Threshold Energy For Impact Ionization Research Articles

Engineering of impact ionization characteristics in In0.53Ga0.47As/Al0.48In0.52As superlattice avalanche photodiodes on InP substrate

We report on engineering impact ionization characteristics of In0.53Ga0.47As/Al0.48In0.52As superlattice avalanche photodiodes (InGaAs/AlInAs SL APDs) on InP substrate to design and demonstrate an APD with low k-value. We design InGaAs/AlInAs SL APDs with three different SL periods (4 ML, 6 ML, and 8 ML) to achieve the same composition as Al0.4Ga0.07In0.53As quaternary random alloy (RA). The simulated results of an RA and the three SLs predict that the SLs have lower k-values than the RA because the electrons can readily reach their threshold energy for impact ionization while the holes experience the multiple valence minibands scattering. The shorter period of SL shows the lower k-value. To support the theoretical prediction, the designed 6 ML and 8 ML SLs are experimentally demonstrated. The 8 ML SL shows k-value of 0.22, which is lower than the k-value of the RA. The 6 ML SL exhibits even lower k-value than the 8 ML SL, indicating that the shorter period of the SL, the lower k-value as predicted. This work is a theoretical modeling and experimental demonstration of engineering avalanche characteristics in InGaAs/AlInAs SLs and would assist one to design the SLs with improved performance for various SWIR APD application.

Tensile CESL-induced strain dependence on impact ionization efficiency in nMOSFETs

Process-induced strain dependence of impact ionization efficiency (IIE) in nMOSFETs with a tensile contact etch stop layer (CESL) is presented for the first time. From the universal relationship between the IIE and the electric field in the pinch-off region, a difference in the IIE of nMOSFETs between without and with the tensile CESL is found. This result can be mainly attributed to the narrowing effect of the bandgap energy caused by the tensile CESL-induced strain into the channel, i.e., the reduced threshold energy for impact ionization. In addition, the wafer-bending experiments can further provide strong evidence for the bandgap energy narrowing. It means that the IIE measurement could serve as a reliable monitor of the process-induced strain into the channel.

Substrate current enhancement in 65 nm metal-oxide-silicon field-effect transistor under external mechanical stress

An approach to get uniaxial tensile stress from the channel by using strained silicon to enhance drain current and mobility was developed. A 65 nm metal-oxide-semiconductor field-effect transistor (MOSFET) was bent by applying external mechanical stress. The drain current and transconductance of the transistor were found to increase 15% and 20%, respectively. In this paper, the behaviors of the substrate current and the impact ionization rate are also investigated. It was found that the substrate current and gate voltage corresponding to the maximum impact ionization current have significantly increased by increasing external mechanical stress. According to the relationship to the strain-induced mobility enhancement, the increase in impact ionization efficiency resulted from the decrease in threshold energy for impact ionization which was due to the narrowing of the bandgap.

Cointegration of In Situ Doped Silicon–Carbon Source and Silicon–Carbon I-Region in P-Channel Silicon Nanowire Impact-Ionization Transistor

The p-channel impact-ionization nanowire multiple- gate field-effect transistors (I-MuGFETs or I-FinFETs), which have a multiple-gate/nanowire-channel architecture, were demonstrated. The superior gate-to-channel coupling reduces the breakdown voltage VBD for enhanced device performance. For the first time, an in situ doped source was incorporated with the impact-ionization MOS transistor. The in situ phosphorus-doped Si source with improved dopant activation and very abrupt junction profile reduces VBD and enhances the on-state current Ion. An additional improvement was also achieved by incorporating a strained Si1-yCy impact-ionization region (I-region) and an in situ doped Si1-yCy source, leading to reduction in Vbd and enhancement in Ion. This is due to strain-induced reduction of the impact-ionization threshold energy Eth. Furthermore, an excellent subthreshold swing of below 3 mV/decade at room temperature was achieved for all devices.

A Theoretical Model for the HgCdTe Electron Avalanche Photodiode

A ballistic model is presented for electron avalanche multiplication in the conduction band of HgCdTe, based upon the concept of an optical phonon limited mean free path for the electron, λ e. The model predicts avalanche gain as a function of applied bias voltage V, and a threshold voltage for impact ionization V th. Impact ionization probabilities are calculated analytically using a simplified band structure model for HgCdTe and used to estimate values for the threshold energy for impact ionization. A simple ballistic model is developed to correlate the relationship between electron energy and applied bias voltage, based upon the relevant electron scattering mechanisms in HgCdTe. A comparison with published gain–voltage data suggests that the process is limited by optical phonon scattering, and the relationship between electron energy and applied bias voltage, for a uniform electric field F = V/W, across a diode depletion width W, is given by E = α(E)V, where α(E) = [λ e(E)/W]. For high electron energies λ e(E) is independent of E and α(E) depends only on the dielectric parameters of the material. Using this simple model it is easy to predict electron avalanche gain versus voltage for any parametric combination of diode geometry, bandgap, and operating temperature.

Simulation and design of a germanium L-shaped impact-ionization MOS transistor

This paper reports a novel L-shaped impact-ionization MOS (LI-MOS) transistor structure that achieves a subthreshold swing of well below 60 mV/decade at room temperature and operates at a low supply voltage. The device features an L-shaped or an elevated impact-ionization region (I-region), which displaces the hot carrier activity away from the gate dielectric region to improve hot carrier reliability and VT stability problems. Germanium, which has a lower bandgap and impact-ionization threshold energy lower than silicon, is chosen as the material of choice for the LI-MOS transistor structure. Device physics and design principles for the LI-MOS transistor are detailed through extensive two-dimensional device simulations. The LI-MOS transistor exhibits excellent scalability, making it suitable for augmenting the performance of standard CMOS transistors in future technology generations.

Reduction of impact-ionization threshold energies for performance enhancement of complementary impact-ionization metal-oxide-semiconductor transistors

We explore the improvement of electrical performance of impact-ionization metal-oxide-semiconductor (I-MOS) transistors by the reduction of impact-ionization threshold energy through incorporation of materials with smaller bandgaps. Silicon-germanium (SiGe) I-MOS transistors were demonstrated. The lower bandgap of SiGe, as compared to Si, contributes to lower electron and hole impact-ionization threshold energies, leading to avalanche breakdown at a much reduced source voltage and enhanced device performance. Both n- and p-channel I-MOS devices were fabricated on Si0.60Ge0.40-on-insulator substrates using a complementary metal-oxide-semiconductor compatible process flow. Excellent subthreshold swings as low as 5mV/decade were achieved for the SiGe I-MOS devices. Reduction in breakdown voltage VBD was as large as 1.3 and 1.6V, respectively, for the n- and p-channel Si0.60Ge0.40 I-MOS devices.

Positive and negative temperature dependences of electron-impact ionization in In0.53Ga0.47As

The electron-impact ionization coefficient (α) in In0.53Ga0.47As increases with temperature at electric fields below 200kV∕cm, in contrast to most crystalline semiconductors. It exhibits conventional negative temperature dependence at higher fields. A four-valley analytical band Monte Carlo model is used to study this anomalous behavior. The simulations show that both alloy scattering and the temperature dependence of the impact ionization threshold energy contribute to the temperature dependence of α. At low fields, we find that most ionization events occur in the first conduction band causing an increase of α with temperature in In0.53Ga0.47As. At high fields, α decreases with temperature again because of the reduced hot-electron population in the second conduction band.

On the Origin of Increase in Substrate Current and Impact Ionization Efficiency in Strained-Si n- and p-MOSFETs

Strain dependence of substrate current and impact ionization efficiency in the strained-Si n- and p-MOSFETs has been systematically investigated with the wide range of strain, by utilizing the universal relationship between the impact ionization efficiency and the electrical field in the pinch-off region. It was found that the impact ionization efficiency increased with increasing strain, leading to the drastic increase of substrate current, and that the strain dependence of impact ionization efficiency was well represented by simply using the strain dependence of the bandgap energy regardless of carrier type. This result strongly suggests that the strain-induced enhancement of impact ionization efficiency is not due to the modification of the carrier transport properties but to the narrowing of the bandgap, and presumably, the resulting decrease in threshold energy for impact ionization. It was also found that the impact ionization efficiency increased with the increase of temperature while its dependence did not exhibit distinct strain dependence. These results are attributable mainly to the temperature dependence of energy distribution function.

Collision Broadening through Sequences of Scattering Events Theory, Consequences and Modeling within Semiclassical Monte Carlo

Recent developments in the understanding of the theory and consequences of collision broadening through sequences of scattering events are reviewed and expanded upon, particularly in regard to semiclassical Monte Carlo simulation. Using basic theory backed up by quantum transport simulations, it is demonstrated that the requirement for energy conservation in the coupled carrier-phonon system prohibits the accumulation of the uncertainty in the carrier energy associated with collision broadening through a sequence of scattering events. This accumulation, however, had been possible with the conventional treatment of collision broadening in semiclassical Monte Carlo. Accordingly, an improved, more physically accurate algorithm for connecting sequences of collision broadened scattering events within semiclassical Monte is described. The potential consequences of collision broadening are then reassessed using both the quantum transport simulations and semiclassical Monte Carlo simulations incorporating the new algorithm, including full-band Monte Carlo simulations for the first time. The broadening assisted runaway of the high-energy tail of the carrier distribution possible with the conventional algorithm is shown to be an artifact of that algorithm, not of the form of the collision broadening used (Lorentzian, etc.) as often assumed. Simulation results obtained with the new algorithm also demonstrate that for important cases, Golden-Rule-based Monte Carlo simulations can remain accurate despite the presence of significant collision broadening. A notable class of exceptions, however, is when collision broadening allows for a threshold or scattering bottle-neck to be overcome, although the ultimate requirement for energy conservation remains critical. Specifically, it is shown that collision broadening lowers but does not eliminate the impact ionization threshold energy, producing simulated impact ionization coefficients for holes in silicon that accurately track experimental results as a function of field. Similarly, the quantum transport simulations illustrate how collision broadening can significantly affect optical absorption, but that simply subjecting low energy carriers to scattering induced collision broadening does not allow them to overcome large potential barriers.

COLLISION BROADENING THROUGH SEQUENCES OF SCATTERING EVENTS: THEORY, CONSEQUENCES AND MODELING WITHIN SEMICLASSICAL MONTE CARLO

Recent developments in the understanding of the theory and consequences of collision broadening through sequences of scattering events are reviewed and expanded upon, particularly in regard to semiclassical Monte Carlo simulation. Using basic theory backed up by quantum transport simulations, it is demonstrated that the requirement for energy conservation in the coupled carrier-phonon system prohibits the accumulation of the uncertainty in the carrier energy associated with collision broadening through a sequence of scattering events. This accumulation, however, had been possible with the conventional treatment of collision broadening in semiclassical Monte Carlo. Accordingly, an improved, more physically accurate algorithm for connecting sequences of collision broadened scattering events within semiclassical Monte is described. The potential consequences of collision broadening are then reassessed using both the quantum transport simulations and semiclassical Monte Carlo simulations incorporating the new algorithm, including full-band Monte Carlo simulations for the first time. The broadening assisted runaway of the high-energy tail of the carrier distribution possible with the conventional algorithm is shown to be an artifact of that algorithm, not of the form of the collision broadening used (Lorentzian, etc.) as often assumed. Simulation results obtained with the new algorithm also demonstrate that for important cases, Golden-Rule-based Monte Carlo simulations can remain accurate despite the presence of significant collision broadening. A notable class of exceptions, however, is when collision broadening allows for a threshold or scattering bottle-neck to be overcome, although the ultimate requirement for energy conservation remains critical. Specifically, it is shown that collision broadening lowers but does not eliminate the impact ionization threshold energy, producing simulated impact ionization coefficients for holes in silicon that accurately track experimental results as a function of field. Similarly, the quantum transport simulations illustrate how collision broadening can significantly affect optical absorption, but that simply subjecting low energy carriers to scattering induced collision broadening does not allow them to overcome large potential barriers.

On-state and off-state breakdown in GaInAs/InP composite-channel HEMT's with variable GaInAs channel thickness

Short-channel Ga/sub 0.47/In/sub 0.53/As high electron mobility transistors (HEMTs) suffer from low breakdown voltages due to enhanced impact-ionization effects in the narrow bandgap channel. This could limit the application of single-channel devices to medium power millimeter-wave systems. A composite Ga/sub 0.47/In/sub 0.53/As/InP channel, which exploits the high electron mobility of Ga/sub 0.47/In/sub 0.53/As at low electric fields, and the low impact-ionization and high electron saturation velocity of InP at high electric fields can overcome this limitation. In this paper we study on-state and off-state breakdown of Ga/sub 0.47/In/sub 0.53/As/InP composite-channel HEMT's with a variable GaInAs channel thickness of 30, 50, and 100 /spl Aring/. Reduction of channel thickness leads to the improvement of both on-state and off-state breakdown voltages. In on-state conditions, the enhancement in the effective Ga/sub 0.47/In/sub 0.53/As channel bandgap that takes place when the channel thickness is reduced to the order of the de Broglie wavelength (channel quantization) effectively enhances the threshold energy for impact-ionization, which is further reduced by real space transfer of electrons from the Ga/sub 0.47/In/sub 0.53/As into the wider bandgap InP. Channel thickness reduction also causes a decrease in the sheet carrier concentration in the extrinsic gate-drain region and therefore, a reduction of the electric field beneath the gate. This, together with the adoption of an Al/sub 0.6/In/sub 0.4/As Schottky layer (increasing the gate Schottky barrier height), leads to excellent values of the gate-drain breakdown voltage. In conclusion, composite channel InAlAs/GaInAs/InP HEMTs, thanks to the combined effect of effective band-gap increase, enhanced real space transfer into InP, and sheet carrier density reduction, allow a good trade-off between current driving capability and both on-state and off-state breakdown voltage.

Theoretical analysis of the breakdown voltage in pseudomorphic HFETs

In this paper a two-dimensional (2-D) model based on a solution to the moments of the Boltzmann transport equation is used to study breakdown in pseudomorphic Heterojunction Field Effect Transistors (HFETs). The effects of the energy conservation equation and the space charge effects of generated carriers are studied in the model. The model is then used to study breakdown in GaAs channel and In/sub 0.53/Ga/sub 0.47/As channel HFETs. The model shows that impact ionization breakdown in these structures is dominated by generation in two regions: (1) the high field region near the corner of the gate, and (2) the channel near the top heterojunction. Next, the effect of a thin pseudomorphic layer, which has a high threshold energy for impact ionization, is studied. This layer is shown to significantly improve the breakdown. voltage of the HFET if used properly. Finally the effects of doping on breakdown voltage of these HFETs are studied. This study shows that increased doping can improve the maximum estimated output power of these devices.

Understanding hot-electron transport in silicon devices: Is there a shortcut?

Results of a Monte Carlo study of carrier multiplication in silicon bipolar and field-effect transistors and of electron injection into silicon dioxide are presented. Qualitative and, in most cases, quantitative agreement is obtained only by accounting for the correct band structure, all relevant scattering processes (phonons, Coulomb, impact ionization), and the highly nonlocal properties of electron transport in small silicon devices. In addition, it is shown that quantization effects in inversion layers cause a shift of the threshold energy for impact ionization which is very significant for the calculation of the substrate current in field-effect transistors. Conservation of parallel momentum, image-force corrections, dynamic screening of the interparticle Coulomb interaction, and improvements to the WKB approximation are necessary to treat correctly the injection of electrons from silicon into silicon dioxide. The validity of models—analytic or Monte Carlo—which treat hot-electron transport with oversimplified physical approximations is argued against. In a few words, there is no shortcut.

Design considerations for high performance avalanche photodiode multiplication layers

We have studied the effect of the thickness of the multiplication region on the noise performance characteristics of avalanche photodiodes (APD's). Our simulation results are based on a full band Monte Carlo model with anisotropic threshold energies for impact ionization. Simulation results suggest that the well known McIntyre expression for the excess noise factor is not directly applicable for devices with a very thin multiplication region. Since the number of events is drastically reduced when the multiplication layer is very thin, the ionization coefficient is not a good physical parameter to characterize the process. Instead effective quantum yield, which is a measure of the total electron-hole pair generation in the device, is a more appropriate parameter to consider. We also show that for the device structure considered here, modeling the excess noise factor using a discrete Bernoulli trial model as opposed to the conventional continuum theory produces closer agreement to experimental measurements. Our results reinforce the understanding that impact is a strong function of carrier energy and the use of simplified field-dependent models to characterize this high energy process fails to accurately model this phenomenon. >

Monte Carlo analysis of band structure influence on impact ionization in InP

The influence of electronic band structure on the impact ionization phenomena in InP was studied using the Monte Carlo method. Realistic band structures evaluated using the nonlocal pseudopotential method were employed. Threshold energies for impact ionization were calculated throughout the first Brillouin zone. A new impact ionization model which explicitly takes into account the wave-vector dependence of impact ionization threshold energies was used to calculate the ionization rate. The simulation results of drift velocity and ionization coefficient agree very well with experimental data. In spite of significant anisotropy in the threshold energies and complex band structures, no significant orientational dependence was found in the ionization coefficients. An important result of the work presented here is the demonstration that the biaxial elastic strain in InP produced by the lattice mismatch between adjacent layers in a heterostructure can significantly alter the impact ionization thresholds.

Monte Carlo simulation of the effect of multiplication layer thickness in wide-bandwidth avalanche photodiodes

Summary form only given. The impact ionization process was studied in thin multiplication layers used in wide-bandwidth SAGM-APDs (separate absorption, grading, and multiplication avalanche photodiodes) using wave-vector-dependent threshold energies for impact ionization and a full-band Monte Carlo model. Results show that the number of ionizing collisions is reduced in thin layers, leading to an increase in the effective beta / alpha ratio and explaining the experimentally observed reduction in the excess noise factor. It is also shown that, even though the values of alpha and beta in III-V semiconductors converge at high electric fields, it is possible to use thin multiplication regions to reduce the excess noise factor and at the same time increase the bandwidth of the device operation. >

Impact ionization in semiconductors: Effects of high electric fields and high scattering rates.

We present a theory of impact ionization in semiconductors that expands an earlier theory of Kane and includes the effects of high electric fields and high scattering rates on the electron-electron collision process. We show that their combined effect, i.e., the intracollisional field effect and collision broadening, leads to a softening of the threshold energy for impact ionization and a marked increase in the anisotropy of the ionization rate with respect to the direction of the electric field.

Observation of anisotropic impact ionization in Si MOSFET

Anisotropy in the substrate current has been observed experimentally in Si MOSFETs on the (100) surface. The impact ionization rate along the (001) axis becomes larger than that along the (011) axis. The ratio of the ionization rate between the two directions has been found to be as high as 5 at 81 K. This anisotropy can be explained by the anisotropic threshold energy for impact ionization. Furthermore, it has been found that the anisotropy ratio increases with a decrease in temperature and the length of the pinch-off region in MOSFETs. These results suggest that the non-stationary transport in the pinch-off region can lead to the enhancement of the anisotropy in the impact ionization.

Hole impact ionization enhancement in AlxGa1−xSb

The threshold energy and scattering rate for impact ionization in AlxGa1−xSb have been calculated for compositions x near the value where the spin-orbit-split gap Δ equals the band gap Eg. We have found that a minimum in the threshold energy for impact ionization near the Δ=Eg composition cannot explain the enhancement of the hole ionization coefficient, although it has been proposed as a possible mechanism. Rather, an increased scattering rate is responsible for this enhancement. It is caused by a minimum change of momentum in the hole ionization near the threshold and the mixing of an s-like state into the valence-like state induced by a composition disorder in this ternary compound. Our results indicate that the maximum hole ionization coefficient will occur at a composition x that lies between zero and the composition at which Δ=Eg.