The channeling avalanche photodiode: A novel ultra-low-noise interdigitated p-n junction detector

A novel avalanche photodiode (APD) concept, the channeling APD, is proposed. Using a new interdigitated p-n junction structure, electrons and holes are spatially separated and impact ionize in layers of different band gap. Thus the effective ionization-rates ratio can be made extremely high (κ = α/β > 100), while maintaining a high gain, by a proper choice of the band gap difference. In the limit of large κ, this device mimics a channeltron photomultiplier. This structure can be fabricated using most III-V lattice matched heterojunctions, including long-wavelength materials for fiber-optical communications (1.3 leq lambda < 1.6µm). The design of three channeling APD's using Al0.45Ga0.55As/ GaAs, InP/In0.53Ga0.47As, and AlAs0.08Sb0.92/GaSb heterojunctions is discussed in detail. Other important features of this structure are the unique capacitance-voltage characteristic, which may be important in varactor diode applications, and the interdigitized geometry which allows the depletion of large volumes of semiconductor materials doped to levels as high as 10¹⁷/cm³. This novel semiconductor device may find interesting applications also for FET's and integrated p-i-n-FET receivers and may be used for studies of high-field transport phenomena (e.g., drift velocities) over a wide range of electric fields.     

The variably spaced superlattice energy filter quantum well avalanche photodiode: A solid-state photomultiplier

A new highly-efficient, single-carrier-type avalanche photodiode is presented based on impact ionization across the band edge discontinuity. A resonant tunneling superlattice structure, the variably spaced superlattice energy filter (VSSEF), is used to selectively tune the incident electron distribution to the quantum well impact excitation energy. The gain is greatly enhanced within the structure for two reasons: most of the incident electrons contribute to the ionization process in each stage of the device, and the lowest lying subbands, which are by far the most highly occupied, can be ionized. It is predicted that multiple ionization, similar to secondary emission in photomultiplier tubes, can occur thereby greatly enhancing the gain of the device. The device behaves in principle more like a photomultiplier than other existing solid-state photodetectors since more than one secondary carrier can be produced per initiating carrier per stage of the device. Representative devices are presented for the Ga0.47In0.53As/ (Al0.48In0.52AsBa0.17Sr0.83F2) and GaAs/(AlGaAs-ZnSe) material systems based on a calculation using first an uncoupled quantum well system and second, a complete resonant tunneling coupled quantum well calculation based on an Airy function approach.     

Microwave performance of GaAs MESFET's with AlGaAs buffer layers—Effect of heterointerfaces

Field-effect transistors consisting of GaAs active layers and Al0.33Ga0.67As buffer layers with abrupt, graded-bulk, and graded superlattice heterointerfaces were fabricated and compared to GaAs buffer transistors. Microwave measurements showed that a good interface is obtained in the graded superlattice interface structure and that there is a small improvement in gain (1-2 dB) over the GaAs buffer structure.     

High transconductance InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

Pseudomorphic In0.15Ga0.85As/Al0.15Ga0.85As modulation-doped field effect transistors (MODFET's) exhibiting extremely good dc characteristics have been successfully fabricated, dc transconductance in these strained-layer structures of 270 mS/mm were measured for 1-µm gate, normally-on devices at 300 K. Maximum drain current levels are 290 mA/mm, with excellent pinch-off and saturation characteristics. The transconductance increased to 360 mS/mm at 77 K while no persistent photoconductivity or drain collapse was observed. Preliminary microwave results indicate a 300-K current gain cutoff frequency of about 20 GHz. These results are equivalent to the best GaAs/AlGaAs MODFET results and are due in part to the improved transport properties and carrier confinement in the InGaAs quantum well.     

Characterization of InGaAs/AlGaAs pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic InyGa1-yAs/Al0.15- Ga0.85As (0.05 le y le 0.2) MODFET's grown by MBE have been characterized at dc (300 and 77 K) and RF frequencies. Transconductances as high as 310 and 380 mS/mm and drain currents as high as 290 and 310 mA/mm were obtained at 300 and 77 K, respectively, for 1-µm gate lengths and 3-µm source-drain spacing devices. Lack of persistent trapping effects,I-Vcollapse, and threshold voltage shifts observed with these devices are attributed to the use of low mole fraction AlxGa1-xAs while still maintaining 2DEG concentrations of about 1.3 × 10¹²cm^-2. Detailed microwave S-parameter measurements indicate a current gain cut-off frequency Of 24.5 GHz Wheny = 0.20, which is as much as 100 percent better than similar GaAs/AlGaAs MODFET structures, and a maximum frequency of oscillation of 40 GHz. These superior results are in part due to the higher electron velocity of InGaAs as compared with GaAs. Velocity field measurement performed up to 3 kV/cm using the magnetoresistance method indicates an electron saturation velocity of greater than 1.7 × 10⁷cm/s at 77 K fory = 0.15, which is 20 percent higher than GaAs/AlGaAs MODFET's of similar structure.     

An In0.15Ga0.85As/GaAs pseudomorphic single quantum well HEMT

This letter describes high electron mobility transistors (HEMT's) utilizing a conducting channel which is a single In0.15Ga0.85AS quantum well grown pseudomorphically on a GaAs substrate. A Hall mobility of 40 000 cm²/V.s has been observed at 77 K. Shubnikov-de Haas oscillations have been observed at 4.2 K which verify the existence of a two-dimensional electron gas at the In0.15Ga0.85As/GaAs interface. HEMT's fabricated with 2-µm gate lengths show an extrinsic transconductance of 90 and 140 mS/mm at 300 and 77 K, respectively-significantly larger than that previously reported for strained-layer superlattice InxGa1-xAs structures which are nonpseudomorphic to GaAs substrates. HEMT's with 1-µm gate lengths have been fabricated, which show an extrinsic transconductance of 175 mS/mm at 300 K which is higher than previously reported values for both strained and unstrained InxGa1-xAs FET's. The absence of AlxGa1-xAs in these structures has eliminated both the persistent photoconductivity effect and drain current collapse at 77 K.     

A new Al0.3Ga0.7As/GaAs modulation-doped FET

A new Al0.3Ga0.7As/GaAs modulation-doped FET fabricated like a MESFET but operating like a JFET was successfully fabricated and tested. This new device replaces the Schottky gate of the MESFET with an n+/p+ camel diode structure, thereby allowing problems associated with the former to be overcome. The devices, which were fabricated from structures grown by molecular beam epitaxy (MBE), had a 1µm gate length, a 290µm gate width, and a 4µm channel length. The room temperature transconductance normalized to the gate width was about 95 mS/mm, which is comparable to that obtained in similar modulation-doped Schottky barrier FET's. Unlike modulation-doped Schottky barrier FET's, fabrication of this new device does not require any critical etching steps or formation of a rectifying metal contact to the rapidly oxidizing Al0.3Ga0.7As. Relatively simple fabrication procedures combined with good device performance make this camel gate FET suitable for LSI applications.     

Theoretical study of multiquantum well avalanche photodiodes made from the GaInAs/AlInAs material system

We present numerical calculations of the electron and hole impact ionization coefficients in bulk Ga0.47In0.53As, Al0.48In0.52As and related multiquantum well structures. It is found that significant enhancement of the electron impact ionization rate over the hole impact ionization rate can be achieved by more than an order of magnitude in simple multiquantum well APD's at low applied electric fields, but only at the expense of severe carrier trapping effects. At large applied fields, ∼ 200 kV/cm, trapping becomes insignificant but the hole ionization rate increases dramatically. An alternative device that has a graded region at the end of the well, grown using a SLAM superlattice structure, is examined. In this structure electron trapping effects are eliminated, but hole trapping can still occur. It is determined that the graded structure shows an improvement over the simple multiquantum well device. The improvement may not be substantial enough to warrant the more complicated development of the graded superlattice device.     

Comparison of multiquantum well, graded barrier, and doped quantum well GaInAs/AlInAs avalanche photodiodes: A theoretical approach

A comparison of the multiquantum well; graded barrier, and doped quantum well Ga0.47In0.53As/Al0.48In0.52As avalanche photodiodes (APD's) is presented based on the calculated gain, excess noise factor, bandwidth, and gain-bandwidth product. A general numerical method, based on an ensemble Monte Carlo calculation, is used to determine the device performance, measured in terms of the electron and hole ionization probabilities, as a function of the device geometries and applied electric field. From a determination of the ionization rates, critical performance figures such as the gain, excess noise factor, and bandwidth can be determined. Various device geometries are examined (different layer widths, dopings, and overall applied electric field strength) among the three device types. The results indicate that the doped quantum well device gives the largest gain-bandwidth product at the lowest noise factor of the three device types. Surprisingly, the highest absolute gain is achievable in a simple multiquantum well APD, but at a much smaller bandwidth than in a doped quantum well device. At comparable device sizes, the doped quantum well device can deliver roughly two orders of magnitude more gain and gain-bandwidth product than either the simple multiquantum well or graded barrier device.     

Characterization of deep levels in modulation-doped AlGaAs/GaAs FET's

Deep levels in modulation-doped field-effect transistors (MODFET's) fabricated from MBE-grown AlGaAs/GaAs heterostructures, have been characterized by a modified deep-level transient spectroscopy (DLTS) technique. Assuming donor-like traps in the AlGaAs layer, it is shown that the threshold voltage Vtvaries exponentially with time under pulsed-biased conditions. This result is verified experimentally by observing the transient in the drain current IDin long-gate FET's biased in saturation. The resulting Δ √{I_{D}} DLTS spectrum reveals an electron trap with an activation energy of 0.472 eV in Si-doped Al0.3Ga0.7As.     

Ultrashort laser: Lasing in MBE GaAs layer with perpendicular-to-film optical excitation and emission

Lasing was observed in a 4.5 μm thick GaAs MBE-grown heterostructure (0.2 μm Al0.42Ga0.58As, 4.1 μm GaAs, 0.21 μm AlGaAs). The laser was driven by pulses from a mode-locked Ar laser (514.5 nm) with a maximum of 20 W peak power in 100 ps long pulses focused to 25 μm. The lasing occured along the pump axis within a cavity defined by the coated AlGaAs surfaces.     

A study of high-speed normally off and normally on Al0.5Ga0.5As heterojunction gate GaAs FET's (HJFET)

The dc, small-signal microwave, and large-signal switching performance of normally off and normally on Al0.5Ga0.5As gate heterojunction GaAs field-effect transistors (HJFET) with submicrometer gate lengths are reported. The structure of both types of devices comprises an n-type 10¹⁷-cm^-3Sn-doped active layer on a Cr-doped GaAs substrate, a p-type 10¹⁸-cm^-3Ge-doped Al0.5Ga0.5As gate layer and a p⁺-type 5 × 10¹⁸-cm^-3Ge-doped GaAs "contact and cap" layer on the top of the gate. The gate structure is obtained by selectively etching the p⁺-type GaAs and Al0.5Ga0.5As. Undercutting of the Al0.5Ga0.5As layer results in submicrometer gate lengths, and the resulting p⁺-GaAs overhang is used to self-align the source and the drain with respect to the gate. Normally off GaAs FET's with 0.5- to 0.7-µm long heterojunction gates exhibit maximum available power gains (MAG) of about 9 dB at 2 GHz. Large-signal pulse measurements indicate an intrinsic propagation delay of 40 ps with an arbitrarily chosen 100-Ω drain load resistance in a 50-Ω microstrip circuit. Normally on FET's with submicrometer gate lengths (∼0.6 µm) having a total gate periphery of 300 µm and a corresponding dc transconductance of 20-30 mmhos exhibit a MAG of 9.5 dB at 8 GHz. The internal propagation delay time measured under the same conditions as above is about 20 ps.     

Ga0.4In0.6As/Al0.55In0.45As pseudomorphic modulation-doped field-effect transistors

High-performance pseudomorphic Ga0.4In0.6As/ Al0.55In0.45As modulation-doped field-effect transistors (MODFET's) grown by MBE on InP have been fabricated and characterized. DC transconductances as high as 271, 227, and 197 mS/mm were obtained at 300K for 1.6-µm and 2.9-µm gate-length enhancement-mode and 2-µm depletion-mode devices, respectively. An average electron velocity as high as 2.36 × 10⁷cm/s has been inferred for the 1.6-µm devices, which is higher than previously reported values for 1-µm gate-length Ga0.47In0.53As/Al0.48In0.52As MODFET's. The higher bandgap Al0.55In0.45As pseudomorphic barrier also offers the advantages of a larger conduction-band discontinuity and a higher Schottky barrier height.     

The p-n junction quantum well APD: A new solid-state photodetector for lightwave communications systems and on-chip detector applications

A novel variation on the doped quantum well avalanche photodiode is presented that provides comparable signal-to-noise performance at more realizable material doping requirements. The device consists of repeated unit cells formed from a p-n Al0.48In0.52As junction immediately followed by near-intrinsic Ga0.47In0.53As and Al0.48In0.52As layers. As in the doped quantum well device, the asymmetric unit cell selectively heats the electron distribution much more than the hole distribution prior to injection into the narrow-gap Ga0.47In0.53As layer in which impact ionization readily occurs. The effects of various device parameters, such as the junction doping, Ga0.47In0.53As and intrinsic Al0.48In0.52As layer widths as well as the overall bias on the electron and hole ionization rates, is analyzed using an ensemble Monte Carlo method. From the determination of the ionization rates and the ionization probabilities per stage, P and Q, an optimal device design can be obtained that provides high gain at low multiplication noise. In addition, a structure that operates at less than 5 V bias is presented that can provide moderate gain at very low noise. It is expected that the device designs presented herein can serve either as high-gain low-noise detectors for lightwave communications systems or as moderate-gain low-noise detectors for on-chip application.     

Temperature dependence of Al/undoped Al0.5Ga0.5As/GaAs capacitors

Undoped Al0.5Ga0.5As is used in place of the insulator layer in the fabrication of MIS-type capacitors with Schottky gates. The current-voltage and capacitance-voltage characteristics of the capacitors were measured as a function of temperature in the range 300-77 K. At high temperatures current is by thermionic emission over the barrier determined by the Schottky contact and the Al0.5Ga0.5As/ GaAs conduction band discontinuity. As the temperature is lowered, Fowler-Nordheim tunneling is observed at sufficiently large gate biases and at 77 K conduction is ohmic. Based on I-V and C-V data the electron accumulation layer density is estimated to be about 1 × 10¹²cm^-2at 77 K when the capacitor is positively biased. The results obtained indicate that for an appropriate choice of parameters it should be possible to fabricate MIS-like transistors suitable for high-speed operation at 77 K.     

Performance of In0.53Ga0.47As/InP avalanche photodiodes

We calculate operating characteristics of high-sensitivity high-speed In0.53Ga0.47As/InP avalanche photodiodes (APD's). We find that significant photocurrent gain is obtained for a total fixed-charge density ofsigma_{tot} > 3 times 10^{12}cm^-2in the depleted InP and0.53Ga0.47As regions. To obtain high quantum efficiency and low tunneling currents, the fixed-charge density in the InP must be in the range2 times 10^{12}cm^-2leq sigma_{B} leq 3 times 10^{12}cm^-2. We calculate the breakdown voltages for APD's with uniformly doped layers and find that practical detectors with avalanche breakdowns as low as 15 V can be realized. High quantum efficiency and fast response are obtained by compositional grading of the In0.53Ga0.47As heterointerface over a distance ofL gsim 380Å, depending on the doping and amount of the In0.53Ga0.47As layer swept out at breakdown. Finally, a comparison of calculations with experimental results is presented.     

Theory of the GaInAs/AlInAs-doped quantum well APD: A new low-noise solid-state photodetector for lightwave communication systems

We present calculations of the electron and hole ionization coefficients, the excess noise factor, and gain for a doped quantum well APD made from the Al0.48In0.52As/Ga0.47In0.53As material systems. The ionization rates are calculated based on an ensemble Monte Carlo method. The effect of all of the device parameters, i.e., doping concentrations, layer widths, and the overall dc bias field, on the carrier ionization coefficients and the deterministic ionization probabilities,PandQ, is determined. These results in conjunction with recent noise theory results are utilized to determine an optimal device design that provides high gain at very low noise. A complete design including number of stages and individual stage design is presented for the lowest noise, highest gain device realizable in this system. It is anticipated that this device can be used as a new ultralow-noise high-gain receiver in lightwave communications systems.     

A low-noise microwave HEMT using MOCVD

Low-noise high-electron-mobility Transistors (HEMT's) with AlGaAs/GaAs heterostructures have been successfully fabricated using normal pressure metal-organic chemical vapor deposition (MOCVD). Hall mobilities of the two-dimensional electron gas at the interface are 8030 and 14 8000 cm²/V . s at 300 and 77 K, respectively, with an undoped Al0.3Ga0.7As spacer layer of 100 Å. The HEMT's with 0.65-µm-long and 200-µm-wide gates have exhibited a noise figure of 1.13 dB with 10.8 dB of associated gain at 12 GHz, and a dc transconductance of 280 mS/mm. These values are comparable to other reported HEMT devices using molecular-beam epitaxy (MBE).     

Device characteristics of (Al,Ga)As lasers with Ga(As,Sb) active layers

Device characteristics of double heterostructure lasers with Al0.4Ga0.6As confinement layers and GaAs0.99Sb0.01active layers are presented. Average emission wavelengths have been increased from 0.87 μm for undoped active layers to 0.88 μm for2 times 10^{17}cm^-3Ge doped active layers with a "wash" melt preceding the growth of the active layer and to 0.89 μ for1 times 10^{18}cm^-3Mg doped active layers grown following a "wash" melt. Threshold currents for lasers from 21 wafers are examined for several growth conditions and compared with Al0.08Ga0.92As active layer devices. Device resistance, external quantum efficiency, device degradation, and pulsed and CW threshold currents as a function of temperature are also discussed.     

Low-resistance ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As modulation-doped structures obtained by halogen lamp annealing

The successful application of short-term halogen lamp annealing to form ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/ In0.53Ga0.47As modulation-doped structures is demonstrated. Use of Ti in the electron-beam evaporated metallization scheme and a two-step annealing cycle give contacts with reproducibly good electrical and morphological characteristics. Minimum values of specific contact resistancerho_{c} = 4.0 times 10^{-7}and6.0 times 10^{-7}Ω.cm²for AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As, respectively, are measured. Corresponding values of the transfer resistance Rcare 0.12 ± 0.02 and 0.18 ± 0.05 Ω.mm. These values are the lowest achieved with lamp annealing and are comparable to the best obtained with transient furnace annealing.