Analysis of I–V measurements on PtSi-Si Schottky structures in a wide temperature range

I–V Measurements on PtSi-Si Schottky structures in a wide temperature range from 90 to 350 K were carried out. The contributions of thermionic-emission current and various other current-transport mechanisms were assumed when evaluating the Schottky barrier height Φ₀. Thus the generation-recombination, tunneling and leak currents caused by inhomogeneities and defects at the metal-semiconductor interface were taken into account.

Taking the above-mentioned mechanisms and their temperature dependence into consideration in the Schottky diode model, an outstanding agreement between theory and experiment was achieved in a wide temperature range.

Excluding the secondary current-transport mechanisms from the total current, a more exact value of the thermionic-emission saturation current I_te and thus a more accurate value ofΦ_b was reached.

The barrier height Φ_b and the modified Richardson constant A^** were calculated from the plot of thermionic-emission saturation current I_te as a function of temperature too. The proposed method of finding Φ_b is independent of the exact values of the metal-semiconductor contact area A and of the modified Richardson constant A^**. This fact can be used for determination of Φ_b in new Schottky structures based on multicomponent semiconductor materials.

Using the experimentally evaluated value A^** = 1.796 × 10⁶ Am⁻²K⁻² for the barrier height determination from I–V characteristics the value of Φ_b = 0.881 ± 0.002 eV was reached independent of temperature.

The more exact value of barrier height Φ_b is a relevant input parameter for Schottky diode computer-aided modeling and simulation, which provided a closer correlation between the experimental and theoretical characteristics.     

Alpha cutoff frequency of junction transistors

The alpha cutoff frequencies of drift and diffusion transistors are analyzed and approximated by simple analytical expressions. The alpha cutoff frequency is determined in terms of the three major contributions: base-region transport cutoff, emitter cutoff due to the emitter capacitance, and the collector cutoff due to the base resistance and the collector capacitance. Denoting the respective cutoff frequencies by ω^*_c, ω_E and ω_c, and the resulting cutoff frequency by ω_c, it is found by computation that

In addition, a new expression is given for the base transport cutoff.

The dependence of the frequency cutoff on the emitter current is also discussed for the current range where the built-in electric field is partially washed out by the high concentration of minority carriers injected into the base. A calculation is presented for the current level at which there is a maximum in the alpha cutoff.

Experimental data are compared with the calculations.     

Electrical studies of the reactions between ethanol vapours and inversion mode Si-thin SiO2 structures

Processes taking place in Si-thin SiO₂ structures (oxide thickness 135 Å) after exposure to ethanol vapours have been investigated by measurements of the inversion channel I-V characteristics. It has been established that the changes in the Si---SiO₂ structure under the action of ethanol vapours relax with time after their removing and the extent of relaxation depends on the drain voltage duration as well as on the time interval separation of the drain voltage pulses. The I_D(t) dependence obeys two exponents, reflecting the presence of two different processes (one of them being slower than the other one), characterized with different time constants. They appear to be a unique function of the ethanol vapour reaction with the oxide surface. It has been shown that the ethanol vapours action at certain drain voltage leads to the introduction of defects in the Si-thin SiO₂ structure. The defects are a source of electrically active centres. The total value of these charges originating after the interaction of ethanol vapours with the SiO₂ surface is of the order of (1.5–1.7) × 10¹¹ cm⁻². It is proposed that the surface oxide charge is mainly responsible for the ethanol vapours-introduced positive charge in the Si---SiO₂ structure. The paper proposes some concepts for the interpretation of the results obtained. Adsorption of ethanol molecules, surface reaction and desorption of decomposition products are included.     

Novel Delta-Doped InAlGaP/GaAs Heterojunction Bipolar Transistor

The first successful demonstration of a delta-doped InAlGaP/GaAs heterojunction bipolar transistor (HBT) is reported. A comparison to a baseline InAlGaP/GaAs HBT without a delta-doping layer is made. Both of these devices exhibit near-ideal current gain (beta) versus the collector current (I _C) characteristics (i.e., beta independent of I _C) at high currents. The delta-InAlGaP/GaAs HBT exhibits a 40% reduction in offset voltage (V _{CE, offset}) and a 250-mV reduction in knee voltage (V _k) without sacrificing beta compared with the baseline InAlGaP/GaAs HBT. At a higher I _C, the decrease in beta of the InAlGaP/GaAs HBTs with increasing temperature is significantly smaller than the corresponding effect measured in the formerly reported GaAs-based HBTs. The rather temperature-insensitive characteristics of these two InAlGaP/GaAs HBTs originate from their large valence-band discontinuity (DeltaE _V) at the emitter-base (E-B) junction. Furthermore, at intermediate base current I _B levels (0.4-1.6 mA), V _{CE, offset} falls as I _B increases, which is a trend contrary to that of most HBTs in the literature. Finally, the experimental dependence of V _{CE, offset} on temperature, I _B, and the effective barrier height at the E-B junction is explained with reference to an extended large-signal model.     

The intervalley X6→Γ6, L6 scattering time in GaAs measured by ultrafast pump-probe infrared absorption spectroscopy

A direct measurement of the dynamics of electrons in the X₆ valley for a GaAs crystal by time-resolved absorption spectroscopy is reported for the first time. IR picosecond probe pulses were used to monitor the growth and decay of the population in the X₆ valley subsequent to excitation by a 527 nm pump pulse. The intervalley X₆→Γ₆, L₆ scattering time t_x of 0.70 ± 0.50 ps is determined and the crossection for the X₆→X₇ transition is estimated to be 1.8 × 10⁻¹⁶ cm².     

Design optimization of stacked layer dielectrics for minimum gate leakage currents

An effective model to evaluate the leakage currents for different stacked gates deep submicron MOS transistors is presented. For a given equivalent oxide thickness of a stacked gate, the gate leakage current decreases with an increase of high-k dielectric thickness or a decrease of interlayer thickness. Turning points at high gate biases of the I–V curves are observed for Si₃N₄/SiO₂, Ta₂O₅/SiO₂, Ta₂O₅/SiO_2−yN_y, Ta₂O₅/Si₃N₄, and TiO₂/SiO₂ stacked gates except for Al₂O₃/SiO₂ structure. Design optimization for the stacked gate architecture to obtain the minimum gate leakage current is evaluated.     

Rise time of silicon p-n-p photodiodes

Exact analytical expressions are derived for the short circuit photodiode currents excited by light pulses, under the assumption that the drift carrier velocity linearly depends on the electric field in the depletion layer. Reflection from the back surface of the photodiode is taken into account. Using the obtained expressions it is possible to establish a connection between the rise time t_rise and the product W_eff of the absorption coefficient (λ) and effective depletion layer width W_eff(W) at various ratios of the diode thickness and the effective depletion layer width. The influence of the RC-constant (where C is the photodiode effective capacity and R is the sum of the diode series and loading resistances) on the rise time is also analyzed.

One of the most important conclusions is that generally the rise time is larger for p-n-n⁺ photodiode configurations than for n-p-p⁺ configurations at the same substrate resistivity.     

A simple analytical model of counter doping in a uniformly and heavily doped channel region of MOSFETS

We analyze counter doping into a heavily and uniformly doped channel MOSFET region, which enabled us to suppress short channel effects with a proper threshold voltage V_th. We derive a model for the relation between the counter doping conditions and V_th and verify its validity with numerical and experimental data. We show that V_th is determined by the centroid, R_p, and dose, Φ_D, of the counter doping and that V_th is independent of the straggle ΔR_p. We show that R_p is almost invariable while (ΔR_p² + 2Dt) is smaller than R_p0², where R_p0 is the initial R_p which is the projected range of ion implantation, D is the diffusion coefficient of the counter doped impurities, and t is the annealing time. Using this technology we can expect superb short channel immunity with a reduced threshold voltage suitable for deep submicron gate length devices, which is shown from numerical analysis.     

Electrical properties of high permittivity ZrO2 gate dielectrics on strained-Si

Deposition and electrical properties of high dielectric constant (high-k) ultrathin ZrO₂ films on tensilely strained silicon (strained-Si) substrate are reported. ZrO₂ thin films have been deposited using a microwave plasma enhanced chemical vapor deposition technique at a low temperature (150 °C). Metal insulator semiconductor (MIS) structures are used for high frequency capacitance–voltage (C–V), current–voltage (I–V), and conductance–voltage (G–V) characterization. Using MIS capacitor structures, the reliability and the leakage current characteristics have been studied both at room and high temperature. Schottky conduction mechanism is found to dominate the current conduction at a high temperature. Observed good electrical and reliability properties suggest the suitability of deposited ZrO₂ thin films as an alternative as gate dielectrics. Compatibility of ZrO₂ as a gate dielectric on strained-Si is shown.     

Reliability of ultra thin ZrO2 films on strained-Si

Ultra thin high-k zirconium oxide (equivalent oxide thickness 1.57 nm) films have been deposited on strained-Si/relaxed-Si_0.8Ge_0.2 heterolayers using zirconium tetra-tert-butoxide (ZTB) as an organometallic source at low temperature (<200 °C) by plasma enhanced chemical vapour deposition (PECVD) technique in a microwave (700 W, 2.45 GHz) plasma cavity discharge system at a pressure of 66.67 Pa. The trapping/detrapping behavior of charge carriers in ultra thin ZrO₂ gate dielectric during constant current (CCS) and voltage stressing (CVS) has been investigated. Stress induced leakage current (SILC) through ZrO₂ is modeled by taking into account the inelastic trap-assisted tunneling (ITAT) mechanism via traps located below the conduction band of ZrO₂ layer. Trap generation rate and trap cross-section are extracted. A capture cross-section in the range of 10⁻¹⁹ cm² as compared to 10⁻¹⁶ cm² in SiO₂ has been observed. The trapping charge density, Q_ot and charge centroid, X_t are also empirically modeled. The time dependence of defect density variation is calculated within the dispersive transport model, assuming that these defects are produced during random hopping transport of positively charge species in the insulating layer. Dielectric breakdown and reliability of the dielectric films have been studied using constant voltage stressing. A high time-dependent dielectric breakdown (TDDB, t_bd > 1500 s) is observed under high constant voltage stress.     

Space and time resolved surface temperature distributions in Si power diodes operating under self-heating conditions

In order to optimize and improve the design of power devices with improved surge current safe operating area it is necessary to obtain a good correlation between measured and simulated space and time resolved temperature distributions. Therefore, an IR microscope capable of measuring the space and time resolved surface temperature distributions in Si power diodes operating under self-heating conditions has been developed. The minimum detectable spot size is 15 μm, while the signal rise time is detector limited to about 1 μs. The lower temperature detectivity limit is about 10°C over room temperature.

Using this instrument dynamic thermal phenomena in fast recovery 3.3 kV Si power diodes having radiation-induced recombination centers [Proceedings of the 7th EPE, Trondheim, 1997] subjected to 1.2 ms 400–2000 A/cm² and 0.3–2 ms 2000 A/cm² current pulses have been studied. The experimental results have been compared to results from 2D device simulations including surface recombination and carrier lifetime temperature dependence. The agreement between experimental and device simulation results (i.e. dynamic IV characteristics and time and space resolved temperature distributions) is very good up to a peak current density of 1500 A/cm², and a reasonable good one for peak current densities up to 2000 A/cm² (1.2 ms current pulses).     

Process and device characterization of high voltage gallium arsenide P-i-N layers grown by an improved liquid phase epitaxy method

GaAs P-i-N layers with an i-region net doping of less than 10¹² cm⁻³ were grown on P⁺ and N⁺ substrates by a modified liquid phase epitaxy (LPE) method. Doping profiles and structural data obtained by varius characterization techniques are presented and discussed. A P⁺-P-i-N-N⁺ diode with a 25 μm-wide i-region exhibits a breakdown voltage of 1000 V, a t_rr of 50 ns, and reverse current densities (at V_R = 800 V) of − 3 × 10⁻⁶ A/cm² at 25°C and 10⁻² A/cm² at 260° C.     

An investigation of LixNi1−x,O as a mixed-valence thermoelectric material

The mixed valence material, Li_xNi_1−xO, has been investigated as a potential thermoelectric material. Measurements of the Seebeck coefficient, (μ V/°C), electrical resistivity, ρ(Ω-cm), and thermal conductivity, k(W/cm°C) have been made as a function of temperature and lithium concentration. The thermoelectric figure of merit, Z(²/ρk), reaches a value of approximately 1·4×10⁻⁴ at 1100°C for the composition Li_0.04Ni_0.96O.     

Approximate calculation of the spectral function for the stimulated recombination radiation in semiconductors

The spectral function of the stimulated emission in semiconductors, proposed in integral form by and ⁽¹⁾, is explicitly evaluated through a simple approximate expression. The approximation is valid for those values of the photon energy that make the stimulated emission dominant over the absorption if it is assumed that the distances between the quasi-Fermi levels and the band edges, F'_n and F'_p, meet the condition:

(F′_n + F′_p)/2kT 1

.

Explicit expressions for two quantities which are of interest in studying injection lasers i.e. peak energy of the stimulated emission and gain per unit length in the active region are also derived. All the calculations have been performed in the hypothesis of parabolic bands.     

Density of States-Based DC $I$– $V$ Model of Amorphous Gallium–Indium–Zinc-Oxide Thin-Film Transistors

The density of states (DOS)-based DC I-V model of an amorphous gallium-indium-zinc oxide (a-GIZO) thin-film transistor (TFT) is proposed and demonstrated with self-consistent methodologies for extracting parameters. By combining the optical charge-pumping technique and the nonlinear relation between the surface potential (phiS) and gate voltage (V _GS), it is verified that the proposed DC model reproduces well both the measured V _GS-dependent mobility and the I _DS-V _GS characteristics. Finally, the extracted DOS parameters are N _TA = 4.4 times 10¹⁷ cm^-3 middot eV^-1, N _DA = 3 times 10¹⁵ cm^-3 middot eV^-1, kT _TA = 0.023 eV, kT _DGA = 1.5 eV, and EO = 1.8 eV, with the formulas of exponential tail states and Gaussian deep states.     

Dynamic surface temperature measurements in SiC epitaxial power diodes performed under single-pulse self-heating conditions

Semiconductor devices development, design and optimization require the use of computer simulation tools able to predict the entire device safe operating area (SOA), something that it is not always possible due to limitations in some of the physical models in predicting certain properties of device operation under extreme conditions (i.e. high carrier injection levels and high temperature). In order to improve our understanding of device operation under these extreme conditions experimental data of the dynamic IV characteristics and temperature time evolution and space distribution are required. The experimental data obtained are then used in the development of improved physical models and simulation tools.

In this work, dynamic surface temperature measurements as a function of current pulse peak density and length were performed on SiC-PiN epitaxial power diodes. The measurements were carried out using an infrared (IR) microscope developed in our lab capable of measuring space and time surface temperature distributions in semiconductor devices operating under self-heating conditions [Solid State Electron 2001;45(12):2057]. The minimum detected spot size is 15 μm, while the signal raising time is detector limited to about 1 μs. The lowest detectable temperature increment is at least 10 °C over room temperature.

Using this instrument, dynamic thermal phenomena in 4.5 kV SiC-PiN epitaxial power diodes [Mater Sci Forum 2001;353–356:727] subjected to 1 ms long 100–6000 A/cm² and 0.1–5 ms long 3000 A/cm² current pulses have been studied. The possibility of obtaining dynamic surface temperature information from SiC electronic devices operating under self-heating conditions with time constants in the order of ms is demonstrated.     

Effect of emitter recombinations on the open circuit voltage decay of a junction diode

It is shown that when the effect of the carriers stored in the emitter of a p-n junction diode is important due to the bandgap narrowing in the emitter, the continuity differential equations in the emitter and the base defining the Open Circuit Voltage Decay (OCVD) form a coupled system. The solutions of the coupled equations are derived and shapes of the minority-carrier profiles are calculated and discussed. It is found that the quasistatic approximation is valid to a high degree of accuracy in the emitter except for very small values of time. However, this approximation is not valid even roughly in the base. The OCVD plots are approximately parallel straight lines for tτ_B (τ_B is the lifetime of the minority-carriers in the base) and the slope of the plots is 1/τ_B. A method is suggested to obtain more accurate values of τ_B from the observed OCVD. For small values of time (0 < t < 2.5 τ_B), the OCVD plot is sharply curved. By fitting the observed OCVD with the theory in this range, the values of bandgap narrowing and emitter lifetime can also be derived. If (τ_B/τ_E) > 20, (τ_E is the lifetime of minority-carriers in the emitter) the value of τ_E derived in this manner is however not accurate. A typical experimental plot is found to be consistent with the theory. The theory based on charge control method and quasistatic approximation does not agree with our results or with the experimental observations discussed in the paper.     

Metal-germanium Schottky barriers

A systematic study has been made of the electrical characteristics of Schottky barriers fabricated by evaporating various metal films on n-type chemically cleaned germanium substrates. The diodes, with the exception of Al---Ge contacts, exhibit near-ideal electrical characteristics and age only slightly towards lower barrier height values. Al---Ge contacts exhibit very pronounced ageing towards higher barrier height values, due to formation of an extra aluminium oxide interfacial layer. Because of this, the barrier height values of aged Al---Ge contacts derived from I-V and C-V characteristics differ significantly. The dependence of the barrier height, (φ_b) on the metal work function, φ_m, for different metal-germanium contacts shows that surface states play an important role in the formation of the barrier. The density of germanium surface states is estimated to be D_s = 2 × 10¹³ eV⁻¹ cm⁻².     

Transient charging currents in insulators

It is shown that charging currents in good insulators can be used to distinguish between different empirical carrier injection laws, if most carriers are trapped according to the Walden model. Schottky injection and power law injection are considered in detail. The current falls as t⁻ⁿ, 0<n<1. For power law injection the isochronal current density j ∞ E_k, where E is the applied field, and it is shown that the apparent power k 1 for 0·6 < n < 0·9, whatever the true power law exponent p. This accords with the experimentally observed superposition theorem. Some limitations of the power law injection process proposed by Levine are discussed.     

Electrical characteristics of GaAsP Schottky barrier diodes

Schottky barrier diodes of chromium on n-type epitaxial gallium arsenide phosphide (GaAsP) were studied from 25°C to 440°C. The diodes showed significant rectification properties up to a temperature of 440°C. At high temperature the reverse leakage current was 1.15 mA at 25 V with a diode area of 1.14×10⁻³ cm² as compared with 0.25-μA current at room temperature. The n factor derived from the slope of the ln I vs. V curves was 1.1. The barrier height for chromium was found to be 1.25 eV from the capacitance measurements and 1.12 eV from the saturation current vs. temperature measurements. The slope of the C-V curves yielded a carrier concentration of 6.0×10¹⁵ carriers per cm³.