LEDs, Photodetectors and Solar Cells

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1 LEDs, Photodetectors and Solar Cells Chapter 7 (Parker) ELEC 424 John Peeples

2 Why the Interest in Photons? Answer: Momentum and Radiation High electrical current density destroys minute polysilicon and metal traces on ICs. Joule heating leads to fusing Electromigration Photons have very low momentum. Electrical particles radiate energy as the 4 th power of their frequency. The source of crosstalk High frequency photons are much less prone to crosstalk. Thus our interest in: LEDs and Lasers as that emit photons in response to electrical input Solar Cells and Photodetectors that emit electricity in response to photonic input

3 Light Emitting Diodes Essentially a diode made from direct band-gap or modified indirect band-gap material. Injection electroluminescence Carriers are injected Electron and hole recombination results in luminescence Photon energy is that of the band-gap of the material. A forward biased diode injects many electrons into the p-side and holes into the n-side where than can recombine and emit a photon if made from the proper material.

4 LEDs Electron diffusion lengths are typically longer Than that of holes, so the majority of the photon emission will be on the p-side Favorite structure is n ++ -p Band-gap determines photon energy and therefore also determines photon frequency. The E-M spectrum covers about ev Gamma rays at ~10 9 ev The weakest photons at about ev Our eyes detect between 2 and 3 ev! Specific Colors require specific semiconductor materials.

5 LED Materials We want devices to emit from infrared to ultraviolet. GaN 2 ev InGaN 3.3 ev The InGaN system (InN, GaN, AlN, InGaN, AlGaN and InAlGaN) can cover the range, but comes in n-type and does not like to be doped to p-type In the real world, no single alloyed system can span the visible light range. Gallium Arsenide System for Reds GaAs 1.43 ev infrared Add Al to increase the band-gap Ga.56 Al.44 As 1.96 ev Red Heterojunctions for Brightness

6 Other LED Materials Fiber Optics have absorption minima at 1.3 and 1.55μm wavelengths. Alloys of GaInAsP (Gallium Indium Arsenide Phosphide) Grown on the InP (Indium Phosphide) crystal Longer Wavelengths (5 μm to 50 μm) PbSnTe (Lead Tin Telluride) PbSnSe (Lead Tin Selenide) Visible Light Reds Gallium Arsenide Phosphide The GaAs 1-x P x is an indirect semiconductor at x > 0.45 Employs Isoelectronic Centers to produce red, yellow, amber and green LEDs

7 Isoelectronic Centers An isoelectronic center is a crystal lattice site at which the atom has been replaced by another of the same valence electron structure. Consider Nitrogen replacing Phosphorous in GaP The center is not a dopant atom It has the same electron structure. Heisenberg The center s location is known It is at a lattice site It must have a wide momentum band The center is a stepping stone to efficient recombination with no momentum change

8 An LED for All Seasons Red Gallium Arsenide Phosphide (direct semiconduction) Orange, Yellow and Green Blue Gallium Arsenide Phosphide Nitride (isoelectronic centers) Silicon Carbide with N and Al doping (indirect, impurity centers) Gallium Nitride with Zn doping (indirect, impurity centers) (Best) Direct Band-to-Band Recombination Recombination via isoelectronic centers Recombination via impurity centers (Worst) Indirect Band-to-Band recombination

Example For the LEDs above a) What is the band-gap for each? b) At 30% radiative recombination efficiency, what optical power is emitted by each? c) What is the input power to each?

9 Example For the LEDs above a) What is the band-gap for each? b) At 30% radiative recombination efficiency, what optical power is emitted by each? c) What is the input power to each? d) What is the electrical to optical efficiency of each? E ev = 1.24 wavelength (μm) and the optical power will be the radiative efficiency time the diode current time the band-gap (photon) energy.

Junction Photodetectors (Photodiodes) LEDs source light and Photodetectors sense light. Life is great. Photodetectors need not be direct-gap semiconductors.

10 Junction Photodetectors (Photodiodes) LEDs source light and Photodetectors sense light. Life is great. Photodetectors need not be direct-gap semiconductors. They must have the right band-gap energy. Photons create hole-electron pairs if their energy is greater than the band-gap. E ev = 1.24 wavelength (μm) A PN junction and a little anti reflection Coating and you have a photodetector.

11 Cut-off Wavelength Photon energy must be greater than the material bandgap. E ev 1.24 = wavelength (μm) So for Silicon, the cut-off wavelength must be infrared or slightly shorter (red). But not too short! The photon intensity at distance x into the silicon is an exponential function, I = I 0 e ax where I 0 = incident photon intensity in watts per unit area. Short wavelength photons have large absorption coefficients. Electron-hole pairs generated within or at the edge of the depletion region will be separated by the electric field at nearly the saturation drift velocity 10 7 cm/s for silicon. Once separated they enter into the n- or p- regions as majority carriers and become current in the external circuit.

12 What Makes a Good Photodetector? A wide depletion region Reverse bias Low absorption Highly doped thin top junction Lowly doped bottom junction Low junction capacitance See both above The only trade off is the increased transit time across the wide depletion region.

13 Photodiode Example A HeNe laser puts out nm light, modulated at 6 GHz. What will the silicon photodiode depletion width be that provides a transit time of onehalf the modulation frequency period? 1/(3x10 9 Hz) = 3.33x10 10 s Remembering that the saturation drift velocity in silicon is about 10 7 cm/s the depletion region width will be 3.33x10 10 x 10 7 = 33.3μm

14 Photodiode Final Thoughts Tailoring the depletion region width is critical because we are often stuck with the incoming photon wavelengths and the photodiode material. Reverse bias and doping are one way, P-I-N (P-Intrinsic-N) structures are another. A lowly doped intrinsic region separate thin p + and n + regions. Virtually all the reverse bias drops across the I region, ensuring rapid carrier sweep-out. Quantum efficiency and frequency response are optimized in this case, but the width of the I region.

15 Photoconductor Not a junction diode. Just a slab of semiconductor material with metal terminals. Can be intrinsic or extrinsic. Chapter 4 doping/resistivity analysis applies. Can yield more electron/hole current carriers than incoming photons (positive gain)!

16 Photoconductor Cell thickness, D, must be a little greater than 1/α. Intrinsic, N-type and P-type charge carrier generation. Intrinsic requires incoming photon with energy greater than the bandgap. N- and P- type photon energies are clearly less than half the band-gap.

17 Photodetector Gain Gedanken Consider a small extrinsic photodetector with only one donor atom sitting unionized at a single donor site. The small photodetector is at very low temperature and has no applied electric field. No current flows until a single photon arrives and ionizes the one donor atom. The fact that the donor electron leaves the device and is replaced by a second electron as shown in case (b) on the prior slide, represents current gain but only for a time until the donor center recaptures the conduction band electron. This gain can be quantized as the carrier lifetime over the time taken by the electron to transit the detector (carrier transit time). Gain = τ t tr Unfortunately if you want a fast device, you employ lifetime killers and therefore limit gain. Fast response means low sensitivity. Everything is a trade-off.

18 Solar Cell A photodetector provided an electrical current when reverse biased but we really don t need any bias. Illuminating a silicon PN junction with full spectrum visible light instigates charge separation in and close to the depletion region. Assumes light energy larger than the band-gap There will be a terminal voltage at zero current, V oc. This light-generated V oc is called the photovoltaic effect.

19 Open Circuit Voltage To find the magnitude of V oc, we solve the ideal diode equation, I = i 0 e qv kt 1 for V. V = kt q ln 1 + I I 0 In terms of photocurrent; V oc = kt q ln 1 + I p I 0 V oc < V bi < E g q V oc is typically less than one volt, and I sc is about 20 ma/cm 2 for silicon. Many cells in parallel provides V and large overall area to provides I.

20 Fill Factor Fill Factor = I mv m I sc V oc

21 The Bottom Line on Solar Cells Useful solar panels must have large surface area and include a number of cells in parallel in order to produce useful electrical output. This makes silicon a favored material. The Good: Sunlight has reasonable power content from 0.3 to 1.5 μm wavelength. The Bad: Silicon is transparent beyond 1 μm and not that good on the short wavelength end. This means we must work hard to absorb all incoming light. Surface area enhanced by pyramidal etching. Reflection damped by coatings. The Ugly: A single-value energy-gap responds only to a portion of the light spectrum. Multijunction structures have limited application and are typically compound semiconductor material. Impressive R & D has yielded impressive efficiencies.

Lecture 18: Photodetectors

Lecture 18: Photodetectors Contents 1 Introduction 1 2 Photodetector principle 2 3 Photoconductor 4 4 Photodiodes 6 4.1 Heterojunction photodiode.................... 8 4.2 Metal-semiconductor photodiode................