Impact ionization wave breakdown of drift step recovery diodes

High frequency IMPATT oscillations followed under certain conditions by reversible impact ionization wave breakdown of the p ⁺-n-n ⁺ diode structure have been experimentally observed for the first time in a drift step recovery diode operating in the avalanche breakdown mode after a fast voltage restoration of the p-n junction.     

Potential and limitations of Schottky-barrier barritt devices

Computer simulation of various Schottky-barrier structures is carried out to investigate the large-signal properties of these devices. Comparison between Schottky-barrier devices and their p-n junction counterparts are also made to evaluate the potential and limitations of these devices and to explain the difference in performance between them. It is shown that among various Schottky-barrier structures, the M-n-i-p⁺ structure is the most powerful one and the M-n-p-p⁺ device is the most efficient one. Furthermore, Schottky-barrier devices with low barrier heights for minority carriers (less than 0.2 eV) are capable of producing power levels close to the generated power of p-n junction devices. Investigation of the temperature dependence of the large-signal performance of these devices shows that Schottky barriers are more sensitive and exhibit their optimum performance close to room temperature value. At low temperature, the output power is limited by the low minority carrier injection, whereas at high temperature the limitation is due to the velocity-modulation losses in the injection and low-field regions of the device.     

The role of impact ionization in the formation of reverse current-voltage characteristics of Al-SiO2-n-Si tunnel structures

Physical processes responsible for the reverse current-voltage (I-V) characteristics of Al-SiO₂-n-Si structures with 1.2–3.2-nm-thick SiO₂ and a silicon doping level of 10¹⁴-10¹⁸ cm^?3 were analyzed. A new model describing the evolution of the hot-electron energy in structures of this kind is suggested. The roles played by Auger ionization and impact ionization are differentiated. The turn-on voltages of a tunnel MOS structure are studied both theoretically and experimentally. The turn-on voltage is shown to decrease with increasing oxide layer thickness.     

Simulation of special features of the drift-mobility saturation in submicrometer silicon structures

Special characteristics of the high-field drift of electrons in submicrometer n⁺-n-n⁺ structures are studied by mathematical simulation methods in the quasi-hydrodynamic approximation. Alternative dependences of the mobility and energy-relaxation time on the electron temperature are used to calculate the profiles of the potential, temperature, drift mobility, and density of the thermal-energy flux of electrons. It is shown that, in a submicrometer configuration, a large part of the thermal energy acquired by an electron in a high-resistivity n-type region is dissipated in a low-resistivity n⁺-type contact. This effect reduces the rate of increase in the electron temperature in the drift region as the voltage increases, brings about an increase in the effective mobility, and prevents saturation of the drift velocity, as is shown by the calculated current-voltage characteristics.     

Voltage dependence of the differential capacitance of a p +-n junction

The dependences of the differential capacitance and current of a p ⁺-n junction with a uniformly doped n region on the voltage in the junction region are calculated. The p ⁺-n junction capacitance controls the charge change in the junction region taking into account a change in the electric field of the quasi-neutral n region and a change in its bipolar drift mobility with increasing excess charge-carrier concentration. It is shown that the change in the sign of the p ⁺-n junction capacitance with increasing injection level is caused by a decrease in the bipolar drift mobility as the electron-hole pair concentration in the n region increases. It is shown that the p ⁺-n junction capacitance decreases with increasing reverse voltage and tends to a constant positive value.     

Giant burst of impact ionization in a <Emphasis Type="Italic">p-n</Emphasis> junction of the 6<Emphasis Type="Italic">H</Emphasis>-SiC polytype

A study of the electron component of impact ionization in the p ⁺-n ^?-n ⁺ junction in the 6HSiC polytype made it possible to detect a giant burst of impact ionization and origination of an extra early avalanche breakdown. The electric field of this breakdown is lower by ～20% than the electric field of the breakdown arising as a result of a steady development of the impact ionization. It is of interest that this phenomenon occurs abruptly, without any apparent causes, in particular, without an increase in the dark current characteristic of a prebreakdown state of the p-n junction. Conditions for origination of an unusual breakdown and its properties made it possible to assume that there are nonlinear processes that give rise to a streamer. In the p-n junction plane, the anomalous breakdown is seen as a narrow glowing track with a width of ≈10 μm. This effect takes place in the conditions of the Wannier-Stark ladder of states. The latter can stimulate a local accumulation of charge and formation of a streamer structure.     

Special features of radiation-defect annealing in silicon <Emphasis Type="Italic">p-n</Emphasis> structures: The role of Fe impurity atoms

Deep-level transient spectroscopy is used to study the formation of complexes that consist of a radiation defect and a residual impurity atom in silicon. It is established that heat treatment of the diffused Si p⁺-n junctions irradiated with fast electrons lead to the activation of a residual Fe impurity and the formation of the FeVO (E_0.36 trap) and FeV₂ (H_0.18 trap) complexes. The formation of these traps is accompanied by the early (100–175°C) stage of annealing of the main vacancy-related radiation defects: the A centers (VO) and divacancies (V₂). The observed complexes are electrically active and introduce new electron (E_0.36: E _t ^e =E _c -0.365 eV, σ _n =6.8×10^?15 cm²) and hole (H_0.18: E _t ^h =E _v +0.184 eV, σ _p =3.0×10^?15 cm²) levels into the silicon band gap and have a high thermal stability. It is believed that the complex FeVO corresponds to the previously observed and unidentified defects that have an ionization energy of E _t ^e =E _c ?(0.34–0.37) eV and appear as a result of heat treatment of irradiated diffused Si p⁺-n junctions.     

On the maximum thickness of the space-charge region of reverse biased <Emphasis Type="Italic">p</Emphasis><Superscript>+</Superscript>-<Emphasis Type="Italic">n</Emphasis> junctions with a positive bevel

Two-dimensional potential and electric-field strength distributions in edge regions of sharply asymmetric reverse biased p ⁺-n junctions with a positive bevel were numerically simulated. It was shown that the maximum thickness of the space-charge region W _nM nonmonotonically depends on the angle θ between the bevel surface and junction plane: the function W _nM (θ) reaches a maximum at θ decreasing from 60° to 35° as the parameter Q _s /ɛ _s ɛ₀ EvM increases from 0 to 0.02 (here Q _s is the surface charge density, ɛ _s ɛ₀ is the absolute permittivity of the semiconductor, and E _vM is the maximum field strength in the space-charge region of the p ⁺-n junction far from the bevel). The results obtained may be useful in designing high-voltage thyristors based on Si, SiC, and other materials.     

Violation of neutrality and occurrence of S-shaped current-voltage characteristic for doped semiconductors under double injection

It is shown that violation of quasi-neutrality and its subsequent recovery (with an increase in the current density) may occur in doped n layers of p ⁺-n-n ⁺ structures under double injection at a high injection level and for a certain combination of electrical parameters. The violation of quasi-neutrality leads to a significant increase in the voltage across the base and subsequent recovery of neutrality gives rise to sharp decrease in voltage drop, as a result of which an S-shaped current-voltage characteristic is formed. The characteristic threshold current density for this effect is proportional to the base doping level N _d .     

Bistable low temperature (77 K) impurity breakdown in <Emphasis Type="Italic">p</Emphasis>-type 4<Emphasis Type="Italic">H</Emphasis>-SiC

Low-temperature (77 K) forward current-voltage characteristics of 4H-SiC p ⁺-p-n ⁺-n (substrate) mesa epitaxial diode structures have been measured. The characteristics are S-shaped, which is accounted for by the bistable nature of the impact ionization of frozen-out acceptor atoms of aluminum.     

Study of photocapacitance in diodes fabricated from silicon doped with vanadium

The levels of vanadium in the band gap of n-and p-Si were determined using photocapacitance measurements. It is shown that vanadium introduces levels only in the upper half of the band gap of n-Si; these levels have ionization energies of about E _c ?0.21 eV, E _c ?0.32 eV, and E _c ?0.52 eV. By contrast, V levels are located both in the upper and lower halves of the p-Si band gap: E _c ?0.26 eV, E _v +0.52 eV, E _v +0.42 eV, and E _v +0.31 eV. It is ascertained that the photoionization cross sections of all vanadium levels are larger for electrons than for holes. It is shown that the concentration of electrically active vanadium centers in n-and p-Si depends on both the concentration of shallow-level impurities and the time of vanadium diffusion into Si.     

The linear stage of evolution of electron-hole avalanches in semiconductors

A complete analytical solution of the problem of the linear stage of evolution of electron-hole avalanches in the uniform time-independent electric field E _ext is derived. The theory accounts for the drift, diffusion, and impact ionization of electrons and holes, thus providing a means for calculating the space-time distributions of fields and charges as well as all the basic parameters of the avalanches up to the onset of nonlinear effects at the time t _a . Formulas for the group velocity of the avalanches and for the velocity of its leading fronts are derived. It is shown that the time t _a must be determined from the condition that the impact ionization coefficient α in the center of the avalanche be reduced by a specified small quantity η. A transcendent equation is derived, which allows the calculation of the time t _a as a function of the quantity η, the unperturbed coefficient α(E _ext), and other parameters of the semiconductor. It is found that, when α(E _ext) is increased by two orders of magnitude, the total number of electron-hole pairs generated up to the point t _a decreases by nearly three orders of magnitude.     

Influence of the electron-hole equilibrium shift on transition-metal diffusion in GaAs

Diffusion of impurities of transition metals Fe, Cu, and Cr in heavily doped p ⁺-, n ⁺-, and intrinsic (at diffusion temperature) GaAs is studied. A technique in which impurity diffuses into GaAs-based structures with heavily doped layers (p ⁺-n or n ⁺-n) was used. It is shown that the impurity diffusivity values in p ⁺-GaAs and n ⁺-GaAs are significantly higher and lower, respectively, than for i-GaAs. The results obtained are discussed taking into account the effect of the electron-hole equilibrium shift in semiconductors on the diffusion of impurities migrating according to the dissociative mechanism. The interstitial-component concentration for Fe, Cu, and Cr impurities in GaAs was determined at the diffusion temperature.     

Tunnel emission of electrons in photo-field detectors and in an auger transistor in very strong electric fields

The effect of a strong electric field [(5–7)×10⁷ V cm^?1] on the electron emission from a semiconductor to vacuum in photo-field detectors and in MIS structures with a tunnel-transparent insulator layer (Al-SiO₂-n-Si Auger transistor) has been studied. It is shown for the first time that the existence of deep self-consistent quantum wells on the semiconductor surface in a strong electric field provides a possibility of controlling the energy of fast electrons responsible for the impact ionization near the base of the Auger transistor and changes the photosensitivity of narrow-gap photo-field cathodes in the IR spectral range due to the formation of a transistor structure at the semiconductor-vacuum interface. It is also demonstrated that, both in photo-field detectors and in tunnel Al-SiO₂-n-Si transistor structures, only the electron tunnel current should be taken into account and the hole current, disregarded. The reason for this is that only the electron current exists in vacuum, and the tunneling of holes in Al-SiO₂-n-Si from the semiconductor into the metal is unlikely because of the large hole effective mass in the valence band of SiO₂.     

Improvement in the quantum sensitivity of InAs/InAsSb/InAsSbP heterostructure photodiodes

InAs/InAs_0.88Sb_0.12/InAs_0.50Sb_0.20P_0.30 heterostructure photodiodes operating at room temperature in the spectral range 1–4.8 μm are developed. It is shown that the formation of a curvilinear reflecting surface constituted by a number of hemispheres on the rearside of the photodiode chip leads to an increase in the quantum sensitivity of the photodiodes by a factor of 1.5–1.7 at wavelengths in the range 2.2–4.8 μm. At an exposed photodiode area of 0.9 mm² and a p-n junction area of 0.15 mm², a zero-bias differential resistance of 30 Ω and a quantum sensitivity of 0.24 electron/photon at a wavelength of 3 μm are obtained. The operation of a photodiode with re-reflection of the photon flux in the crystal due to reflection from the curvilinear surface of the rearside of the photodiode chip is theoretically analyzed. The possibility of effective conversion of the re-reflected flux of photons into a photocurrent, with a simultaneous decrease in the p-n junction area, is demonstrated. An increase in the quantum sensitivity in the short-wavelength spectral range 1–2.2 μm by 35% relative to the calculated data is observed, which is probably due to impact ionization in the narrow-gap active region.     

Investigation of energy levels of Er-impurity centers in Si by the method of ballistic electron emission spectroscopy

The method of ballistic electron emission spectroscopy is used for the first time to study the energy spectrum of Er-impurity complexes in Si. The features are observed in the ballistic electron spectra of mesa diodes based on p ⁺-n ⁺ Si structures with a thin (∼30 nm) p ⁺-Si:Er surface layer in the region of ballistic-electron energies eV _t lower than the conduction-band-edge energy E _c in this layer. They are associated with the tunnel injection of ballistic electrons from the probe of the scanning tunnel microscope to the deep donor levels of the Er-impurity complexes in the p ⁺-Si:Er layer with subsequent thermal excitation into the conduction band and the diffusion to the p ⁺-n ⁺ junction and the direct tunneling in it. To verify this assumption, the ballistic-electron transport was simulated in the system of the Pt probe, native-oxide layer SiO₂-p ⁺-Si:Er-n ⁺, and Si substrate. By approximating the experimental ballistic-electron spectra with the modeling spectra, the ground-state energy of the Er complex in Si was determined: E _d ≈ E _c − 0.27 eV. The indicated value is consistent with the data published previously and obtained from the measurements of the temperature dependence of the free-carrier concentration in Si:Er layers.     

Optimizing high voltage bipolar transistors in a smart-power complementary BiCMOS technology

The implementation of high voltage vertical bipolar transistors in a BiCMOS technology requires sufficient space for the extension of the collector-base depletion region. Assuming that layout design rules for high voltage devices are used, the open base breakdown voltage BV_ceo is only defined by the one-dimensional vertical doping profile through the n⁺-emitter, the p-base, the n-intrinsic and the n⁺-extrinsic collector, i.e. lateral effects can be neglected for this type of brakdown. This paper describes the derivation of simple equations for optimizing the n⁺pn⁻n⁺-structure. Closed-form analytical equations based on the impact ionization model from Fulop ([1] Solid St. Electron. 10, 39 (1967)) yield the dependence of the open base breakdown voltage BV_ceo on the transistor gain, doping level and width of the intrinsic collector.