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Lecture 8: Solar Cell, LED, Metal/Semiconductor Junction and Heterojunction Requirement: understand and explain in word. * Some of the content from C.

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Presentation on theme: "Lecture 8: Solar Cell, LED, Metal/Semiconductor Junction and Heterojunction Requirement: understand and explain in word. * Some of the content from C."— Presentation transcript:

1 Lecture 8: Solar Cell, LED, Metal/Semiconductor Junction and Heterojunction Requirement: understand and explain in word. * Some of the content from C. Hu : “Modern Semiconductor devices for Integrated Circuits”

2 Solar Cells is also known as photovoltaic cells. Converts 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. 5.7 Solar Cells

3 Solar Cell Basics V 0.7 V –I sc Maximum power-output Solar Cell IV I Dark IV 0 Eq.(4.9.4) Eq.(4.12.1) N P - Short Circuit light I sc + (a) E c E v

4 Direct-Gap and Indirect-Gap Semiconductors Electrons have both particle and wave properties. An electron has energy E and wave vector k. indirect-gap semiconductor direct-gap semiconductor Direct-gap semiconductor: Absorption coefficient is larger. Si is most prevalent for solar cell because of low cost.

5 Light Absorption α(1/cm): absorption coefficient 1/α : light penetration depth A thinner layer of direct-gap semiconductor can absorb most of solar radiation than indirect-gap semiconductor. Si solar cell > 50 um in thickness to absorb most of the photons because of low α

6 Short-Circuit Current and Open-Circuit Voltage If light shines on the N-type semiconductor and generates holes (and electrons) at the rate of G s -1 cm -3, If the sample is uniform (no PN junction), d 2 p’/dx 2 = 0  p’ = GL p 2 /D p = G  p

7 Solar Cell Short-Circuit Current, I sc Assume very thin P+ layer and carrier generation in N region only. G is really not uniform. L p needs be larger than the light penetration depth to collect most of the generated carriers.

8 Open-Circuit Voltage Total current is I SC plus the PV diode (dark) current: Solve for the open-circuit voltage (V oc ) by setting I=0 How to raise V oc ?

9 Output Power Theoretically, the highest efficiency (~24%) can be obtained with 1.9eV >Eg>1.2eV. Larger Eg lead to too low Isc (low light absorption); smaller Eg leads to too low Voc. Tandem solar cells gets 35% efficiency using large and small Eg materials tailored to the short and long wavelength solar light. A particular operating point on the solar cell I-V curve maximizes the output power (I V). Si solar cell with 15-20% efficiency dominates the market now

10

11 NRL’s new triple-junction solar cells could achieve 50 percent efficiency

12 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. EcEc EvEv Radiative recombination Non-radiative recombination through traps 5.8 Light Emitting Diodes and Solid-State Lighting

13 Direct and Indirect Band Gap Direct band gap Example: GaAs Direct recombination is efficient as k conservation is satisfied. Indirect band gap Example: Si Direct recombination is rare as k conservation is not satisfied Trap

14 LED Materials and Structure

15 LED Materials and Structure red yellow blue Wavelength (μm) Color Lattice constant (Å) InAs InN infrared 3.45 InP violet 5.87 GaAs GaP AlP GaN AlN UV 3.11 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) E g (eV) Red Yellow Green Blue

16 Common LEDs Spectral range Material System SubstrateExample Applications InfraredInGaAsPInPOptical communication Infrared -Red GaAsPGaAs 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-UVAlInGaN GaN or sapphire Solid-state lighting Red- Blue Organic semicon- ductors glassDisplays AlInGaP Quantun Well

17 Two kinds of metal-semiconductor contacts: Rectifying Schottky diodes: metal on lightly doped silicon Low-resistance ohmic contacts: metal on heavily doped silicon 5.9 Metal-Semiconductor Junction

18  Bn Increases with Increasing Metal Work Function Theoretically,   Bn =  M –  Si  M  Si : Work Function of metal : Electron Affinity of Si  q  Bn E c E v E f E 0 q  M  Si = 4.05 eV Vacuum level,

19 Schottky Barriers Energy Band Diagram of Schottky Contact Schottky barrier height,  B, is a function of the metal material.  B is the most important parameter. The sum of q  Bn and q  Bp is equal to E g.

20 Schottky barrier heights for electrons and holes  Bn increases with increasing metal work function  Bn +  Bp  E g

21 A high density of energy states in the bandgap at the metal- semiconductor interface pins E f to a narrow range and  Bn is typically 0.4 to 0.9 V Question: What is the typical range of  Bp ? Fermi Level Pinning (Schottky barrier lowering)  q  Bn E c E v E f E 0  q  M  Si = 4.05 eV Vacuum level, + 

22 Using C-V Data to Determine  B Question: How should we plot the CV data to extract  bi ? EvEv EfEf EcEc q  bi q  Bn

23 Once  bi is known,   can be determined using Using CV Data to Determine  B V 1/C 2  bi EvEv EfEf EcEc q  bi q  Bn

24 Thermionic Emission Theory E fn - q(  B  V)  q  B qV Metal N-type Silicon V E fm E v E c x v thx

25 Schottky Diodes V I Reverse bias Forward bias V = 0 Forward biased Reverse biased

26 Applications of Schottly Diodes I 0 of a Schottky diode is 10 3 to 10 8 times larger than a PN junction diode, depending on  B. A larger I 0 means a smaller forward drop V. A Schottky diode is the preferred rectifier in low voltage, high current applications. I V PN junction Schottky  B I V PN junction Schottky diode  B diode

27 5.10 Heterojunction Heterojunction gives us additional parameters to manipulate the ratio of electron/hole current More will be discussed in ECE684: HEMT

28 What is the energy band diagram at thermal equilibrium? What is V bi


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