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PHYSICAL ELECTRONICS(ECE3540) APPLICATIONS OF APPLICATIONS OF PHYSICAL ELECTRONICS – PART I PHYSICAL ELECTRONICS – PART I 1 Tennessee Technological University Monday, October 28, 2013

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 , the Zener 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. 2 Tennessee Technological University Monday, October 28, 2013

PHYSICAL ELECTRONICS(ECE3540) Explanation of the Reading Assignment Zener Diodes and Tunnel Diodes 3 Tennessee Technological University Monday, October 28, 2013

Junction Breakdown Dominant if both sides of a junction are very heavily doped. Filled States - Empty States E c Can be classified into two: 1. Zener Breakdown E v 2. Avalanche Breakdown I   6 V 10 V/cm E E p crit Breakdown 4 Tennessee Technological University Monday, October 28, 2013

1. Zener Breakdown I Forward Current V B , breakdown voltage V R Small leakage Current A P N R C A 3.7 V IC Zener diode B D A Zener diode is designed to operate in the breakdown mode. 5 Tennessee Technological University Monday, October 28, 2013

Peak Electric Field Deletion Neutral Region layer increasing reverse bias N+ N a P x p 0 1 / 2   qN  (a) 2    ( 0 )  ( | V |)  E E E  p bi r   E p s increasing reverse bias  2 E    s crit V x B bi 2 qN x p (b) 6 Tennessee Technological University Monday, October 28, 2013

2. Avalanche Breakdown • Impact ionization: an energetic Ec electron generating electron and original hole, which can also cause electron impact ionization. EFp Ev • Impact ionization + positive feedback  avalanche breakdown  2 E  s crit V electron-hole B 2 qN pair generation Ec 1 1 1    EFn V B N N N a d Ev 7 Tennessee Technological University Monday, October 28, 2013

Quantum Mechanical Tunneling Fig. 1 Quantum Mechanical Tunneling  2 8 ( ) m Tunneling probability:   H  exp 2 ( ) P T V E 2 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. 8 Tennessee Technological University Monday, October 28, 2013

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. 9 Tennessee Technological University Monday, October 28, 2013

Tunnel Diode  In the reverse direction, tunnel diodes are called back diodes (or backward diodes ) and can 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. 10 Tennessee Technological University Monday, October 28, 2013

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 11 Tennessee Technological University Monday, October 28, 2013

PHYSICAL ELECTRONICS(ECE3540) APPLICATIONS OF PN JUNCTION APPLICATIONS OF PN JUNCTION DIODES DIODES 12 Tennessee Technological University Monday, October 28, 2013

The PN Junction as a Temperature Sensor   ( 1 ) q V kT I I e 0   D D   2   p n I Aqn   0 i L N L N   p d n a Fig. 3: PN Junction diode as a Temperature Sensor What causes the IV curves to shift to lower V at higher T ? 13 Tennessee Technological University Monday, October 28, 2013

Solar Cells •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. Fig. 4: World Energy Consumption 14 Tennessee Technological University Monday, October 28, 2013

Solar Cell Basics I Short Circuit Dark IV Eq.(4.9.4) light N Isc P 0.7 V 0 V - Solar Cell E c IV –I sc E v Maximum + power-output (a)    ( 1 ) q V kT I I e I 0 sc 15 Tennessee Technological University Monday, October 28, 2013

Light Absorption   - x Light intensity (x) e α (1/cm): absorption coefficient hc  Photon Energy (eV)  1 . 24   ( ) m  Fig. 5: Photon Energy vs. Absorption Coefficient A thinner layer of direct-gap semiconductor can absorb most of solar radiation than indirect-gap semiconductor. Compare Si and Ge. 16 Tennessee Technological University Monday, October 28, 2013

Output Power A particular operating point on the solar cell I-V curve maximizes the output power (I x V).    Output Power I V FF sc oc •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. 17 Tennessee Technological University Monday, October 28, 2013

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 Non-radiative Radiative recombination recombination through traps E v 18 Tennessee Technological University Monday, October 28, 2013

LED Materials and Structure Fig. 7: LED Materials and Structure 1 . 24 1 . 24    LED wavelengt h ( m) photon energy ( ) E g eV 19 Tennessee Technological University Monday, October 28, 2013

LED Materials and Structure compound semiconductors Lattice ( eV ) Wavelength E Color constant g ( μ m) binary semiconductors: (Å) - Ex: GaAs, efficient emitter InAs 0.36 3.44 6.05 ternary semiconductor : InN 0.65 1.91 3.45 infrared - Ex: GaAs 1-x P x , tunable E g (to vary InP 1.36 0.92 5.87 the color) red GaAs 1.42 0.87 5.66 quaternary semiconductors: yellow 5.46 GaP 2.26 0.55 blue - Ex: AlInGaP , tunable E g and lattice violet constant (for growing high quality AlP 3.39 0.51 5.45 epitaxial films on inexpensive GaN 2.45 0.37 3.19 substrates) AlN 6.20 0.20 3.11 UV Table: Light-emitting diode materials 20 Tennessee Technological University Monday, October 28, 2013

Common LEDs Spectral Material Substrate Example Applications range System Infrared InGaAsP InP Optical communication Infrared- Indicator lamps. Remote GaAsP GaAs Red control Optical communication. Red- GaA or AlInGaP High-brightness traffic Yellow GaP signal lights High brightness signal Green- GaN or InGaN lights. Blue sapphire Video billboards GaN or Blue-UV AlInGaN Solid-state lighting sapphire Organic Red- semicon- glass Displays Blue ductors 21 Tennessee Technological University Monday, October 28, 2013

Solid-State Lighting luminosity (lumen, lm) : a measure of visible light energy normalized to the sensitivity of the human eye at different wavelengths Compact Tube Theoretical limit at Incandescent White Theoretical limit fluorescent fluorescent peak of eye sensitivity lamp LED (white light) lamp lamp ( λ =555nm) 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 22 Tennessee Technological University Monday, October 28, 2013

Diode Lasers Light Amplification (d) Net Light Absorption (a) Absorption (e) Net Light Amplification (b) Spontaneous Emission Light amplification requires population inversion: electron occupation probability is larger (c) Stimulated for higher E states than lower E Emission states. Stimulated emission: emitted photon has identical frequency and directionality as the stimulating photon; light wave is amplified. 23 Tennessee Technological University Monday, October 28, 2013