PHYSICAL ELECTRONICS(ECE3540) APPLICATIONS OF PHYSICAL ELECTRONICS PART I Tennessee Technological University Monday, October 28, 2013 1
Introduction In the following slides, we will discuss the summary of the Reading Assignment: the concepts of a large reverse-bias voltage that cause a Junction Breakdown, thezener Effect and the Avalanche Effect. 1. Junction Breakdown, Avalanche Breakdown, Tunneling Breakdown 2. Zener Diodes 3. Tunnel Diodes 4. Applications of Physical Electronics I: PN Junction Diodes. More discussion on these concepts in Chapter 12. Tennessee Technological University Monday, October 28, 2013 2
PHYSICAL ELECTRONICS(ECE3540) Explanation of the Reading Assignment Zener Diodes and Tunnel Diodes Tennessee Technological University Monday, October 28, 2013 3
Junction Breakdown Filled States - Empty States E c E v Dominant if both sides of a junction are very heavily doped. Can be classified into two: 1. Zener Breakdown 2. Avalanche Breakdown I V E p E crit 10 6 V/cm Breakdown Tennessee Technological University Monday, October 28, 2013 4
1. Zener Breakdown I V B, breakdown voltage Small leakage Current A Forward Current V R P N R A Zener diode B 3.7 V A Zener diode is designed to operate in the breakdown mode. C D IC Tennessee Technological University Monday, October 28, 2013 5
Peak Electric Field Deletion layer N+ P N a Neutral Region increasing reverse bias E p E 0 x p (a) increasing reverse bias E p 2qN E( 0) ( bi Vr ) s 1/2 x p (b) x V B 2 secrit 2 qn bi Tennessee Technological University Monday, October 28, 2013 6
E c E E Fp v electron-hole pair generation 2. Avalanche Breakdown Impact ionization: an energetic original electron electron generating electron and hole, which can also cause impact ionization. Impact ionization + positive feedbackavalanche breakdown V B secrit 2qN 2 E c E Fn V B 1 N 1 N a 1 N d E v Tennessee Technological University Monday, October 28, 2013 7
Quantum Mechanical Tunneling Fig. 1 Quantum Mechanical Tunneling Tunneling probability: 2 ( 8 m P exp 2T ( V E ) 2 H h A tunnel diode or Esaki diode is a type of semiconductor that is capable of very fast operation, well into the microwave frequency region, made possible by the use of the quantum mechanical effect called tunneling. ) Tennessee Technological University Monday, October 28, 2013 8
Tunnel Diode Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow p n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the p-n junction. As voltage increases further these states become more misaligned and the current drops this is called negative resistance because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode. Tennessee Technological University Monday, October 28, 2013 9
Tunnel Diode In the reverse direction, tunnel diodes are called back diodes (or backward diodes) andcan act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction). Under reverse bias, filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the PN junction barrier in reverse direction. Tennessee Technological University Monday, October 28, 2013 10
Tunnel Diode Fig. 2: a) Simplified Energy band diagram of a tunnel diode with a reverse bias voltage b) I-V Characteristic of a Tunnel Diode with a reverse-bias voltage Tennessee Technological University Monday, October 28, 2013 11
PHYSICAL ELECTRONICS(ECE3540) APPLICATIONS OF PN JUNCTION DIODES Tennessee Technological University Monday, October 28, 2013 12
The PN Junction as a Temperature Sensor I qv kt I0( e 1) I 0 Aqn 2 i L D p p N d Dn L N n a Fig. 3: PN Junction diode as a Temperature Sensor What causes the IV curves to shift to lower V at higher T? Tennessee Technological University Monday, October 28, 2013 13
Solar Cells Fig. 4: World Energy Consumption Solar Cells are also known as photovoltaic cells (PV). Convert sunlight to electricity with 10-30% conversion efficiency. 1 m 2 solar cell generate about 150 W peak or 25 W continuous power. Low cost and high efficiency are needed for wide deployment. Tennessee Technological University Monday, October 28, 2013 14
Solar Cell Basics light N Short Circuit P I sc I Dark IV Eq.(4.9.4) - 0 0.7 V V E c Solar Cell IV E v + (a) I sc Maximum power-output I qv kt I ( 0 e 1) I sc Tennessee Technological University Monday, October 28, 2013 15
Light Absorption Light intensity (x) α(1/cm): absorption coefficient e -x Fig. 5: Photon Energy vs. Absorption Coefficient hc Photon Energy (ev) 1.24 ( m) A thinner layer of direct-gap semiconductor can absorb most of solar radiation than indirect-gap semiconductor. Compare Si and Ge. Tennessee Technological University Monday, October 28, 2013 16
Output Power A particular operating point on the solar cell I-V curve maximizes the output power (I x V). Output Power I sc V oc FF Si solar cell with 15-20% efficiency dominates the market now Theoretically, the highest efficiency (~24%) can be obtained with 1.9eV >E g >1.2eV. Larger E g lead to too low I sc (low light absorption); smaller E g leads to too low Voc. Tandem solar cells gets 35% efficiency using large and small E g materials tailored to the short and long wavelength solar light. Tennessee Technological University Monday, October 28, 2013 17
Light Emitting Diodes and Solid-State Lighting Light emitting diodes (LEDs) LEDs are made of compound semiconductors such as InP and GaN. Light is emitted when electron and hole undergo radiative recombination. E c Radiative recombination E v Non-radiative recombination through traps Tennessee Technological University Monday, October 28, 2013 18
LED Materials and Structure Fig. 7: LED Materials and Structure LED wavelength ( m) 1.24 photon energy 1.24 ( ev ) E g Tennessee Technological University Monday, October 28, 2013 19
LED Materials and Structure E g (ev) Wavelength (μm) Color Lattice constant (Å) InAs 0.36 3.44 6.05 InN 0.65 1.91 infrared 3.45 InP 1.36 0.92 5.87 GaAs 1.42 0.87 red 5.66 yellow GaP 2.26 0.55 blue 5.46 AlP 3.39 0.51 violet 5.45 GaN 2.45 0.37 3.19 AlN 6.20 0.20 UV 3.11 Table: Light-emitting diode materials compound semiconductors binary semiconductors: - Ex: GaAs, efficient emitter ternary semiconductor : -Ex: GaAs 1-x P x, tunable E g (to vary the color) quaternary semiconductors: - Ex: AlInGaP, tunable E g and lattice constant (for growing high quality epitaxial films on inexpensive substrates) Tennessee Technological University Monday, October 28, 2013 20
Common LEDs Spectral range Material System Substrate Example Applications Infrared InGaAsP InP Optical communication Infrared- Red GaAsP GaAs Indicator lamps. Remote control Red- Yellow AlInGaP GaA or GaP Optical communication. High-brightness traffic signal lights Green- Blue InGaN GaN or sapphire High brightness signal lights. Video billboards Blue-UV AlInGaN GaN or sapphire Solid-state lighting Red- Blue Organic semiconductors glass Displays Tennessee Technological University Monday, October 28, 2013 21
Solid-State Lighting luminosity (lumen, lm): a measure of visible light energy normalized to the sensitivity of the human eye at different wavelengths Incandescent lamp Compact fluorescent lamp Tube fluorescent lamp White LED Theoretical limit at peak of eye sensitivity ( λ=555nm) Theoretical limit (white light) 17 60 50-100 90 683 ~340 Luminous efficacy of lamps in lumen/watt Organic Light Emitting Diodes (OLED) : has lower efficacy than nitride or aluminide based compound semiconductor LEDs. Terms: luminosity measured in lumens, luminous efficacy Tennessee Technological University Monday, October 28, 2013 22
(a) Absorption Diode Lasers Light Amplification (d) Net Light Absorption (b) Spontaneous Emission (c) Stimulated Emission (e) Net Light Amplification Light amplification requires population inversion: electron occupation probability is larger for higher E states than lower E states. Stimulated emission: emitted photon has identical frequency and directionality as the stimulating photon; light wave is amplified. Tennessee Technological University Monday, October 28, 2013 23
Laser Applications Red diode lasers: CD, DVD reader/writer Blue diode lasers: Blu-ray DVD (higher storage density) 1.55 m infrared diode lasers: Fiber-optic communication Photodiodes Photodiodes: Reverse biased PN diode. Detects photo-generated current (similar to I sc of solar cell) for optical communication, DVD reader, etc. Avalanche photodiodes: Photodiodes operating near avalanche breakdown amplifies photocurrent by impact ionization. Tennessee Technological University Monday, October 28, 2013 24
Picture Credits Semiconductor Physics and Devices, Donald Neaman, 4th Edition, McGraw Hill Publications. Modern Semiconductor Devices for Integrated Circuits, Prof. Chenming Calvin Hu, UC Berkeley. http://www.eecs.berkeley.edu/~hu/book-chapters-and-lecture-slidesdownload.html Tennessee Technological University Monday, October 28, 2013 25