Setback Layer Research Articles

High-Photocurrent and Wide-Bandwidth UTC Photodiodes With Dipole-Doped Structure

InP-based unitraveling-carrier photodiodes (UTC-PDs) with novel dipole-doped structure to achieve high photocurrent as well as wide bandwidth are demonstrated in this letter. The dipole-doped layers in combination with a 22-nm-thick undoped InGaAs setback layer were employed at the InGaAs/InP absorption and collection interface to reduce the current blocking effect. A high photocurrent of 160 mA with 1.9 GHz 3-dB bandwidth from a 70-\(\mu \) m-diameter top-illuminated UTC-PD is achieved. A large 3-dB bandwidth of 62.5 GHz, extracted using an equivalent circuit model, has also been obtained from a 12-\(\mu \) m-diameter UTC-PD device. The results demonstrate that the dipole-doping can serve as an effective alternative to the quaternary InGaAsP layer at InGaAs/InP interface for InP-based UTC-PD to suppress the current blocking and reduce the complexity in epilayers growth and device fabrication.

Polarization-Induced Electric Fields Make Robust n-GaN/i-InGaN/p-GaN Solar Cells

We study n-GaN/i-InGaN/p-GaN solar cells on Ga-face substrates. We find that polarization charges at the heterointerfaces pin the value of <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OC</sub> , <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">J</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">SC</sub> , and the fill factor to nearly optimal levels when the InGaN layer thickness exceeds a value <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> , even as the p- and n-layer thicknesses and doping concentrations vary widely. We verify an analytical approximation for <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> . We report nearly undiminished performance when an i-GaN setback layer is added between a p-doped layer and the i-InGaN layer - an addition that may be necessary to obtain good quality heterointerfaces. Additionally, the 2-D electron gas formed at the n-GaN/i-InGaN interface facilitates thinner n-GaN window layers for improved external quantum efficiency.

Base charge accumulation and push-out effects on nonlinearity of Type-I InP/InGaAs/InP and Type-I/II AlInP/GaAsSb/InP double heterojunction bipolar transistors

The influence of energy band alignment on carrier transport and signal integrity is investigated on fabricated Type-I InP/InGaAs/InP and Type-I/II AlInP/GaAsSb/InP DHBTs. The Type-I double heterojunction bipolar transistor (DHBT) requires the use of a transition region (setback and superlattice layer) in base-collector hetero-interface to minimize the conduction band discontinuity that can cause current blocking in the collector I-V characteristics. Despite the effort, Type-I DHBT exhibits gain compression and base charge accumulation at high current injection, giving a nonlinear microwave operation. In contrast, the Type-I/II DHBT has a favorable band alignment and permits hot electron injection without impedance in both emitter-base and base-collector junctions, resulting in considerable microwave linearity improvement.

On the design of base-collector junction of InGaAs/InP DHBT

The effects of the composite-collector structure and the δ-doping density in InGaAs/InP DHBT on the Kirk current have been studied. The effective grade layer in the discretely graded layer goes into the InGaAs setback layer. The formulas of the maximum doping density, the maximum Kirk current, and the corresponding δ-doping density are derived under different Kirk-effect conditions. Both the maximum collector doping density and the maximum Kirk current are dependent on the δ-doping layer. By optimizing the delta doping, the Kirk current density can be greatly increased.

The role of doping type in setback layers on wafer‐fused AlGaAs/GaAs/GaN HBTs

AbstractThe role of doping type in the setback layer of wafer fused AlGaAs/GaAs/GaN HBTs has been investigated. The calculation assuming an ideal GaAs/GaN interface indicates that the current blocking effect due to the conduction band discontinuity between GaAs and GaN can be effectively alleviated by inserting a lightly doped GaAs setback layer with either n‐type or p‐type doping. And the n‐type setback is expected to work better than the p‐type setback for the same doping concentration. In experiments encouraging current gain of ∼ 8 was observed in wafer fused HBTs containing 30 nm GaAs setback layers of either n‐type or p‐type doping, substantially improved from the HBTs without any setback, but still much smaller than annealed as‐grown AlGaAs/GaAs/GaAs HBTs. It was proposed that the non‐ideal fused interface with a thin disordered material and interface charges/traps dominates the device performance, and that likely both types of fused HBTs (setback doping concentration is ∼ 1 × 1017 cm–3) have similar band diagrams after taking into account impurity redistribution near the interface. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

High-Breakdown-Voltage Submicron InGaAs/InP Double Heterojunction Bipolar Transistor with ft = 170 GHz and fmax = 253 GHz

The layer structure of InGaAs/InP double heterojunction bipolar transistor (DHBT) is designed to enhance the frequency performance and breakdown voltage. The composition-graded base structure is used to decrease the base transit time. The InGaAs setback layer and two highly doped InGaAsP layers are used to eliminate the conduction band spike of the collector. The submicron-emitter InGaAs/InP DHBT is fabricated successfully. The base contact resistance is greatly decreased by optimization of contact metals. The breakdown voltage is more than 6 V. The current gain cutoff frequency is as high as 170 GHz and the maximum oscillation frequency reached 253 GHz. The DHBT with such high performances can be used to make W-band power amplifier.

The role of setback layers on the breakdown characteristics of AlGaAs/GaAs/GaN HBTs

AbstractBreakdown voltages (VCBO) of AlGaAs/GaAs/GaAs and AlGaAs/GaAs/GaAs‐setback/GaN HBTs have been compared both theoretically and experimentally with respect to setback layer thicknesses and the doping type. VCBO was calculated using a non‐local energy model. The hard breakdown voltage was measured on as‐grown GaAs homojunctions and AlGaAs/GaAs/GaN HBTs formed by direct wafer fusion. The calculation showed an increase of VCBO from ∼ 20 V to ∼ 325 V by replacing the GaAs collector with GaN, and VCBO ∼90 V was indeed measured. The smaller than predicted VCBO is attributed to the large leakage current currently present in the fused junctions. Insertion of a lightly doped GaAs setback layer has resulted in improved current gain of the fused HBTs, but it also degrades the breakdown characteristics. 20‐30 nm setback layers were found to maintain VCBO significantly higher than that of GaAs homojunctions and likely reasonable current gains. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Thermal Redistribution of Oxygen and Carbon in Sub-50 NM Strained Layers of Boron Doped SiGeC

This work is the second report on a localized elevated oxygen concentration (or pileup), which occurs with thermal anneals of strained, boron doped SiGeC layers. The thermally activated oxygen updiffusion into thin strained layers of SiGeC is correlated to the preferential outdiffusion of carbon from the SiGe setback or spacer layers. A strong dependency on both prebake temperature and post-growth anneal is demonstrated, which correlates to available oxygen at the surface of the p-type silicon substrate. Additionally, evidence is provided with SIMS (secondary ion mass spectrometry) data that carbon diffusivity in SiGeC:B is significantly reduced as compared to SiGeC without boron.

Thermal Redistribution of Oxygen and Carbon in Boron-Doped Pseudomorphic SiGeC Heterojunction Nanometer Base Layers

This work is the second report on a localized elevated oxygen concentration (or pileup), which occurs with thermal anneals of strained, boron-doped SiGeC layers. The thermally activated oxygen updiffusion into thin strained layers of SiGeC is correlated to the preferential outdiffusion of carbon from the SiGe setback or spacer layers. A strong dependency on both prebake temperature and postgrowth anneal is demonstrated, which correlates to available oxygen at the surface of the p-type silicon substrate. Additionally, evidence is provided with secondary ion mass spectrometry data that carbon diffusivity in SiGeC:B is significantly reduced as compared to SiGeC without boron.

Analyses of delta-doped emitter bipolar transistor

In this article, the theoretical model of delta-doped emitter bipolar transistor that included the effect of bandgap shrinkage is presented. The recombination in emitter set-back layer is also considered. We also present the calculated results for the current gain as a function of barrier height, barrier width, and emitter set-back layer thickness. The experimental results of the device are also given.

Predicted electrical properties of modulation-doped ZnO-based transparent conducting oxides

A one-dimensional Poisson/Schrödinger program has been used to predict the effect of layer thicknesses, donor concentration, and band-gap offset on the electrical properties of transparent conducting modulation-doped ZnO∕ZnMgO multilayer structures. Mobilities as high as 145cm2∕Vs were predicted for a structure with an average carrier density of 3.8×1018cm−3 and a resistivity of 1×10−2Ωcm; for a comparable resistivity in monolithic ZnO, the mobility would be lower ∼30cm2∕Vs and the carrier density would be higher, leading to higher optical absorption. However, it was found that the maximum sheet electron density that could be transferred from the doped to the undoped layers was ∼1013cm−2, limiting the lowest calculated resistivity to ∼1.5×10−3Ωcm. The optimal thicknesses to simultaneously achieve high mobility and low resistivity were 2–5nm for both the pure ZnO and ZnMgO:Al layers. For ZnO thicknesses above this range the resistivity steadily increased, and below 2nm the mobility decreased. For ZnMgO:Al thicknesses increased above this range, the mobility rapidly decreased, whereas decreasing below 2nm increased the resistivity. The effect of the pure ZnMgO set-back layer thickness on mobility is discussed and a spacer layer of ∼1.5nm is proposed for ZnO∕ZnMgO multilayers. The effect of ZnO layer thickness on possible intersubband scattering is also discussed.

Effect of thin emitter set-back layer on GaAs delta-doped emitter bipolar junction transistor

GaAs delta-doped emitter bipolar junction transistors (δ-BJT) with different emitter set-back layer thicknesses of 10to50nm were fabricated to study the emitter set-back layer thickness effect on device dc performance. We found that the current gain decreases following decrease in the emitter set-back layer thickness. A detailed analysis was performed to explain this phenomenon, which is believed to be caused by reduction of the effective barrier height in the δ-BJT. This is due to change in the electric-field distribution in the delta-doped structure caused by the built-in potential of the base-emitter (B-E) junction. Considering the recombination and barrier height reduction effects, the thickness of the emitter set-back layer should be designed according to the B-E junction depletion width with a tolerance of ±5nm. The dc performance of a δ-BJT designed based on this criteria is compared to that of a Al0.25Ga0.75As∕GaAs heterojunction bipolar transistor (HBT). Both devices employed base doping of 2×1019cm−3 and base-to-emitter doping ratio of 40. Large emitter area (AE≈1.6×10−5cm−2) and small emitter area (AE≈1.35×10−6cm−2) device current gains of 40 and 20, respectively, were obtained in both types of transistors passivated by (NH4)2S treatment. The measured current gain of the GaAs δ-BJT is the highest reported for a homojunction device with such high base-to-emitter doping ratio normally used in HBT devices.

Comprehensive studies of InGaP/GaAs heterojunction bipolar transistors with different thickness of setback layers

The performances of InGaP/GaAs heterostructure bipolar transistors (HBTs) with different thickness of setback layers are theoretically studied. The appropriate thickness of the setback layer is an important factor in high-speed HBTs. In this work, it is found that the HBT device with a 60–90 Å setback layer has better DC and RF characteristics due to the absence of potential spike and reduced transit time. In addition, the studied devices with appropriate setback layer thickness have lower offset voltage, reverse saturation voltage, and base and collector current ideality factor.

Characteristics of an InP–InGaAs–InGaAsP HBT

The dc characteristics of an interesting heterojunction bipolar transistor with an InGaAs-InGaAsP composite-collector structure are studied and reported. Due to the insertion of an InGaAsP setback layer at the base-emitter heterojunction, the potential spikes as well as the electron blocking effect are suppressed significantly. In addition, the presence of an effective base-collector homojunction substantially reduces the current blocking effect. The studied device shows good dc performances including the small offset and saturation voltage without degrading the breakdown behavior. A typical dc current gain of 118 and the desired current amplification over 11 decades of the magnitude of collector current I/sub C/ are obtained.

Improved characteristics of metamorphic InAlAs∕InGaAs high electron mobility transistor with symmetric graded InxGa1−xAs channel

A metamorphic high electron mobility transistor with symmetric graded InxGa1−xAs channel has been successfully grown by molecular beam epitaxy system. Due to the lower interface roughness scattering, the improved electron mobility as high as 9500(30600)cm2∕Vs at 300(77)K is achieved. By using the self-consistent method, three subbands in the graded channel are found, which is matched to the Shubnikov-de Haas data. By using the graded channel, In0.425Al0.575As Schottky layer, and undoped InP setback layer, a high gate breakdown voltage of 24V is obtained. Meanwhile, the measured current gain cutoff frequency fT and maximum oscillation frequency fmax for a 1.5μm gate device are 18.9 and 48.4GHz, respectively.

Wafer-fused n-AlGaAs/p-GaAs/n-GaN Heterojunction Bipolar Transistor with uid-GaAs Base-Collector Setback

ABSTRACTRecently we reported the first AlGaAs-GaAs-GaN heterojunction bipolar transistor (HBT), a device that potentially combines the high-breakdown voltage of an n-GaN collector with the high mobility of an AlGaAs-GaAs emitter-base. Because of the high degree of lattice mismatch between GaAs (lattice constant of 5.65A) and GaN (3.19A), we formed these devices through wafer fusion, also called direct wafer bonding. Measurements on the first generations of wafer fused HBTs revealed good current modulation, with modest output current (0.83 KA/cm2) and a current gain of 1.2. Limitations to the current gain may be related to traps and defects introduced by the fusion process, or may be a consequence of the natural conduction band offset between GaAs and GaN, which is not well known. This paper describes our new HBT structure that included a thin (20nm) uid-GaAs base-collector “setback” layer. The setback layer shifted the fused GaAs-GaN interface slightly into the collector. This new HBT structure also incorporated a reduced base thickness of 100 nm. HBTs with setback layers demonstrate increased output current (1.7 KA/cm2) and increased current gain (1.9).

Design tradeoff guidelines for n-channel delta-doped Si/SiGe heterojunction MOSFETs

A design trade-off study for n-channel δ-doped Si/SiGe heterojunction MOSFETs has been performed using a combination of numerical simulation and analysis. A few design guidelines resulted from our study. We have shown that the cap and channel layer must always be made as thin as possible to reduce the separation of the mobile charge centroid from the surface, in which case better short-channel immunity, better leakage and driving ability will result. A deep potential well does not improve driving ability by much and leads to high leakage, although the short-channel immunity is better. A thick setback layer results in better leakage and driving ability but leads to poor short-channel immunity. A high δ-doping dose provides excellent short-channel immunity but leakage is high and driving ability is low. We summarize our study by constructing viable design regions enclosed by surfaces corresponding to performance constraints.

Characteristics of InGaP/GaAs delta-doped heterojunction bipolar transistor

An InGaP/GaAs heterojunction bipolar transistor with a δ-doped sheet structure has been fabricated successfully. High current gain, especially in the very low collector current regime, low collector-emitter offset voltage (ΔVCE) characteristics are obtained. There is an extremely small offset voltage of about 55 meV due to the elimination of the potential spike at the emitter-base heterojunction by a δ-doped sheet and a 50 Å very thin setback layer. On the other hand, the high current gain of 280 also reveals that the degradation of current gain due to the potential spike can be avoided. In addition, the influence of the δ-doped sheet and the undoped setback on device properties is studied.

High breakdown voltage symmetric double δ-doped In0.49Ga0.51P/In0.25Ga0.75As/GaAs high electron mobility transistor

A double δ-doped In0.49Ga0.51P/In0.25Ga0.75As/GaAs high electron mobility transistor has been successfully fabricated by metalorganic chemical-vapor deposition. Improved electron mobility as high as 5410 (19 200) cm2/V s at 300 (77) K along with turn-on voltage as high as 2.3 V and reverse gate-to-drain voltage up to 75 V are achieved. These characteristics are attributed to the use of the δ-doped, undoped InGaP Schottky layer, and undoped GaAs setback layer. Moreover, the parasitic parallel conduction can be eliminated. The activation energy is also deduced.

Study of In0.49Ga0.51P/GaAs/In0.49Ga0.51P doubleδ-doped heterojunction bipolar transistor

A lattice-matched In0.49Ga0.51P/GaAs/In0.49Ga0.51P doubleδ -doped heterojunction bipolar transistor, prepared by low-pressure metal organic chemical vapor deposition (LP-MOCVD), is fabricated successfully and reported. Due to the insertion of δ -doped sheets and setback layers both at base–emitter (B–E) and base–collector (B–C) heterojunctions, the potential spikes are suppressed significantly. In addition, the electron blocking effect is removed and a dramatic improvement of current gain is obtained. A modified Ebers–Moll model is employed to study and analyse the device performances. The experimental results show that the common-emitter current gain over 210 at the collector current of 35 mA and an offset voltageΔVCE smaller than 50 mV are obtained. Also, a lower knee-shaped voltage of 1.4 V at the collector current of 40 mA is observed. These results indicate that the device studied is a good candidate for high-speed and high-power circuit applications.