1. Theory
- Definition: Generation-recombination models supply the term of the continuity equations: Shockley-Read-Hall recombination through traps, Auger recombination at high carrier density, impact ionization at high field and band-to-band tunnelling at very high field (Ayalew; Allpix; Ansys).
- SRH: The rate depends on the trap level and on electron and hole lifetimes, which fall with doping and are fitted by Fossum-Lee or Scharfetter forms (Ayalew; Allpix).
- Auger: Three-carrier collisions dominate the lifetime in heavily doped emitters and contacts (Allpix).
- Impact ionization: Hot carriers create electron-hole pairs at a rate per unit length that rises steeply with field, setting avalanche breakdown (Allpix; Ansys).
- Tunnelling: At fields near , electrons tunnel directly from valence to conduction band, causing GIDL and Zener breakdown (Wong et al.; Ansys).
2. Working Principle
- Trap-mediated capture: Traps near midgap capture electrons and holes alternately, so SRH is fastest for midgap levels and limited by the slower capture (Ayalew).
- Lifetime combination: Recombination rates add, so inverse lifetimes add, and the shortest mechanism dominates (Allpix).
- Field heating: Carriers gaining more than the ionization energy between optical-phonon collisions create new pairs; higher temperature means more phonon collisions and less ionization (Allpix; background knowledge for the mechanism).
- Local versus non-local: Local models evaluate rates from the field at each point, while non-local models integrate along tunnelling or acceleration paths, which is more accurate but slower (Wong et al.).
3. Applications
- Bipolar and solar devices: SRH and Auger lifetimes for base transport, emitter saturation current and cell efficiency (Allpix; Ayalew).
- Breakdown design: Avalanche voltage of power devices, ESD structures and avalanche photodiodes (Allpix; Ansys).
- Leakage: GIDL and junction tunnelling leakage in scaled MOSFETs and FinFETs (Wong et al.).
- Detector simulation: Charge trapping and gain in silicon particle detectors (Allpix).
Recombination
| Name | Formula | Related Formulae | Variables/Constants |
|---|---|---|---|
| Shockley-Read-Hall recombination | |||
| Doping-dependent SRH lifetime | |||
| Auger recombination |
Impact Ionization and Tunnelling
| Name | Formula | Related Formulae | Variables/Constants |
|---|---|---|---|
| Impact ionization: Chynoweth-type laws | |||
| Impact ionization: Okuto-Crowell and Bologna | |||
| Band-to-band tunnelling (Hurkx form) |
Important Questions
Ques: How fast does SRH lifetime fall with doping?
Roughly inversely with doping above the reference concentration. Allpix uses the Fossum-Lee form with electron values of s and and hole values of s and , giving electron lifetimes of 9.1 at , 0.91 at , 99 at and 10 at , and hole lifetimes of 351, 26.5, 2.8 and 0.28 (computed). Ayalew describes the generalized Scharfetter form with a minimum lifetime and an exponent, fitted to the same silicon data of Fossum. The fall comes from more defects and dopant-related traps in heavily doped material (background knowledge). Allpix adds a low-temperature factor after Schenk. Reference: Allpix Squared collaboration, ‘Charge Carrier Lifetime & Recombination’, Allpix Squared documentation, CERN (after Fossum and Lee 1982 and 1983, Schenk 1992, Kerr 2002 and Dziewior and Schmid 1977) (fetch-tool summary); T. Ayalew, ‘SiC Semiconductor Devices Technology, Modeling, and Simulation’, doctoral dissertation, Institute for Microelectronics, TU Wien, Section 3.5.1 (year not stated; Eqs. 3.107 and 3.108 truncated in the extract) (fetch-tool summary).
Ques: Is the hole SRH lifetime s or s?
Both appear, for different purposes. Allpix gives a hole of s with , the Fossum-Lee fit to measured lifetimes in high-quality silicon, while Bortolossi’s TCAD test case uses constant lifetimes of s for electrons and s for holes, which are typical simulator defaults (background knowledge). The Fossum-Lee value describes the intrinsic material limit, about 130 times the default (computed); the default represents processed device silicon, where contamination and process damage shorten lifetimes. Allpix’s electron value, s, matches the default, so only the hole value differs. For a real process, lifetimes should be calibrated from measured diode leakage or minority-carrier lifetime, since they vary by orders of magnitude between wafers and processes (background knowledge). Reference: Allpix Squared collaboration, ‘Charge Carrier Lifetime & Recombination’, Allpix Squared documentation, CERN (after Fossum and Lee 1982 and 1983, Schenk 1992, Kerr 2002 and Dziewior and Schmid 1977) (fetch-tool summary); A. Bortolossi, ‘3D Finite Element Drift-Diffusion Simulation of Semiconductor Devices’, Politecnico di Milano thesis (2014) (fetch-tool summary).
Ques: Are the Auger coefficients or cm^6/s in total?
They are close, from different fits. Allpix uses after Dziewior and Schmid, whose individual values are and , summing to (background knowledge for the split). Bortolossi uses and , summing to (computed), 3.6 per cent higher. Since the Auger lifetime is 26 at with the Allpix value (computed), the difference shifts it by about 1 , negligible against other uncertainties. Dziewior and Schmid’s values are the more commonly cited set (background knowledge), so is the safer default; more recent work by Kerr, which Allpix cites for the rate form, includes Coulomb-enhanced Auger at lower doping (background knowledge). Reference: Allpix Squared collaboration, ‘Charge Carrier Lifetime & Recombination’, Allpix Squared documentation, CERN (after Fossum and Lee 1982 and 1983, Schenk 1992, Kerr 2002 and Dziewior and Schmid 1977) (fetch-tool summary); A. Bortolossi, ‘3D Finite Element Drift-Diffusion Simulation of Semiconductor Devices’, Politecnico di Milano thesis (2014) (fetch-tool summary).
Ques: When does Auger recombination dominate over SRH?
Only in very heavily doped regions with good material. Allpix states that Auger matters above about , citing Fossum and Lee. With , the Auger lifetime is 2.6 at , 105 at and 26 at (computed). With the electron SRH lifetime of the Fossum-Lee form, the combined lifetimes are 95, 17 and 7.2 (computed): SRH still dominates for electrons at , while for holes, whose SRH lifetime is 0.57 at that doping (computed), Auger already controls the lifetime. Auger therefore governs emitter recombination in solar cells and bipolar transistors and the lifetime under high injection, where the excess carrier density, not the doping, enters (background knowledge). Reference: Allpix Squared collaboration, ‘Charge Carrier Lifetime & Recombination’, Allpix Squared documentation, CERN (after Fossum and Lee 1982 and 1983, Schenk 1992, Kerr 2002 and Dziewior and Schmid 1977) (fetch-tool summary).
Ques: Do the impact-ionization models agree?
Near they agree, but at lower fields they differ several-fold. With Allpix’s silicon parameters at 300 , the electron coefficient at is from Van Overstraeten-De Man, from Massey and from Okuto-Crowell; at they are , and only 262, and at , and (computed). The Okuto-Crowell form, , falls faster at low field than the Chynoweth-type . Since breakdown depends on the integral of through the junction, models fitted to different field ranges give different breakdown voltages; the RD50-optimized variants in Allpix were recalibrated for detectors. Ansys’s Selberherr defaults equal the low-field Van Overstraeten values. Reference: Allpix Squared collaboration, ‘Impact Ionization’, Allpix Squared documentation, CERN (after Massey et al. 2006, Van Overstraeten and De Man 1970, Okuto and Crowell 1975, the Bologna model 1999 and the RD50 optimization, arXiv:2211.16543) (fetch-tool summary); Ansys Optics, ‘Silicon diode’ application example, Lumerical CHARGE knowledge base (fetch-tool summary).
Ques: Why does avalanche breakdown voltage rise with temperature?
Because phonon scattering reduces impact ionization. In the Van Overstraeten-De Man model, with multiplies both the prefactor and the exponent, and the Massey model raises B with temperature by 499 V/cm per kelvin for electrons (Allpix). At 400 , , which lowers the electron coefficient at from to , 40 per cent (computed). A higher field, and hence a higher voltage, is needed for the same multiplication integral, so the breakdown voltage has a positive temperature coefficient (background knowledge for the circuit consequence). This distinguishes avalanche from Zener tunnelling breakdown, whose voltage falls with temperature as the band gap narrows. Reference: Allpix Squared collaboration, ‘Impact Ionization’, Allpix Squared documentation, CERN (after Massey et al. 2006, Van Overstraeten and De Man 1970, Okuto and Crowell 1975, the Bologna model 1999 and the RD50 optimization, arXiv:2211.16543) (fetch-tool summary).
Ques: How steep is band-to-band tunnelling, and why are local models inaccurate?
Extremely steep. With the Hurkx parameters in the Ansys silicon-diode example, A = , B = and exponent 2.5, and an assumed prefactor D of 1, the generation rate is at , at and at (computed), nine orders of magnitude for a doubling of field. Tunnelling is non-local: an electron leaves the valence band at one point and arrives in the conduction band some distance away, so evaluating the rate from the local field misrepresents the path. Wong et al. note that the local Hurkx model is cheap but underestimates leakage at large bias, and their modified local model matches non-local results for diode leakage and GIDL while running six times faster in a 3-D FinFET. Meshes of 4 or less are needed at such junctions (Ansys). Reference: Ansys Optics, ‘Silicon diode’ application example, Lumerical CHARGE knowledge base (fetch-tool summary); H. Y. Wong, D. Dolgos, L. Smith and R. V. Mickevicius, ‘Modified Hurkx band-to-band-tunneling model for accurate and robust TCAD simulations’, Microelectronics Reliability 104, 113552 (2020), doi:10.1016/j.microrel.2019.113552 (abstract only) (fetch-tool summary).
References
- T. Ayalew, “SiC Semiconductor Devices Technology, Modeling, and Simulation,” doctoral dissertation, Institute for Microelectronics, TU Wien, Section 3.5.1 “Shockley-Read-Hall Recombination.” Available: https://www.iue.tuwien.ac.at/phd/ayalew/node72.html
- Allpix Squared collaboration, “Charge Carrier Lifetime & Recombination,” Allpix Squared documentation, CERN. Available: https://allpix-squared.docs.cern.ch/docs/06_models/03_lifetime_recombination/
- A. Bortolossi, “3D Finite Element Drift-Diffusion Simulation of Semiconductor Devices,” Laurea Magistrale thesis, Politecnico di Milano, 2014. Available: https://www.politesi.polimi.it/retrieve/a81cb05a-eff6-616b-e053-1605fe0a889a/2014_7_Bortolossi.pdf
- Allpix Squared collaboration, “Impact Ionization,” Allpix Squared documentation, CERN. Available: https://allpix-squared.docs.cern.ch/docs/06_models/05_impact_ionization/
- Ansys Optics, “Silicon diode,” Lumerical CHARGE application example. Available: https://optics.ansys.com/hc/en-us/articles/360042272934-Silicon-diode
- H. Y. Wong, D. Dolgos, L. Smith and R. V. Mickevicius, “Modified Hurkx band-to-band-tunneling model for accurate and robust TCAD simulations,” Microelectronics Reliability, vol. 104, 113552, 2020, doi:10.1016/j.microrel.2019.113552. Available: https://scholarworks.sjsu.edu/faculty_rsca/1605
Related pages in this vault: TCAD Mobility Models, Drift-Diffusion Device Equations, Hydrodynamic and Energy-Balance Transport, HgCdTe Photodiode, Band-Gap Narrowing and Quantum Corrections, Thermal and Electro-Thermal Simulation, Lifetime Testers, PN Junction, Deep-Level Transient Spectroscopy (DLTS).