor level E A (p-SC). SCs (intrinsic or extrinsic) are lousy conductors (compared to metals). SCs (intrinsic or extrinsic) are lousy electrical insulators (compared to ceramics and polymers). Typical conductivities: Si (intrinsic) 10 -5 per ohm.cm, Si (doped), 10 -1 to 10 2 , Cu 10 6 , Quartz 10 -19 .

6. Semiconductor Properties Intrinsic/Extrinsic SCs cannot be used as electrical wires, too much power loss… Intrinsic/Extrinsic SCs cannot be used in charge storage or electrical insulation, too much charge leakage… What are they good for???? Absolutely nothing on their own… But, how can a multi-billion $ industry be based on such materials? SC devices exploit asymmetry in the band diagrams at the junction (interface) when differently doped SCs are brought together.

7. Semiconductor Junctions When materials with dissimilar electrical properties are brought in contact, a depletion layer forms at the junction (interface). e- h+

19. Photodiode A photodiode is an n-p junction . When light of sufficient photon energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. If the absorption occurs in the junction's depletion region, these carriers are swept from the junction by the built-in field of the depletion region, producing a photocurrent . basic design more efficient design

20. Photodiode Boeing’s 31% efficient photovoltaic cell. Farming sunlight…

21. Light-Emitting Diodes (LEDs) A light-emitting diode ( L E D ) is a semiconductor device that emits incoherent narrow-spectrum light when electrically biased in the forward direction . Like a normal diode, L E D consists of a p-n junction . As in other diodes, current flows easily from the p-side to the n-side in the forward but not in the reverse direction. Charge-carriers — electrons and holes — flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and therefore, its c o l o r , depends on the band gap energy of the materials forming the p-n junction .

22. Light-Emitting Diodes (LEDs) AlGaAs - red and IR AlGaP - green AlGaInP - high-brightness orange-red , orange, yellow, and green GaAsP - red , orange , and yellow GaP - red , yellow and green GaN - green , blue , white InGaN - near UV , bluish-green and blue SiC as substrate - blue Sapphire (Al 2 O 3 ) as substrate - blue ZnSe - blue Diamond (C) - UV

23. Light-Emitting Diodes (LEDs) Applications: traffic lights/signals, motorcycle lights, flashlights, light bars on emergency vehicles, fiber optics, optical computer mice, display panels… A large LED screen at the University of Arkansas stadium.

24. Transistors Transistors are back-to-back p-n or n-p diodes. This one is a bipolar junction transistor (BJT) . There are many other kinds of transistors. The transistor is used in a wide variety of digital and analog functions, including signal amplification , switching (logic) , voltage regulation , and signal modulation ,

25. Transistors – Band Diagrams A transistor looks like two diodes back-to-back and the band diagram is symmetric . No current could flow through a transistor because back-to-back diodes would block current both ways . However, when you apply a small current to the BASE transistor, a much larger current can flow through it as a whole. This gives a transistor its switching behavior and also amplifies the incoming signal. A small current can turn a larger current on and off.

26. Transistors – Signal Amplification

27. Transistors – Switch with a Built-in Logic Metal Oxide Semiconductor Field Effect Transistor (MOSFET) This is a depletion MOSFET that is “ normally on .” You need to apply reverse bias to pinch off the conduction from the p -channel . There are MOSFETs that are normally off as well (called enhancement type MOSFETs).

30. April 1972 Name of Processor: 8008 Clock speed: 200 kilohertz Number of transistors: 3,500 September 1978 Name of Processor: 8086 Clock speed: 10 MHz Number of transistors: 29,000 February 1982 Name of Processor: 286 Clock speed: 12 MHz Number of transistors: 134,000 October 1985 Name of Processor: 386 Clock speed: 16 MHz Number of transistors: 275,000 June 1991 Name of Processor: 486 Clock speed: 50 MHz Number of transistors: 1,200,000 January 1996 Name of Processor: Pentium Clock speed: 166 MHz Number of transistors: 3.3 million August 1998 Name of Processor: Pentium II Clock speed: 450 MHz Number of transistors: 7.5 million March 2000 Name of Processor: Pentium III Clock speed: 1.0 GHz Number of transistors: 28 million Nov 2002 Name of Processor: Pentium 4 Clock speed: 3.0 GHz Number of transistors: 55 million

31. IBM 650 that "became the most popular medium-sized computer in America in the 1950's - rental cost was $5000 per month - 1500 were installed - able to read punched cards or magnetic tape - used rotating magnetic drum main memory unit that could store 4000 words!!! Nokia 9210 Communicator is part of the latest wave of web cell phones Dual 2GHz PowerPC G5 8 GB RAM

34. APPLICATION: SENSORS • Ex: Oxygen sensor: ZrO 2 • A concentration gradient causes diffusion of Oxygen through the ceramic. • This creates a measurable voltage. In order to make sensor response more rapid, increase diffusion rate by introducing more vacancies. • Add Ca impurity, requiring extra e - : --create Ca interstitials (not likely in this case) --create O 2- vacancies (dominant) ∆ V = – e k T · ln c 1 / c 2

35. P-N RECTIFYING JUNCTION REVIEW • Allows flow of electrons in one direction only (e.g., useful to convert alternating current to direct current. --No applied potential: no net current flow. --Forward bias: carrier flow through p-type and n-type regions; holes and electrons recombine at p-n junction; current flows. --Reverse bias: carriers flow away from p-n junction; carriers depleted; little current flow.

36. Popular culture