Thermionic emission

About: Thermionic emission is a research topic. Over the lifetime, 6099 publications have been published within this topic receiving 97892 citations. The topic is also known as: Edison effect.

The dc voltage dependence of semiconductor grain‐boundary resistance

Abstract: A model is developed to describe the potential barriers which often occur at grain boundaries in polycrystalline semiconductors. The resistance of such materials is determined by thermionic emission over these barriers. The dc grain‐boundary current density as a function of applied voltage is calculated using several forms for the density of defect states within the boundary region. In all cases, the currents are Ohmic at low voltages; they can attain a quasisaturated level at intermediate voltages, and they display a sharp bias dependence at high voltages. The details of the intermediate and high‐voltage characteristics are found to depend strongly on the grain‐doping density and on the density and energy distribution of defect states at the grain boundary. Contrary to previous assertions, we find that the large current‐voltage nonlinearities found in real materials are most likely associated with defect‐state densities that decrease above the zero‐bias Fermi level. The results of the model are compared ...

Solar cell contact resistance—A review

Abstract: An overview of ohmic contacts on solar cells is presented The fundamentals of metal-semiconductor contacts are reviewed, including the Schottky approach, Fermi level pinning by surface states, and the mechanisms of thermionic emission, thermionic/field emission, and tunneling for current transport The concept of contact resistance is developed and contact resistance data for several different contact materials on both silicon and gallium arsenide over a range of doping densities are summarized Finally, the requirements imposed by solar cells on contact resistance are detailed

Photon-enhanced thermionic emission for solar concentrator systems

Abstract: Solar-energy conversion usually takes one of two forms: the ‘quantum’ approach, which uses the large per-photon energy of solar radiation to excite electrons, as in photovoltaic cells, or the ‘thermal’ approach, which uses concentrated sunlight as a thermal-energy source to indirectly produce electricity using a heat engine. Here we present a new concept for solar electricity generation, photon-enhanced thermionic emission, which combines quantum and thermal mechanisms into a single physical process. The device is based on thermionic emission of photoexcited electrons from a semiconductor cathode at high temperature. Temperature-dependent photoemission-yield measurements from GaN show strong evidence for photon-enhanced thermionic emission, and calculated efficiencies for idealized devices can exceed the theoretical limits of single-junction photovoltaic cells. The proposed solar converter would operate at temperatures exceeding 200 °C, enabling its waste heat to be used to power a secondary thermal engine, boosting theoretical combined conversion efficiencies above 50%. The conversion of solar energy into electricity usually occurs either electrically or through thermal conversion. A new mechanism, photon-enhanced thermionic emission, which combines electric as well as thermal conversion mechanisms, is now shown to lead to enhanced conversion efficiencies that potentially could even exceed the theoretical limits of conventional photovoltaic cells.

Normalized thermionic-field (T-F) emission in metal-semiconductor (Schottky) barriers

Abstract: Thermionic field (T-F) emission in uniformly doped metal-semiconductor (Schottky) barriers is analyzed to yield a normalized solution in closed form for the forward and reverse current (I)-voltage (V) relationship. A quasi one-dimensional approach and Maxwell-Boltzmann statistics are used. The formulation is expressed in terms of the ‘flat-band’ current density Im, the band bending Eb in the semiconductor depletion region, the materials constant E0 0 ( ln [ I I m ] = − E b E 0 0 at 0°K in the WKB approximation), and kT. The kinetic energy in units of Eb at which the maximum injection of carriers occurs in the semiconductor is shown to be cosh -2(kT/E0 0). Current flow in the temperature range between pure thermionic emission (kT/E0 0 ⪢ 1) and pure field emission (kT/E0 0 ⪡ 1) is analyzed and criteria for the transition of T-F emission to thermionic and to field emission are given. Computer solutions for the energy distribution of the injected carriers and for the normalized I-V characteristic are presented in graphical form. The results permit a straightforward calculation of the barrier height and the impurity concentration in the semiconductor from measurements of current density and differential resistance at a single applied bias. Application of these results explains a reported discrepancy between barrier heights deduced from photothreshold, C-V and I-V characteristics of WGaAs and AuGaAs Schottky barriers. A relatively constant excess temperature T0 (i.e., ln I ∝ (T + T0) when V ⪢ kT/q) is predicted in the case of large Eb/E0 0 in the higher kT/E0 0 range where thermionic emission is nearly predominant. I ∝ [exp(qV/kT) − 1] is shown to be a general expectation for all Schottky barriers near zero bias when the I-V characteristic is dominated by either thermionic or thermionic-field emission. The assumption of a Gaussian energy distribution of carriers leads to values for the slope of ln I vs. V in reasonable agreement with the results of the computer analysis, but the prediction of the absolute value of the current density deviates rapidly from the computed value when kT/E0 0 departs appreciably from unity. The Gaussian distribution also does not provide the smooth transition from T-F to thermionic emission characteristic of the computer solution.

Current transport in metal-semiconductor-metal (MSM) structures

Abstract: The current-voltage characteristics of a metal-semiconductor-metal structure (essentially two metal-semiconductor contacts connected back to back) have been studied based on the thermionic emission theory. When a uniformly doped semiconductor is thin enough that it can be completely depleted before avalanche breakdown occurs, the structure can exhibit many novel transport behaviors. Two outstanding features of the structure are that (1) a wide range of high-level injection of minority carriers can be achieved by varying the barrier heights of the two contacts and (2) the critical voltage at which the minority carrier injection increases rapidly can be varied by varying the semiconductor doping and thickness. Experimental silicon MSM structures of PtSi-Si-PtSi have been made from n-type silicon with doping of 4×1014 cm−3 and thickness of 12 μm. The critical voltage at room temperature is about 30 V. The current increases over five orders of magnitude with only 10 per cent increase of the voltage. The above results and other measurements over wide temperature range do substantiate the theoretical predictions.