Semiconductor Devices Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Semiconductor Devices Optical Devices Electronic Devices Light Emitting Diode (LED) Laser Diode (LD) Energy Band Structures PN Junction Diode Bipolar Junction Transistor MOSFET Devices Conduction band hΩ(phonon) Electron hν (photon) Direct Band Gap Source: http://electronics.howstuffworks.com/led1.htm Hole hν Valence band Indirect Band Gap Hwang Group (GIST) Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Extrinsic, Elemental Semiconductors Intrinsic Semiconductor : Pure material (4 valence electrons) ex) Si, Ge Extrinsic Semiconductor : Impurity additions (Dopants) Extrinsic Semiconduction : Resulting from dopants (1) n-type : negative charge carriers dominates Intrinsic Semiconductors (4 valence electrons) + V A elements l t (5 valence l electrons) l t ) = One extra electron = A conduction electron (2) p-type : positive charge carriers dominates Intrinsic Semiconductors (4 valence electrons) + III A elements (3 valence electrons) = Deficiency of one electron = An electron hole Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Extrinsic, Elemental Semiconductors n-type Semiconductors (1) Intrinsic Semiconductors + V A elements = Extra electrons = Conduction electrons (2) The addition of a group V A atom into solid solution in a crystal of group IV A → Modified energy band structure n-type Semiconductor II II II = Si = Si = Si = • II II II e= Si = As = Si = II II II = Si = Si = Si = II II II “N-type” Source: http://electronics.howstuffworks.com/led1.htm Diamond a o d cubic cub c structure Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Extrinsic, Elemental Semiconductors p-type Semiconductors (1) Intrinsic Semiconductors + III A elements = Deficiency of electrons = Electron holes (2) The addition of a group III A atom into solid solution in a crystal of group IV A → Modified energy band structure p-type Semiconductor II II II = Si = Si = Si = II II II = Si = B = Si = I ◦h+ II II = Si = Si = Si = II II II Source: http://electronics.howstuffworks.com/led1.htm “P-type” P type Light Emitting Diodes (LEDs) Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved 1. What is a Diode? • At the junction, free electrons from the N-type material fill holes from the P-type material. • This creates an insulating layer in the middle of the diode called the depletion zone. Source: http://electronics.howstuffworks.com/led1.htm Light Emitting Diodes (LEDs) Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved 1. What is a Diode? • A diode is the simplest sort of semiconductor device. • Broadly speaking, a semiconductor is a material with a varying ability to conduct electrical current. • Most semiconductors are made of a poor conductor that has had impurities (atoms of another material) added to it. The process of adding add g impurities pu t es is s called ca ed dop doping. g Source: http://electronics.howstuffworks.com/led1.htm Light Emitting Diodes (LEDs) Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved 1. What is a Diode? When the negative end of the circuit is hooked up to the N-type layer and the positive end is hooked up to P-type layer, electrons and holes start moving and the depletion zone disappears. Source: http://electronics.howstuffworks.com/led1.htm When the positive end of the circuit is hooked up to the N-type layer and the negative end is hooked up to the P-type layer, free electrons collect on one end of the diode and holes collect on the other other. The depletion zone gets bigger. Light Emitting Diodes (LEDs) 2. How Can a Diode Produce Light? Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Light Emitting Diodes (LEDs) Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved 3. Advantages • LEDs have several advantages over conventional lamps. • For one thing, they don't have a filament that will burn out, so they last much longer. • Additionally, their small plastic bulb makes them a lot more durable. durable • They also fit more easily into modern electronic circuits. Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Organic Light Emitting Diodes (OLEDs) 1. OLED Structure • Substrate (clear plastic, glass, foil) Th substrate The b t t supports t the th OLED. OLED • Anode (transparent) - The anode removes electrons (adds electron "holes") when a current flows through the device. • Organic layers - These layers are made of organic molecules or polymers. Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Organic Light Emitting Diodes (OLEDs) 1. OLED Structure • Conducting layer - This layer is made of organic plastic molecules that transport "holes" from the anode. (Polyaniline) • Emissive layer - This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made. (Polyfluorene) j • Cathode - The cathode injects electrons when a current flows through the device. Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Organic Light Emitting Diodes (OLEDs) 2. Fabrication of OLED • Vacuum deposition or vacuum thermal evaporation (VTE) - In a vacuum chamber, the organic molecules are gently heated (evaporated) and d allowed ll d to condense d as thin hi films fil onto cooled l d substrates. • Organic g vapor p phase p deposition p (OVPD) ( ) - In a low-pressure, hot-walled reactor chamber, a carrier gas transports evaporated organic molecules onto cooled substrates, where they condense into thin films. • Inkjet printing - OLEDs are sprayed onto substrates just like inks are sprayed onto paper during printing. Inkjet technology greatly reduces the cost of OLED manufacturing and allows printed onto very y large g films for large g OLEDs to be p displays like 80-inch TV screens or electronic billboards. Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Organic Light Emitting Diodes (OLEDs) 3. How Can OLEDs Produce Light? OLEDs emit light in a similar manner to LEDs, through g a process p called “Electrophosphorescence”. Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Organic Light Emitting Diodes (OLEDs) 4. Type of OLEDs Passive-matrix OLED (PMOLED) • PMOLEDs have strips of cathode, organic layers and strips of anode. • The anode strips are arranged perpendicular p p to the cathode strips. p • The intersections of the cathode and anode make up the pixels where light is emitted. emitted • External circuitry applies current to selected strips of anode and cathode, determining which pixels get turned on and which pixels remain off. • Again, g , the brightness g of each pixel p is proportional to the amount of applied current Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Organic Light Emitting Diodes (OLEDs) Active-matrix OLED (AMOLED) • AMOLEDs AMOLED h have ffull ll layers l off cathode, th d organic molecules and anode, but the anode layer overlays a thin film transistor (TFT) array that forms a matrix matrix. • The TFT array itself is the circuitry that determines which pixels get turned on to form an image. • AMOLEDs consume less power than PMOLEDs because the TFT array requires less power than external circuitry, so they are efficient for large displays. • AMOLEDs also have faster refresh rates suitable for video. • The best uses for AMOLEDs are computer monitors, large-screen TVs and electronic signs or billboards. Source: http://electronics.howstuffworks.com/led1.htm Copyright © 2010 Nano Devices Kyung-Won Park. All rights reserved Organic Light Emitting Diodes (OLEDs) 5. Structure of OLED Source: http://electronics.howstuffworks.com/led1.htm