I-region Width Research Articles

Nearly Ideal Breakdown Voltage Observed in Lateral p-i-n Diodes Fabricated on a SiC High-Purity Semi-Insulating Substrate

Lateral p-i-n diodes with small i-region width (less than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5 ~\mu \text{m}$ </tex-math></inline-formula> ) were fabricated by direct ion implantation into a high-purity semi-insulating silicon carbide (SiC) substrate. The breakdown voltage of the diodes increased with increasing the i-region width and a lateral effective breakdown electric field of 1.7–1.9 MV/cm was obtained. The experimental breakdown voltage well agreed with the technology computer-aided design (TCAD) simulation, indicating that deep levels contributing the semi-insulating property do not seriously deteriorate the breakdown voltage in case of the small i-region width.

Physics based non-linear large-signal analysis of multiple-graphene layer exotic pin (p++–n−–n–n++) devices and ultra-fast SPST/SPDT/SPMT switches on Si/3C-SiC (100) substrates for application in THz-communication

Multiple-Graphene-Layer (MGL) on Si/3C-SiC (100) substrate is proposed for designing of a vertically doped p++–n−–n–n++ device/switch array for Terahertz communication systems. In addition to the superior mechanical properties in Graphene, conducive–electrical and electronic properties, especially, high carrier mobility in MGL has made this a promising material for different novel applications in THz domain. A quantum-modified classical drift–diffusion (QMCLDD) mathematical model, incorporating quantum size effects, arising due to Multiple-Graphene-Layer, is developed by the authors for analysing the THz-characteristics of exotic-pin p++–n−–n–n++ devices and switches. The validity of the model is further established by comparing the simulation results with those of the experimental observations under similar electrical/thermal/structural operating conditions. The comprehensive study reveals that the switching behaviour of the device improves considerably due to the incorporation of MGL structure in the central i-region of the device. The device has shown considerably faster reverse recovery time (~ 0.5 ns), low forward RF series resistances (0.29Ω) and low power dissipation (0.45 dB). Central i-region width, doping concentration and mesa diameter of the device are optimized by studying the punch-through effects on the electric field profiles under large signal operation. It is observed that compared to the High-Punch-Through Device (HPTD) and Non-Punch-Through Device (NPTD), Low-Punch-Through Device (LPTD) is showing better high-frequency performances under similar operating conditions. A tradeoff between device width and doping concentration is further established for ultra-fast switching operation in the THz-regime. A comparative analysis of Insertion loss (IL) and Isolation (ISO) in Single-Pole-Single-Throw (SPST), Single-Pole-Double-Throw (SPDT) and Single-Pole-Multi-Throw (SPMT) THz-switches (with series–shunt and Shunt type arrays) is reported in the paper. The study clearly reveals the suitability of LPTD variant for the development of SPST switches in terms of lowest IL (0.015 dB) and SPMT shunt switches in terms of highest ISO (88.0 dB) at around 4.0 THz. Moreover, the authors have made quasi 2D-thermal analysis of the Device-Under-Test (DUT). The results will be extremely useful for developing ultrafast solid-state switches. To the best of authors’ knowledge this is the first ever report on vertically grown MGL-exotic pin (p++–n−–n–n++) device on Si/3C-SiC (100) substrate, as a THz ultrafast switch.

Non-monotonic dependence of current upon i-width in silicon p–i–n diodes

Silicon p–i–n diodes with different i-region widths are fabricated and tested. It is found that the current shows the non-monotonic behavior as a function of i-region width at a bias voltage of 1.0 V. In this paper, an analytical model is presented to explain the non-monotonic behavior, which mainly takes into account the diffusion current and recombination current contributing to the total current. The calculation results indicate that the concentration ratio of p-region to n-region plays a crucial role in the non-monotonic behavior, and the carrier lifetime also has a great influence on this abnormal phenomenon.

An Improved Physics-Based Formulation of the Microwave p-i-n Diode Impedance

An improved formulation of the frequency-dependent impedance for p-i-n diodes from physical and geometrical parameters is presented. This work is addressed to diode designers and allows them to evaluate quickly and accurately the diode impedance. It comes in parallel with existing SPICE p-i-n diode model used in CAD software. Under forward bias conditions, important recombinations occur in the heavily doped end regions of thin p-i-n diodes that seriously affects the diode impedance. This effect is taken into account to increase the accuracy of existing numerical models and to extend their validity domain to any I-region thicknesses. This improvement has been validated by measurement results on a 5-mum I-region width silicon p-i-n diode

Reactance influence on PIN diode attenuators

The reactive component of a PIN diode is known to be a function of current due to conductivity modulation in the I-region. This reactive component can negatively influence predicted attenuation levels in PIN diode attenuators if not properly accounted for. A figure of merit based on the PIN diode parameters, DC forward current, and operation frequency, is proposed that determines the operating conditions where this reactive component can be safely ignored. Simulations and measurement data on PIN diodes of various I-region widths verify the proposed figure of merit.

Avalanche Multiplication and Breakdown in 4H-SiC Diodes

The measured photomultiplication and excess noise characteristics of two 4H-SiC p-i-n diodes, with i-region widths of 0.105 μm and 0.285 μm, were modelled using a nonlocal multiplication model to determine the ionization threshold energies and the impact ionization coefficients of 4H-SiC. The modelled ionization coefficients accurately predicted the breakdown voltage of a 0.485 μm p-i-n structure. The breakdown voltage of 4H-SiC calculated using these parameters is approximately 7.2 and 4.6 times higher than that of similar Si and Al0.8Ga0.2As structures respectively. Dead space, the minimum distance required for carriers to equilibrate in the electric field, has to be taken into account to accurately predict the breakdown voltage in thin devices.

Nonlocal effects in thin 4H-SiC UV avalanche photodiodes

The avalanche multiplication and excess noise characteristics of 4H-SiC avalanche photodiodes with i-region widths of 0.105 and 0.285 /spl mu/m have been investigated using 230-365-nm light, while the responsivities of the photodiodes at unity gain were examined for wavelengths up to 375 nm. Peak unity gain responsivities of more than 130 mA/W at 265 nm, equivalent to quantum efficiencies of more than 60%, were obtained for both structures. The measured avalanche characteristics show that /spl beta/>/spl alpha/ and that the /spl beta///spl alpha/ ratio remains large even in thin 4H-SiC avalanche regions. Very low excess noise, corresponding to k/sub eff/<0.15 in the local noise model, where k/sub eff/=/spl alpha///spl beta/(/spl beta///spl alpha/) for hole (electron) injection, was measured with 365-nm light in both structures. Modeling the experimental results using a simple quantum efficiency model and a nonlocal description yields effective ionization threshold energies of 12 and 8 eV for electrons and holes, respectively, and suggests that the dead space in 4H-SiC is soft. Although dead space is important, pure hole injection is still required to ensure low excess noise in thin 4H-SiC APDs owing to /spl beta///spl alpha/ ratios that remain large, even at very high fields.

Temperature dependence of avalanche multiplication in submicron Al0.6Ga0.4As diodes

Photomultiplication measured in Al0.6Ga0.4As p+–i–n+ diodes falls with temperature in a manner that becomes less marked with decreasing i-region width and increasing electric field. Simple arguments relate this result to the effects of the ionization dead space, which is expected to depend weakly on temperature, and to the reduced scattering in the high electric fields present in thin avalanching structures. The temperature dependence of the “enabled” ionization coefficient can be approximately accounted for by simply subtracting the ballistic dead space from the width of the avalanche region. The simple arguments suggest the effect is independent of materials system.

Multiplication and excess noise characteristics of thin 4H-SiC UV avalanche photodiodes

The avalanche multiplication and excess noise characteristics of thin 4H-SiC avalanche photodiodes with an i-region width of 0.1 /spl mu/m have been investigated. The diodes are found to exhibit multiplication characteristics which change significantly when the wavelength of the illuminating light changes from 230 to 365 nm. These multiplication characteristics show unambiguously that /spl beta/>/spl alpha/ in 4H-SiC and that the /spl beta///spl alpha/ ratio remains large even in thin 4H-SiC diodes. Low excess noise, corresponding to k=0.1 in the local model where k=/spl alpha///spl beta/ for hole injection, was measured using 325-nm light. The results indicate that 4H-SiC is a suitable material for realizing low-noise UV avalanche photodiodes requiring good visible-blind performance.

Excess noise characteristics of Al/sub 0.8/Ga/sub 0.2/As avalanche photodiodes

The avalanche noise characteristics of Al/sub 0.8/Ga/sub 0.2/As have been measured in a range of p-i-n and n-i-p diodes with i-region widths /spl omega/ varying from 1.02 to 0.02 μm. While thick bulk diodes exhibit low excess noise from electron initiated multiplication, owing to the large /spl alpha///spl beta/ ratio (1/k), the excess noise of diodes with /spl omega/ < 0.31 μm were found to be greatly reduced by the effects of dead space. The thinnest diodes exhibit very low excess noise, corresponding to k = 0.08, up to a multiplication value of 90. In contrast to most III-V materials, it was found that both thick and thin Al/sub 0.8/Ga/sub 0.2/As multiplication layers can give very low excess noise and that electrons must initiate multiplication to minimize excess noise, even in thin structures.

Electrical characterization and carrier transport mechanisms of GaAs p/i/n devices for photovoltaic applications

GaAs p/i/n diodes made by Metal-Organic Vapour Phase Epitaxy were examined by electrical measurements for evaluating the optimum i-region for use as solar cells. Four series of samples were prepared and studied each one with a different i-region width. The performance of the devices was examined by means of Admittance spectroscopy as well as classical current–voltage and capacitance–voltage characterization, allowing the calculation of the minority carriers lifetime ( τ eff) and the diodes ideality factors. The values of the τ eff were found to lie between 8.7 ps and 0.14 ns for i-region widths between 0 and 0.8 μm. These results were used to model the multilayer structure with the two-diode representation and explain the conductance mechanisms inside the diodes. This modeling showed that the recombination/generation currents were dominating in forward biased diodes and the ohmic loss current in reverse bias.

Avalanche multiplication characteristics of Al/sub 0.8/Ga/sub 0.2/As diodes

The avalanche multiplication characteristics of Al/sub 0.8/Ga/sub 0.2/As have been investigated in a series of p-i-n and n-i-p diodes with i-region widths, w, varying from 1 /spl mu/m to 0.025 /spl mu/m. The electron ionization coefficient, /spl alpha/, is found to be consistently higher than the hole ionization coefficient, /spl beta/, over the entire range of electric fields investigated. By contrast with Al/sub x/Ga/sub 1-x/As (x/spl les/0.6) a significant difference between the electron and hole initiated multiplication characteristics of very thin Al/sub 0.8/Ga/sub 0.2/As diodes (w=0.025 /spl mu/m) was observed. Dead space effects in the diodes with w/spl les/0.1 /spl mu/m were found to reduce the multiplication at low bias below the values predicted from bulk ionization coefficients. Effective /spl alpha/ and /spl beta/ that are independent of w have been deduced from measurements and are able to reproduce accurately the multiplication characteristics of diodes with w/spl ges/0.1 /spl mu/m and breakdown voltages of all diodes with good accuracy. By performing a simple correction for the dead space, the multiplication characteristics of even thinner diodes were also predicted with reasonable accuracy.

Thin multiplication region InAlAs homojunction avalanche photodiodes

Low excess noise in avalanche photodetectors (APDs) is desired for improved sensitivity and high-frequency performance. Gain and noise characteristics are measured for InAlAs p-i-n homojunction APDs that were grown with varying i-region widths on InP by molecular beam epitaxy. The effective ionization ratio k (β/α) determined by noise measurements shows a dependence on multiplication region width, reducing from 0.31 to 0.18 for multiplication region thicknesses of 1600–200 nm. This trend follows previously shown results in AlGaAs-based APDs, which exhibit reduced excess noise due to nonlocal multiplication effects. These results show that this effect is a characteristic of thin avalanche regions and is not a material-specific phenomenon.

Frequency and Time Response of Pin Photodiode

Frequency and time responses of the PIN photodiode have been derived for front-and back-surface illumination. The analysis is based on solvign the continuity equation in the depletion-region, subject to boundary conditions, and gives the drift current component which is transit-time dependent. To get the diffusion current component, the continuity equation is solved in the n and p regions, respectively. The frequency and time responses of the PIN photodiode with different depletion region widths has been calculated. It is also found that the bandwidth is dependent on the side of illumination. With lighter illumination from the n-side, the bandwidth of homo junction (HJ) photodiode of 1 µm i-region width is found to be 18 GHz for an n-region width of 0.5 µm, and 29GHz for an n-region width of 0.2 µm. However, the corresponding bandwidths are 5 GHz, and 21 GHz for the same p-region widths of 0.5µm and 0.2 m and light illumonation from the P-side. The bandwidth of single hetero-structure junction (SHJ) PIN photodiode with 1 µm i-region width, where the layer facing the light is made from a material with wider band gap, is 26 GHx for both sides of illuminations. The bandwidth of double hetreo-structure junctions (DHJ) PIN photodiode with 1 µm i-

Switching phenomena in reverse-biased gold-diffused silicon p+-i-n+diodes

A novel mode of operation for a reverse-biased narrow i-region gold-diffused silicon p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -i-n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> diode exhibiting switching negative resistance is reported. The i-region width is in the range of 12-16 µm, and gold has been used to generate deep trapping levels in the i-region.