Advanced Analytical Chemistry Lecture 7 Chem 4631 pn junctions Formed by contact between a p-type and n-type semiconductor. The junction formed has rectifying properties – current can flow in one direction easily but limited in the other direction. Chem 5570 pn junctions When the pn junction forms some of free electrons at the interface diffuse and combine with the holes creating a depletion layer. Chem 5570 pn junctions Coulomb force from ions prevents further migration across the p-n junction. The electrons which had migrated across from the n to the p region in the forming of the depletion layer reach equilibrium. Other electrons from the n region cannot migrate because they are repelled by the negative ions in the p region and attracted by the positive ions in the n region. Chem 5570 pn junctions – Reverse Bias An applied voltage in the reverse direction further impedes the flow of electrons across the junction. A reverse voltage drives the electrons away from the junction, preventing conduction. Chem 5570 pn junctions – Forward Bias An applied voltage in the forward direction assists electrons in overcoming the coulomb barrier of the space charge in depletion region. Electrons will flow with very small resistance in the forward direction. Chem 5570 pn junctions – Forward Bias Chem 5570 pn junctions – Applications Diode applications: – Rectifiers – Switching diodes – Zener diodes – Varactor diodes (Control Capacitance) Photodiodes – pn junction photodiodes – p-i-n and avalanche photodiodes Solar Cells Light Emitting Diodes Lasers Chem 5570 pn junctions – Applications Rectifiers and Switching diodes As diodes in circuits - to allow an electric current to pass in one direction (the diode's forward direction) while blocking current in the opposite direction (the reverse direction). This unidirectional behavior is called rectification, and is used to convert alternating current to direct current, and to extract modulation from radio signals in radio receivers. Chem 5570 pn junctions – Applications Zener diodes A type of diode that permits current not only in the forward direction, but also in the reverse direction if the voltage is larger than the breakdown voltage known as the "Zener voltage". The breakdown characteristics of diodes can be tailored by controlling the doping concentration. I Heavily doped p+ and n+ regions result in low breakdown voltage (Zener effect). V Used as reference voltage Region of in voltage regulators . operation Chem 5570 pn junctions – Applications Devices to convert optical energy to electrical energy – photodetectors: generate electrical signal – Solar cells: generate electrical power Devices to convert electrical energy to optical energy – light emitting diodes (LEDs) – laser diodes Chem 5570 pn junctions – Applications Photodiodes – pn junction photodiodes – p-i-n and avalanche photodiodes Chem 5570 Photodiodes Detectors (pn junction photodiodes) A reverse-biased pn junction (pn diode) on a silicon chip can be fabricated to act as a detector. When a reverse-bias is applied, a depletion layer forms. UV and vis photons have enough energy to create holes or electrons when striking the depletion layer of a pn junction. The holes and electrons formed in the depletion layer migrate to the connecting leads and produce a current. Chem 5570 Chem 5570 Photodiodes Detectors (pn junction photodiodes) The spectral range: 190 to 1100 nm Spectral response - an important characteristic of any photo-detector. Measures how the photocurrent, IL varies with the wavelength of incident light. Frequency response - measures how rapidly the detector can respond to a time varying optical signal. The generated minority carriers have to diffuse to the depletion region before an electrical current can be observed externally. Since diffusion is a slow process, the maximum frequency response is a few tens of MHz for pn junctions. Higher frequency response (a few GHz) can be achieved using p-i-n diodes. Chem 5570 Photodiodes Detectors (p-i-n and avalanche photodiodes) The i-region is very lightly doped (it is effectively intrinsic). The diode is designed such that most of the light is absorbed in the i- region. Under small reverse bias, the i-region is depleted, and the carriers generated in the i-region are collected rapidly due to the strong electric field. p-i-n diodes operating at 1.3 m and 1.55 m are used extensively in optical fiber communications. Chem 5570 Photodiodes Detectors (p-i-n and avalanche photodiodes) p-i-n photodiode has a thicker depletion region to allow for more efficient collection of carriers and higher quantum efficiency. Example: p-i-n photodiodes operating at 1.55 um made of In0.53Ga0.47As deposited on InP substrate. Noise in a p-i-n photodiode (or p-n) is primarily due to shot noise; the random nature of the generation of carriers in the photodiode yields also a random current fluctuation. Chem 5570 pn junctions – Applications Solar Cells Converts the energy of sunlight directly into electricity by the photovoltaic effect. Solar cells are large area pn-junction diodes designed specifically to avoid energy losses. Typically have a narrow heavily doped n-region to allow illumination through the n-side. The depletion region or SCL extends into the p-side. Chem 5570 pn junctions – Applications Neutral Neutral n-region E Solar Cells o p-region Diffusion Drift Long Medium L e Back electrode Short Finger electrode Lh Depletion region n W p Voc Fig. 6.49: The principle of operation of the solar cell (exaggerated features to highlight principles) From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002) http://Materials.Usask.Ca Chem 5570 pn junctions – Applications Solar Cells There are electrodes attached to the n-side which allow illumination and form an array of electrodes. Finger electrodes Bus electrode for current collection n p Fig. 6.50: Finger electrodes on the surface of a solar cell reduce the series resistance From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002) http://Materials.Usask.Ca Chem 5570 pn junctions – Applications Solar Cells The photocurrent produces a voltage drop across the resistive load, which forward biases the pn junction. Ideally, each incident photon with Ehν > Eg will create one electron flowing in the external device. Ehν < Eg : the device is transparent to the incident light. Ehν ≥ Eg : photons are absorbed and EHP are photogenerated in the device. Ehν > Eg : energy generated is lost as heat to the device. Chem 5570 pn junctions – Applications Solar Cells Chem 5570 pn junctions – Applications Solar Cells EHPs exp(−x) x Lh W Le Iph Fig. 6.51: Photogenerated carriers within the volume Lh + W + Le give rise to a photocurrent Iph. The variation in the photegenerated EHP concentration with distance is also shown where is the absorption coefficient at the wavelength of interest. From Principles of Electronic Materials and Devices, Second Edition, S.O. Kasap (© McGraw-Hill, 2002) http://Materials.Usask.Ca Chem 5570 pn junctions – Applications Solar Cells Si with a bandgap of 1.1 eV corresponds to a threshold wavelength of 1.1 um. Incident energy greater than 1.1 um is wasted (~25%). High-energy photons absorbed near the surface are lost by recombination. These loses can be close to 40%. Chem 5570 Chem 5570 pn junctions – Applications Solar Cells A High Efficiency Device GaInP/GaInAs/Ge by Spectrolab (A Boeing Company) achieved 40.7% efficiency in 2007. Top cell – 1.8 eV = 689 nm Middle cell – 1.4 eV = 866 nm Bottom cell – 0.67 eV = 1850 nm Chem 5570 pn junctions – Applications Solar Cells A concentrator triple-junction compound device (InGaP top, GaAs middle, and InGaAs bottom) by Sharp Corporation achieved 44.4% efficiency in 2013. Chem 5570 pn junctions – Applications Solar Cells Team from the National Renewable Energy Laboratory (NREL) achieved 47.1% in 2020. The new solar conversion efficiency record was achieved with a six-junction solar cell and was measured under concentrated illumination. Chem 5570 pn junctions – Applications Solar Cells Organic solar cells break new efficiency record - 2018 Test cells made in the laboratory reach power conversion efficiencies (PCEs) of 17.3%, a value that is significantly higher than the current 14 to 15%. Chem 5570 pn junctions – Applications Solar Cells Here are the top five best solar panel manufacturers in 2019 ranked based on the highest efficiency solar panel they have to offer: SunPower (22.2%) LG (21.1%) Solartech Universal (20.2%) Silfab (20.0%) Solaria (19.4%) Chem 5570 pn junctions – Applications Solar Cells Chem 5570 pn junctions – Applications Solar Cells Efficiency is limited due to several factors. The energy of photons decreases at higher wavelengths. Radiation with higher wavelength causes only heating up of solar cell and does not produce any electrical current. Each photon can cause only production of one electron-hole pair. The highest efficiency of silicon solar cell is around 23 %, some other semi-conductor materials up to 30 %, which is dependent on wavelength and semiconductor material. Self loses are caused by metal contacts on the upper side of a solar cell, solar cell resistance and due to solar radiation reflectance on the upper side (glass) of a solar cell. Lab solar cells have the efficiency of up to 20 %, and classically produced solar cells up to 15 %. Chem 5570 pn junctions – Applications Light Emitting Diodes Made from a direct bandgap semiconductor, i.e. GaAs, where the electron-hole pair (EHP) recombination results in the emission of a photon. The emitted photon energy hv is ~equal to the bandgap energy, Eg. The emitted photons must escape without being reabsorbed, so the p-side has to be narrow. Chem 5570 pn junctions – Applications Light Emitting Diodes A simple LED is a pn junction on a suitable substrate (GaAs or GaP). Light output p n+ Epitaxial layers n+ Substrate Fig.