Satadal Dutta

Opto-electronic modeling of light emission from avalanche-mode silicon p+n junctions

This work presents the modeling of light emission from silicon based pþn junctions operating in avalanche breakdown. We revisit the photon emission process under the influence of relatively high electric fields in a reverse biased junction (>105 V/cm). The photon emission rate is described as a function of the electron temperature Te, which is computed from the spatial distribution of the electric field. The light emission spectra lie around the visible spectral range (k 300–850 nm), where the peak wavelength and the optical intensity are both doping level dependent. It is theoretically derived that a specific minimum geometrical width (170 nm) of the active region of avalanche is required, corresponding to a breakdown voltage of 5V, below which the rate of photon emission in the desired spectrum drops. The derived model is validated using experimental data obtained from ultra-shallow pþn junctions with low absorption through a nm-thin pþ region and surface coverage of solely 3 nm of pure boron. We observe a peak in the emission spectra near 580 nm and 650 nm for diodes with breakdown voltages 7V and 14 V, respectively, consistent with our model.

Monolithic optical link in silicon-on-insulator CMOS technology

This work presents a monolithic laterally-coupled wide-spectrum (350 nm < λ < 1270 nm) optical link in a silicon-on-insulator CMOS technology. The link consists of a silicon (Si) light-emitting diode (LED) as the optical source and a Si photodiode (PD) as the detector; both realized by vertical abrupt n+p junctions, separated by a shallow trench isolation composed of silicon dioxide. Medium trench isolation around the devices along with the buried oxide layer provides galvanic isolation. Optical coupling in both avalanche-mode and forward-mode operation of the LED are analyzed for various designs and bias conditions. From both DC and pulsed transient measurements, it is further shown that heating in the avalanche-mode LED leads to a slow thermal coupling to the PD with time constants in the ms range. An integrated heat sink in the same technology leads to a ∼ 6 times reduction in the change in PD junction temperature per unit electrical power dissipated in the avalanche-mode LED. The analysis paves way for wide-spectrum optical links integrated in smart power technologies.

An integrated optical link in 140 nm SOI technology

A silicon-on-insulator based optical link is introduced. Higher opto-coupling efficiency and temperature-resilience are obtained via avalanche-mode light-emitting diode operation against forward-mode operation. Self-heating induced thermo-coupling in steady-state is de-embedded by calibrating the photo-detectors photovoltaic characteristics.

An Improved Analytical Model for Carrier Multiplication Near Breakdown in Diodes

The charge carrier contributions to impact ionization and avalanche multiplication are analyzed in detail. A closed-form analytical model is derived for the ionization current before the onset of breakdown induced by both injection current components. This model shows that the ratio of both injection current components affects the multiplication factor at relatively low fields before breakdown, but does not affect the reverse breakdown voltage. Furthermore, the model indicates that in case the ionization coefficients of electrons and holes are quite different in value, which depends upon the semiconductor material, the ionization coefficient of the charge carrier with the highest value can be extracted at those low fields. The one with the lowest value can be obtained by fitting the current close to breakdown. The model is compared and verified with TCAD simulations, and to some extent with experimental data, for silicon p-i-n diodes.

Charge carrier transport and electroluminescence in atomic layer deposited poly-GaN/c-Si heterojunction diodes

In this work, we study the charge carrier transport and electroluminescence (EL) in thin-film polycrystalline (poly-) GaN/c-Si heterojunction diodes realized using a plasma enhanced atomic layer deposition process. The fabricated poly-GaN/p-Si diode with a native oxide at the interface showed a rectifying behavior (Ion/Ioff ratio ∼ 103 at ±3 V) with current-voltage characteristics reaching an ideality factor n of ∼5.17. The areal (Ja) and peripheral (Jp) components of the current density were extracted, and their temperature dependencies were studied. The space charge limited current (SCLC) in the presence of traps is identified as the dominant carrier transport mechanism for Ja in forward bias. An effective trap density of 4.6 × 1017/cm3 at a trap energy level of 0.13 eV below the GaN conduction band minimum was estimated by analyzing Ja. Other basic electrical properties of the material such as the free carrier concentration, effective density of states in the conduction band, electron mobility, and dielectric relaxation time were also determined from the current-voltage analysis in the SCLC regime. Further, infrared EL corresponding to the Si bandgap was observed from the fabricated diodes. The observed EL intensity from the GaN/p-Si heterojunction diode is ∼3 orders of magnitude higher as compared to the conventional Si only counterpart. The enhanced infrared light emission is attributed to the improved injector efficiency of the GaN/Si diode because of the wide bandgap of the poly-GaN layer and the resulting band discontinuity at the GaN/Si interface.In this work, we study the charge carrier transport and electroluminescence (EL) in thin-film polycrystalline (poly-) GaN/c-Si heterojunction diodes realized using a plasma enhanced atomic layer deposition process. The fabricated poly-GaN/p-Si diode with a native oxide at the interface showed a rectifying behavior (Ion/Ioff ratio ∼ 103 at ±3 V) with current-voltage characteristics reaching an ideality factor n of ∼5.17. The areal (Ja) and peripheral (Jp) components of the current density were extracted, and their temperature dependencies were studied. The space charge limited current (SCLC) in the presence of traps is identified as the dominant carrier transport mechanism for Ja in forward bias. An effective trap density of 4.6 × 1017/cm3 at a trap energy level of 0.13 eV below the GaN conduction band minimum was estimated by analyzing Ja. Other basic electrical properties of the material such as the free carrier concentration, effective density of states in the conduction band, electron mobility, and die...

TCAD analysis of wide-spectrum waveguides in high-voltage SOI-CMOS

A TCAD based analysis is presented on the transmission efficiency η of silicon-on-insulator (SOI) and silicon nitride slab waveguides in a high-voltage standard SOI-CMOS technology, for the spectral range of 480 nm–1300 nm, and isotropic optical excitation via monolithic Si-based LEDs. The effects of geometry, wavelength and galvanic isolation on η are reported.

Low power wide spectrum optical transmitter using avalanche mode LEDs in SOI CMOS technology

This paper presents a low power monolithically integrated optical transmitter with avalanche mode light emitting diodes in a 140 nm silicon-on-insulator CMOS technology. Avalanche mode LEDs in silicon exhibit wide-spectrum electroluminescence (400 nm < λ < 850 nm), which has a significant overlap with the responsivity of silicon photodiodes. This enables monolithic CMOS integration of optocouplers, for e.g. smart power applications requiring high data rate communication with a large galvanic isolation. To ensure a certain minimum number of photons per data pulse (or per bit), light emitting diode drivers must be robust against process, operating conditions and temperature variations of the light emitting diode. Combined with the avalanche mode light emitting diodes steep current-voltage curve at relatively high breakdown voltages, this conventionally results in high power consumption and significant heating. The presented transmitter circuit is intrinsically robust against these issues, thereby enabling low power operation.

Random Telegraph Signal phenomena in avalanche mode diodes: Application to SPADs

The current-voltage (IV ) dependency of diodes close to the breakdown voltage is shown to be governed by Random Telegraph Signal (RTS) phenomena. We present a technology independent approach to accurately characterize the bias dependent statistical RTS properties and show that these can fully describe the steep IV -dependency in avalanche. The statistical properties also allow to more accurately describe e.g. the value of the self sustaining avalanche current that is crucial in designing optical detection systems using avalanche photo diodes or single photon avalanche diodes (SPADs). More accurate modelling is shown to allow improving on e.g. count rates, dead time and afterpulsing in quenching and recharge circuits for SPADs. Measurements are performed on diodes in a 140 nm SOI CMOS technology.