GaAs Homojunction Research Articles

Excess noise in GaAs avalanche photodiodes with thin multiplication regions

It is well known that the gain-bandwidth product of an avalanche photodiode can be increased by utilizing a thin multiplication region. Previously, measurements of the excess noise factor of InP-InGaAsP-InGaAs avalanche photodiodes with separate absorption and multiplication regions indicated that this approach could also be employed to reduce the multiplication noise. This paper presents a systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses. It is demonstrated that there is a definite size effect for multiplication regions less than approximately 0.5 /spl mu/m. A good fit to the experimental data has been achieved using a discrete, nonlocalized model for the impact ionization process.

A spectroscopic study of GaAs homojunction internal photoemission far infrared detectors

We report a spectroscopic study of absorption and photoconductivity in GaAs homojunction interfacial workfunction internal photoemission (HIWIP) far-infrared (FIR) detectors utilizing molecular beam epitaxy (MBE) grown multilayer (p +−p −−p +−p −) structures. Strong FIR (50–200 μm) free carrier absorption has been observed and analyzed for a p + GaAs thin film, revealing the suitability for FIR detection. The basic physical mechanism of free carrier absorption in the HIWIP FIR detectors has been determined to be an acoustic phonon-emission assisted process. A simple recombination model is proposed to account for the bias dependence of the responsivity and the saturation behavior. Using the measured responsivity and dark current data, detectivity (D λ ∗) of the FIR detectors has also been estimated.

Perimeter and bulk recombination currents in GaAs homojunction diodes and [formula omitted] heterojunction bipolar transistors after surface processing

The emitter-mesa surface (perimeter) and emitter-base bulk recombination currents, in GaAs AlGaAs heterojunction bipolar transistors (HBTs), are known to degrade device performance at low applied voltages, where these currents dominate. To study the effects of the etchant (used in defining the diode mesas) on the perimeter recombination current, GaAs homojunction diodes were given a short time etch in a selection of wet chemical etchants and the effect on the edge leakage current investigated. It was observed that the edge leakage current could be modified by the process used to fabricate the mesa. Following encouraging results, the work was extended to include GaAs AlGaAs HBTs, and an investigation was undertaken of the effects on recombination at the perimeter of the emitter-mesa and on bulk recombination current, with surface treatments. A simple model was used to separate the bulk and perimeter contributions from the recombination current. The surface recombination current was strongly dependent on the process used to define the mesa, and can therefore, by appropriate processing, be reduced.

Noise characteristics of thin multiplication region GaAs avalanche photodiodes

It is well known that the gain-bandwidth product of an avalanche photodiode can be increased by utilizing a thin multiplication region. Previously, measurements of the excess noise factor of InP/InGaAsP/InGaAs avalanche photodiodes with separate absorption and multiplication regions indicated that this approach could also be employed to reduce the multiplication noise. This letter presents a systematic study of the noise characteristics of GaAs homojunction avalanche photodiodes with different multiplication layer thicknesses. It is demonstrated that there is a definite ‘‘size effect’’ for multiplication regions less than approximately 0.5 μm. A good fit to the experimental data has been achieved using a discrete, nonlocalized model for the impact ionization process.

Investigation of impact ionization in thin GaAs diodes

The electron and hole multiplication coefficients, M/sub e/ and M/sub h/, respectively, have been measured in thin GaAs homojunction PIN and NIP diodes and from conventional ionization analysis the effective electron and hole ionization coefficients, /spl alpha/ and /spl beta/, respectively, have been determined. The nominal intrinsic region thickness w of these structures ranges from 1.0 /spl mu/m down to 25 nm. In the thicker structures, bulk-like behavior is observed; however, in the thinner structures, significant differences are found. As the i-regions become thinner and the electric fields increase, the M/sub e//M/sub h/ ratio is seen to approach unity. The experimental results are modeled and interpreted using a semianalytical solution of the Boltzmann equation. In thin (w/spl les/0.1 /spl mu/m) devices the dead space effect reduces effective ionization coefficients below their bulk values at low values of carrier multiplication. However, overshoot effects compensate for this at extremely high fields (/spl ges/1/spl times/10/sup 3/ kV/cm).

Unambiguous photoreflectance determination of electric fields using phase suppression

Photoreflectance measurements have been performed on a δ-doped GaAs homojunction. Two Franz–Keldysh oscillation features originating from two different regions (a buffer layer and a top layer) of the structure superimpose with each other in the photoreflectance spectrum. By properly selecting the reference phase, one of the features can be suppressed, thus enabling us to determine the electric fields from the two regions unambiguously. The electric field in the top layer is 3.5±0.2×105 V/cm, which is in good agreement with theoretical calculation. The electric field in the buffer layer is 1.2±0.1×104 V/cm.

The Electric Field Measuring by Phase Selective Photoreflectance

AbstractThe built-in electric fields in a MBE grown δ-doped GaAs homojunction have been investigated by the techniques of photoreflectance and phase suppression. Two Franz-Keldysh oscillation features originating from two different fields in the structure superimpose with each other in the photoreflectance spectrum. By properly selecting the reference phase of the lock-in amplifier, one of the features can be suppressed, thus enabling us to determine the electric fields from two different regions. We have demonstrated that only two PR spectra, in-phase and outphase components, are needed to find the phase angle which suppresses one of the features. The electric field in the top layer is 3.5 ± 0.2 × 105 V/cm, which is in good agreement with theoretical calculation. The electric field in the buffer layer is 1.2 ± 0.1 × 104 V/cm, which suggests the existence of interface states at the buffer/substrate interface.

Evidence for a photocurrent Fano resonance in an artificial nanostructure.

We present internal-photoemission (photocurrent) experimental evidence for a Fano resonance at an n-p GaAs homojunction with a 0.5-ML Si intralayer (delta doping). This is one of a very few cases in which Fano resonances have been observed in artificial nanostructures. Our results show that this fundamental class of phenomenona plays a relevant role in band-gap engineering, by affecting the transport and phototransport properties of delta-doping nanostructures.

Photodetector with subwavelength spatial resolution

A photodetector operating with subwavelength spatial resolution is presented. The active region of this detector is defined by the pn-interface region of a highly doped GaAs homojunction. The cleavage plane normal to the interface defines a line detector with high spatial resolution. The actual resolution was tested using a near-field scanning optical microscope. An Al-coated tapered optical fiber of nominal aperture of 60 nm was scanned in the optical near-field vicinity across the active region of the junction. The photocurrent was measured as a function of the position of the fiber tip relative to the cleaved GaAs pn-junction. The width of the observed peak in photocurrent is found to be highly asymmetric with respect to the p or n side of the junction. The reason for this asymmetry is discussed in terms of diffusion length of minority carriers in the n- and p-GaAs close to the suface.

Investigation of GaAs homojunction bipolar transistor with delta doping emitter structure

A GaAs homojunction bipolar transistor with a delta doping emitter structure is proposed and demonstrated. The proposed device makes use of a delta doping structure inducing a triangular barrier for minority carrier confinement, resulting in a high emitter injection efficiency. Based on the minority carrier transport in the bulk emitter region with drift-diffusion mechanisms, and in the triangular barrier region with tunnelling and thermionic-emission mechanisms, an analytical derivation of current-voltage characteristics, including the effect of bandgap shrinkage, was obtained. The calculated results show that the triangular barrier is the key parameter in determining the electrical properties. In addition, due to the absence of heterojunction, the proposed device exhibits more nearly constant current gain with collector current than for the AlGaAs/GaAs heterojunction bipolar transistor (HBT). The proposed device, grown by molecular beam epitaxy, shows a differential current gain of 13 and an offset voltage of 60 mV at a base-to-emitter doping ratio of 10. The offset voltage is attributed mostly to the geometric limits. With a simple chemical treatment of Na/sub 2/S/spl middot/9H/sub 2/O, the differential current gain is enhanced to be 16 due to the reduced surface recombination. Theory and experiment indicate the potential application of the proposed device.

Thermal velocity limits to diffusive electron transport in thin-base np+n GaAs bipolar transistors

We present experimental evidence that minority electron transport across a thin, quasineutral p+ GaAs region is limited by the thermal velocity of the electrons rather than by conventional diffusive transport. A set of GaAs homojunction np+n transistors with base widths of 4000, 2000, 1000, and 500 Å was fabricated and characterized. The diffusive model predicts that the dc collector current of the 500-Å base width transistors should be eight times larger than the collector current of transistors with a 4000-Å-wide base. The experimental results, however, show only a factor of ∼3.5 increase in collector current. The measured collector current versus base width characteristic agrees well with theoretical treatments of thin-base transport. These new results present evidence of quasiballistic electron transport in p+ GaAs and have important implications for GaAs transistor design.

A COMPOUND EMITTER HETEROJUNCTION BIPOLAR TRANSISTOR

A compound emitter heterojunction bipolar transistor (HBT) structure that incorporates an additional heterojunction within the emitter for minority carrier confinement has been proposed. In this new device configuration, the single wide band‐gap emitter layer in a conventional HBT is replaced by two sub‐layers of wide band‐gap material, with the sub‐layer nearer the base having a narrower band‐gap. By means of numerical simulations, the compound emitter HBT was found to perform better than comparable conventional HBTs. With the AlGaAs(n) / GaAs heterostructure system, the optimum compound emitter HBT structure was found to be Al0.3Ga0.7As(n) ‐ Al0. 2Ga0.8As(n) / GaAs with grading at the two hetero‐interfaces. It has a low turn‐on voltage that is almost identical to that of a homojunction GaAs bipolar transistor with similar doping conditions. Compared with a conventional single emitter layer Al0.3Ga0.7As/GaAs HBT, the optimum compound emitter HBT has an enhancement in the current gain by approximately 2 folds, an improvement in the uniform current gain region from 2 to 4 decades of collector current density, and a slight increase in the unity‐gain cut‐off frequency fT by about 7 %.

Effects of heavy doping on numerical simulations of gallium arsenide bipolar transistors

Using the best available physical models is essential for predictive numerical simulations of advanced, high performance GaAs transistors. Among the many input parameters for numerical simulations, the effective intrinsic carrier concentrations, n ie, minority carrier mobilities, μ, and recombination lifetimes, τ, are very critical parameters. The results from recent theoretical calculations for n ie were implemented into a two-dimensional, drift-diffusion simulator for GaAs transistors. In order to compare predicted and measured d.c. common emitter gains, NPN GaAs homojunction bipolar transistors with different but heavily doped bases and similarly doped emitters that have widths between 0.1 and 0.45 μm were fabricated by molecular beam epitaxy. The predicted gains of 8, 25, 46, 72 for these transistors agreed very well with their measured gains of 9, 22, 42 and 70 at high current, respectively. Without using the new theoretical data for n ie but setting n ie equal everywhere to the intrinsic carrier concentration, n i, the predicted gains became 4, 14, 27 and 35, respectively. Sensitivity analyses on mobilities, lifetimes, and n ie showed that physically correct n ie values are quantitatively very important for predictive simulations of GaAs bipolar transistors.

AlGaAs/GaAs heterojunction bipolar transistor with a two-dimensional electron gas emitter

We report the operation of the first two-dimensional electron gas (2-DEG) emitter heterojunction bipolar transistor. This device, which was grown by molecular beam epitaxy, incorporates an undoped GaAs spacer between the emitter and base of a standard AlGaAs/GaAs single-heterojunction bipolar transistor. The introduction of the spacer layer causes the formation of a 2-DEG at the AlGaAs/GaAs interface. This 2-DEG defines the emitter side of the junction and produces an emitter-base characteristic similar to that of the collector GaAs homojunction. Using a 300 Å GaAs spacer, offset voltages as low as 30 mV have been attained. These devices also exhibit current gains greater than 10 at emitter current densities of 3 A/cm2 and gains up to 400 in the high current density regime.

Transistor-based studies of heavy doping effects in n-GaAs

The n2ieDp product (where n2ie is the np product and Dp is the minority hole mobility) in heavily doped n-GaAs has been measured by electrical characterization of p-n-p GaAs homojunction transistors with base dopings ranging from approximately 1×1017 to 9×1018 cm−3. The measured n2ieDp product decreases as the doping density increases. These results suggest that nie is roughly constant with doping density, in sharp contrast to the large increase observed for p-type GaAs. This work shows that when designing GaAs bipolar devices, it is important to consider the large difference in effective band gap between n+ and p+ regions.

Surface and perimeter recombination in GaAs diodes: an experimental and theoretical investigation

Surface and perimeter recombination in GaAs heteroface diodes was studied experimentally and by two-dimensional numerical simulation including Fermi-level pinning. Perimeter and bulk current components were experimentally extracted, and the numerical model was used to study the origin of perimeter current in these devices. Under moderate bias, perimeter recombination occurs primarily within the junction depletion region, but as the bias is increased the perimeter outside of the junction depletion region becomes increasingly important. A bias dependence of the perimeter current ideality factor was observed both experimentally and theoretically and attributed to the perimeter recombination of injected carriers diffusing to the perimeter from the bulk regions. Fermi-level pinning was shown to increase the effective surface-recombination velocity, but conduction along the surface channel plays little role in these devices. The results demonstrate that a simple numerical treatment of Fermi-level pinning and surface recombination can accurately account for surface and perimeter recombination in GaAs homojunction diodes. >

Cryogenic operation of GaAs bipolar transistors with inverted base doping

The low temperature characteristics of GaAs homojunction bipolar transistors with heavily-doped base layers are investigated. An increase in current gain of about a factor of five is observed when the transistors are cooled from 300 K to 77 K. This improvement is attributed to improved emitter injection efficiency and reduced perimeter recombination.

Studies of low-surface 2-kT recombination current of the emitter-base heterojunction of heterojunction bipolar transistors

The 2-kT current at the emitter-base heterojunction of AlGaAs/GaAs heterojunction bipolar transistor grown by liquid phase epitaxy is found to be much lower than that of a GaAs homojunction diode. By measuring spectral response and current-voltage characteristics of (n)AlxGa1−xAs-(p)Al0.05Ga0.95As diodes having a similar structure to the emitter-base heterojunction, it is found that the 2-kT surface recombination current is determined by the lower doped side of the heterojunction. Therefore, a higher 1- to 2-kT current ratio and thus higher current gain can be obtained when a high-band-gap emitter with lower doping concentration is used. It is also found that by inserting an undoped AlGaAs spacer layer in between the emitter-base heterojunction and applying an edge-thinning design, a high-gain heterojunction bipolar transistor can be routinely obtained.

The Effect on Turn-On Voltage (VBE) of AlGaAs/GaAs HBT's due to the Structure of the Emitter-Base Heterojunction

The effects on the turn-on voltage (VBE) of collector current in AlGaAs/GaAs HBT's due to Be diffusion and discontinuity in the conduction band are investigated. It is clarified that VBE increases significantly with the decrease in saturation current due to Be diffusion into the AlGaAs emitter during MBE growth. It is also found that the VBE's of HBT's with Al compositionally abrupt and graded emitter-base junctions and homojunction GaAs bipolar transistors are the same. These results indicate that the spike-like potential barrier at the heterojunction has no influence on the electron injection from emitter to base.

A 27.3% efficient Ga0.5In0.5P/GaAs tandem solar cell

A two-terminal, monolithic cascade solar cell with an efficiency of 27.3% is reported. The device structure consists of a Ga0.5In0.5P homojunction grown epitaxially upon a GaAs homojunction, with a GaAs tunnel diode interconnect. The tandem combination of these two materials is lattice matched, and has a theoretical efficiency of 34%. The device was grown by metalorganic chemical vapor deposition at 700 °C, using trimethylgallium, trimethylindium, arsine, and phosphine as sources. The minority-carrier transport properties of the Ga0.5In0.5P are shown to be relatively insensitive to variations of the growth temperature and phosphine overpressure. Other factors that affect the efficiency of the device are presented and discussed.