R.C. Tozer

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.

Avalanche noise measurement in thin Si p+-i-n+ diodes

The avalanche multiplication and excess noise properties of a range of submicron Si diodes have been investigated. In these thin diodes the excess noise is found to fall below that predicted by conventional local noise theory. Modeling of the multiplication and excess noise using a recurrence method, which includes the dead space for carrier ionization, gives good agreement with experiment. This suggests that the dead space can reduce the excess noise in submicron Si diodes.

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.

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.

Excess noise measurement in avalanche photodiodes using a transimpedance amplifier front-end

We report a versatile system for measuring excess noise and multiplication in avalanche photodiodes, using a transimpedance amplifier front-end and based on phase-sensitive detection, which permits accurate measurement in the presence of a high dark current. The system, which we have used successfully on a wide variety of materials and device structures, can measure reliably the excess noise factor of devices with a capacitance of up to ∼50 pF.

A dynamic collisional-radiative model of a low-pressure mercury-argon discharge lamp: a physical approach to modeling fluorescent lamps for circuit simulations

A dynamic collisional-radiative model of a low-pressure mercury (Hg)-argon (Ar) discharge lamp for application in circuit simulations is presented. The model is implemented in MATLAB as a set of coupled rate equations that are solved simultaneously to give the electron density, the density of the 6/sup 3/P/sub 0,1,2/ states and the electron temperature. These parameters are used to compute the electrical parameters of the discharge positive column and hence the time-dependent voltage-current characteristics of the lamp. The parameters predicted by the model are compared with published experimental data and show good agreement. Calculated lamp voltage-current characteristics based on the model are shown to be in good agreement with direct measurements on commercial fluorescent lamps.

Avalanche noise characteristics of single Al/sub x/Ga/sub 1-x/As(0.3<x<0.6)-GaAs heterojunction APDs

Avalanche multiplication and excess noise have been measured on a series of Al/sub x/Ga/sub 1-x/As-GaAs and GaAs-Al/sub x/Ga/sub 1-x/As (x=0.3,0.45, and 0.6) single heterojunction p/sup +/-i-n/sup +/ diodes. In some devices excess noise is lower than in equivalent homojunction devices with avalanche regions composed of either of the constituent materials, the heterojunction with x=0.3 showing the greatest improvement. Excess noise deteriorates with higher values of x because of the associated increase in hole ionization in the Al/sub x/Ga/sub 1-x/As layer. It also depends critically upon the carrier injection conditions and Monte Carlo simulations show that this dependence results from the variation in the degree of noisy feedback processes on the position of the injected carriers.

Impact ionization coefficients of Al0.8Ga0.2As

The impact ionization coefficients in bulk Al0.8Ga0.2As have been determined from photomultiplication measurements over the electric field range of 328–519 kV/cm. Unlike in AlxGa1−xAs (x⩽0.6), where the electron to hole ionization coefficients ratios (1/k) are less than 2, the 1/k value in Al0.8Ga0.2As was found to be greater than 10. Excess noise measurements corroborated the multiplication results, suggesting that this material may be a suitable multiplication medium for low noise avalanche photodiodes.

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 /spl mu/m. 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 /spl mu/m 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.

Treatment of soft threshold in impact ionization

The avalanche multiplication and excess noise properties of a range of submicron Si diodes have been measured and analyzed using a model for impact ionization which includes the effect of dead space, modified to allow for a gradual onset of ionization, with realistic threshold energies. Good agreement is achieved between the predictions of this “soft dead space” model and measurements of multiplication and excess noise, both on a range of submicron diodes with uniform electric fields and also on a p+n diode with a highly nonuniform electric field.