Noise in near-ballistic n+nn+ and n+pn+ gallium arsenide submicron diodes

Direct-current (d.c.) characteristics and noise measurements in the range 1 Hz-25 kHz are reported for n⁺nn⁺ and n⁺pn⁺ near-ballistic devices, with n regions (p regions) of 0.4 μm (0.45 μm), fabricated by molecular beam epitaxy at Cornell. The n⁺nn⁺ mesa structures show very low 1/? noise. indicating a Hooge parameter α_H = 6.0 × 10^?. This very low noise is attributed to the near absence of phonon collisions. The thermal (? like) noise above 1 kHz is equal to Nyquist noise at the lowest currents, rising to slightly above Nyquist noise for high currents, indicating the presence of carrier drag effects. The n⁺pn⁺ noise, on the contrary, is quite high. It seems to be associated with the ambipolar effects occurring for low injection of electrons in the p region. The importance of noise measurements for confirming ballistic or near-ballistic behavior is discussed.     

An analytical model for the epitaxial bipolar transistor

In the n⁺pn^?n⁺ transistor, high-current effects in the base and collector regions are linked within the current ranges of practical interest. To describe such effects, we have derived an analytical model that is based primarily on five assumptions: (1) the structure is approximately one-dimensional; (2) recombination is negligible in the base and collector quasi-neutral regions, and in the three space-charge regions; (3) high-current effects are negligible in the emitter and n⁺-substrate regions; (4) the Fletcher boundary conditions (or the Misawa boundary conditions) can be used for the three space-charge regions; and (5) the ambipolar approach can be used for the base and collector quasi-neutral regions. The primary findings predicted by the n⁺pn^?n⁺ transistor model are: In current ranges of practical interest (usable current gain), the electron concentration profile has a significant “vertical step” located at the collector-base metallurgical junction for all values of collector current. In the limit of extremely-high-current operation, this step tends to vanish. In the current range where the current gain begins to decline rapidly with increasing collector current, the electron concentration at the base boundary of the collector-base space-charge region goes approximately as the square of the hole concentration at the collector boundary of the same region. Because of this relationship, a charge-control calculation is more difficult than a straightforward calculation of carrier concentration for a given degree of accuracy. The n⁺pn^?n⁺ transistor model (which consists of twelve algebraic equations) is particularly useful for the practically important case of an epitaxial bipolar transistor having a very thin, heavily-doped base region.     

Electrical characteristics of laser-contacted diodes

We have fabricated p⁺-n and Schottky diodes with contacts made of laser-formed palladium-silicide. The electrical characteristics of these diodes are presented. The reverse currents and breakdown voltages are comparable to conventionally contacted p⁺-n diodes. The barrier height of laser-formed Schottky diodes agrees well with published values for Pd₂Si. The promising results point out the potential applications of contact formation by laser irradiation in device manufacture.     

High performance n/p and p/n germanium diodes at low-temperature activation annealing

In this work we demonstrate the fabrication and characterization of high performance junction diodes using annealing temperatures within the temperature range of 300-350 °C. The low temperature dopant activation was assisted by a 50 nm platinum layer which transforms into platinum germanide during annealing. The fabricated diodes exhibited high forward currents, in excess of 400 A/cm² at ∼|0.7| V for both p⁺/n and n⁺/p diodes, with forward to reverse ratio I_F/I_R greater than 10⁴. Best results for the n⁺/p junctions were obtained at the lower annealing temperature of 300 °C. These characteristics compare favorably with the results of either conventional or with Ni or Co assisted dopant activation annealing. The low-temperature annealing in combination with the high forward currents at low bias makes this method suitable for high performance/low operating power applications, utilizing thus high mobility germanium substrates.     

Numerical analysis on abnormal thyristor forward voltage increase due to heavy doping in gated p-base layer

An abnormal forward voltage increase was observed for a p-base gated double diffused n⁺pn^?p⁺ high power thyristor with high impurity concentration at the n⁺-p emitter-base junction. Accurate numerical analysis shows that heavy doping effects are the most responsible mechanism for the abnormality and that depletion layer formation at the center junction accompanies it.It will be shown that appropriate control of the impurity concentration at the emitter-base junction is necessary to avoid this abnormality by realizing the common base transistor current gain of greater than 0.73 for n⁺n^?-portion.     

A study of n +-6H/n-3C/p +-6H-SiC heterostructures grown by sublimation epitaxy

The n ⁺-6H/n-3C/p ⁺-6H-SiC structure was fabricated for the first time by sublimation epitaxy, with mesa diodes formed on its base and their electrical characteristics were studied. It was found that a green band dominates in the spectrum of injection electroluminescence (EL) of these diodes. The band is close by its parameters to that associated with free-exciton recombination in bulk 3C-SiC, but is shifted by ～0.06 eV to shorter wavelengths. A similar effect was observed previously for triangular quantum wells in an n ⁺-6H-SiC/p-3C-SiC heterojunction. An analysis of the experimental data obtained demonstrated that the structure can be regarded as two independent heterojunctions. The EL spectrum observed may be associated with radiative recombination at the n ⁺-6H-SiC/n-3C-SiC heterointerface.     

CdHgTe heterostructures for new-generation IR photodetectors operating at elevated temperatures

The parameters of multilayer Cd_xHg_1–xTe heterostructures for photodetectors operating at wavelengths of up to 5 μm, grown by molecular-beam epitaxy (MBE) on silicon substrates, are studied. The passivating properties of thin CdTe layers on the surface of these structures are analyzed by measuring the C–V characteristics. The temperature dependences of the minority carrier lifetime in the photoabsorption layer after growth and thermal annealing are investigated. Samples of p ⁺–n-type photodiodes are fabricated by the implantation of arsenic ions into n-type layers, doped with In to a concentration of (1–5) × 10¹⁵ cm^–3. The temperature dependences of the reverse currents are measured at several bias voltages; these currents turn out to be almost two orders of magnitude lower than those for n ⁺–p-type diodes.     

Minority carrier lifetimes in silicon solar cells determined from spectral and transient measurements

Measurements of the spectral collection efficiency and short circuit current decay rate following an X-ray pulse have been made on three types of single crystal silicon solar cells. The cell types were n⁺ - p, p⁺ - n, and p⁺ - n - n⁺ with base resistivities of 0.3, 10 and 10 Ω-cm, respectively. Minority carrier lifetimes were determined from both experiments using analytical or device code calculations, as required. For the n⁺ - p and p⁺ - n cells, nominal lifetimes of 2 and 5 ωsec, respectively, were obtained. A lifetime greater than 100 ωsec was inferred for the p⁺ - n - n⁺ device. This value represents a minimum estimate since our analysis is inaccurate when the diffusion length exceeds the cell thickness, as is the case here. The difference in base lifetime for the p⁺ - n and p⁺ - n - n⁺ structures is attributed to gettering during the phosphorus diffusion to form the back surface field layer.     

Dark Current Characteristics of a Radiation Detector Array Developed Using MOVPE-Grown Thick CdTe Layers on Si Substrate

We present reverse bias current (dark current) characteristics of a two-dimensional monolithic pixel-type nuclear radiation detector array fabricated using metalorganic vapor-phase epitaxy (MOVPE)-grown thick CdTe epitaxial layers on Si substrate. The (14?×?8) pixel array was formed by cutting deep vertical trenches using a dicing saw, where each pixel possesses a p-CdTe/ n-CdTe/n ⁺-Si heterojunction diode structure. The dark currents showed pixel-to-pixel variations when measured at higher applied biases exceeding 100?V. The dark current had a dependence on the pixel thickness, where pixels with lower CdTe thickness exhibited higher currents. Moreover, the temperature dependence of the dark current revealed that a deep level with activation energy of around 0.6?eV is responsible for the observed dark currents and their pixel-to-pixel variation. We discuss that the effective ratio of Te to Cd at the growth surface is a major factor that controls the thickness variation, and is also responsible for the formation of 0.6?eV deep levels.     

Effect of the fabrication conditions of SiGe LEDs on their luminescence and electrical properties

SiGe-based n⁺–p–p⁺ light-emitting diodes (LEDs) with heavily doped layers fabricated by the diffusion (of boron and phosphorus) and CVD (chemical-vapor deposition of polycrystalline silicon layers doped with boron and phosphorus) techniques are studied. The electroluminescence spectra of both kinds of LEDs are identical, but the emission intensity of CVD diodes is ～20 times lower. The reverse and forward currents in the CVD diodes are substantially higher than those in diffusion-grown diodes. The poorer luminescence and electrical properties of the CVD diodes are due to the formation of defects at the interface between the emitter and base layers.     

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 .     

High injection phenomena in p+in+ silicon solar cells

A very simple analytic analysis of p⁺in⁺ solar cell structures is undertaken based in the quasi-neutrality condition (n = p), which validity is verified. The only differential equation to be solved in the model is the classical continuity one. Non linearities, appea ronly in the boundary conditions. This model can be applied to p⁺nn⁺ or n⁺pp⁺ cells under very high injection conditions. High carrier concentrations are required to support the current flow even in short-circuit conditions, so enhancing the recombination which, on the other hand, is reduced by high injection lifetime increase. Under-linearity between the short-circuit current and the photon flux is also deduced at very high irradiance. The short-circuit current under bifacial illumination is higher than the sum of those currents under front and back illumination alternatively, so leading to inherently better bifacial cells. In the V_oc vs J_sc curve values of m in the logarithmic slope mkT/q range from values slightly below one to two. The value m = 1 is not to be expected even if the base recombination is negligible due to Dember potential effects.     

Some aspects of LEC transistor behaviour

Some properties of bipolar transistors with low emitter concentrations are investigated both theoretically and experimentally. It turns out that the base current of LEC transistors at medium and high injection levels is the same as in double diffused transistors and can be explained by Auger recombination in the emitter n⁺ region. The cut-off frequency f_T is rather low, due to extra charge storage in the lightly doped emitter region. Small n⁺ emitter areas, surrounded by a p-ring may introduce anomalies such as kinks in the (_cb, V_be) characteristics and negative resistances.     

Properties of barrier contacts with nanosize TiB x layers to InP

Structural and electrical properties of Au-TiB _x -nn ⁺ n ⁺⁺-InP and TiB _x -nn ⁺ n ⁺⁺-InP multilayer barrier structures on standard (“rigid”) and soft (“porous”)n ⁺⁺-InP substrates have been studied, with the semiconductor layers deposited by vapor-phase epitaxy, metallic layers formed by magnetron sputtering, and porous substrates fabricated by electrochemical etching of the standard InP. Samples on porous substrates have the following advantages: leakage currents in their reverse current-voltage characteristics are ten times lower; the range of the exponential rise in current in the forward characteristics is an order of magnitude wider; the changes in the ideality factor and the Schottky barrier height, observed as the contact area varies by a factor of 100, are three and ～10 times smaller, respectively; and the structure of the layers is more stable in annealing at up to 800°C.     

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.     

Negative capacitance (impedance of the inductive type) of silicon p +-n junctions irradiated with fast electrons

Silicon p ⁺-n junction diodes irradiated with 3.5-MeV electrons (with the dose of 4 × 10¹⁶ cm^?2) are studied. The diodes’ inductance (L) was measured at a frequency f = 1 MHz with the amplitude of alternating current equal to 0.25 mA. Simultaneously with measurements of L at alternating current, a direct current was passed through the forward-biased diode, which brought about the injection of minority charge carriers into the base. In order to identify both of the mechanisms that give rise to the inductive-type impedance in irradiated diodes with the p ⁺-n junction and the main radiation defects that are directly involved in the formation of this impedance, irradiated samples were annealed isochronously in the temperature range T _a = 225–375°C with sub-sequent study of the main characteristics of the defects by deep-level transient spectroscopy. It is shown that the inductive-type impedance in irradiated diodes is caused by the processes of capture and retention of charge carriers injected into the base at the trapping centers for a time ～1/2f, i.e., for a half-period of oscillations. It is also shown that the trapping centers are the vacancy-oxygen complexes introduced by irradiation with electrons.     

An analytical model for the edge-illuminated silicon solar cell under concentrated sunlight

An analytical model for the edge-illuminated p⁺nn⁺ solar cell is derived. The model employs the Fletcher boundary conditions for the p⁺n and nn⁺ space-charge regions and the ambipolar approach for the low region, the lightly-doped n-type base region. For high-level condition, the ambipolar approach yields complete information about the low region, including the ohmic drop, the Dember voltage, and the hole concentration profile.     

Theory of VHF detection and frequency multiplication with space-charge-limited current (SCLC) silicon diodes

A non-linear theory of transit-time effects upon VHF detection and frequency multiplication with SCLC silicon diodes, is put forward. Diffusion is neglected and carrier mobility is assumed to be field-independent. The theory applies to the SCLC resistor (n⁺νn⁺ structure) and the punch-through diode (n⁺πn⁺). An analytical theory for relatively small signal-amplitudes is developed. Then, computer calculations yield the frequency and bias dependence of the detected current and the second-harmonic amplitude.It is shown that by properly biasing the n⁺νn⁺ device (the transition region between the ohmic and square-law regions), the detected current is almost frequency-independent up to extremely high frequencies. On the other hand, the transit-time effects upon the detection characteristics of the punch-through diode are by far more important and limit the device usefulness to frequencies below the transit-time frequency. The amplitude of the second harmonic strongly depends upon frequency for both n⁺νn⁺ and n⁺πn⁺ structures.     

Static characteristics of high-barrier Schottky diode under high injection level

Static J–V, σ–J, γ–J characteristics based on numerical 1D simulation for Me–n–n⁺ and Me–n structures are given. The forward voltage drop of the former structure is less then the last one under high level injection because of LH (nn⁺) junction reflecting properties with respect to minority carriers. Holes stimulate base conductivity modulation but, in spite of this, the hole current gives a negligible contribution to the total current. It is shown that the injection ratio is a less significant parameter compared to base conductivity modulation if it is necessary to estimate charge storage. Influence of substrate is investigated. In addition to forming an LH barrier, the substrate acts as a source of majority carriers and defines the forward voltage drop across the diode structure. Bandgap narrowing in the substrate is taken into account. A comparison with experimental results is also given.     

Junction potentials of strongly illuminated n+ - p - p+ solar cells

The low-junction (LHJ) model is applied to an n⁺ - p - p⁺ solar cell having finite dimensions, in order to investigate its performance under intense illumination. Ambipolar transport equations are solved in the three sections of the cell using appropriate boundary conditions. Expressions for junction currents are derived, and the junction potentials under open-circuit conditions are computed by the Newton-Raphson method.The theory presented here includes the effects of high level injection. The generalized current density equations which are derived here for an n⁺ - p - p⁺ device are shown to reduce to the ideal Shockley diode equation with appropriate modifications. The effects of p - p⁺ low-high junction on the open-circuit voltage of the cell are explained. The theoretical results of this paper are consistent with the experimental results of others.