R.A. Milano

An analysis of the performance of heterojunction phototransistors for fiber optic communications

The theory of operation of the heterojunction phototransistor (HPT) is reviewed and the limitations on gain and speed-of-response are examined in the context of fiber optic systems requirements. The response of the base potential is shown to depend on the input optical power, and this dependence results in a power-dependent gain-bandwidth product, fT. Model calculations assuming optimized device structures suitable for multimode and single-mode systems operating in the 1.3-1.55-µm spectral region are used to demonstrate the consequences of this dependence. The results indicate that the HPT can have sufficient gain and speed-of-response for particular applications if a dc bias (optical or electrical) is used. The noise sources,S/N, and sensitivity of the optimized devices are discussed to clarify system applicability.

(Ga,Al)As/GaAs bipolar transistors for digital integrated circuits

(Ga,A1)As/GaAs npn bipolar transistors have been made using MBE growth. The epitaxial layer structure has been designed for high speed integrated circuit operation, and base regions as thin as 1000Å with p = 1019cm-3have been utilized. Fabrication processes based on both selective etching and Be implantation (to achieve a planar structure) have been demonstrated. Effects which influence current gain of the transistors are discussed. It is projected that current-mode-logic ICs based on scaled transistors of this type will achieve propagation delay times of 20-25 ps at an associated speed-power product of 70 fJ (unity fan-in and fan-out).

Effect of heterojunction spike on the quantum efficiency of an AlGaAs/GaAs heterojunction charge coupled device

The ’’spike’’ in the conduction resulting from the band‐gap discontinuity in a p‐ GaAs/n‐Al0.3Ga0.7As heterojunction is observed via the blocking of photoexcited electron flow from the p‐GaAs to the n‐Al0.3Ga0.7As. The spike height deduced from photocurrent versus bias results is 0.14 eV above the conduction‐band minimum of p‐GaAs. This value is 0.13 eV lower than that predicted by theory and suggests a positive interface charge density of 4×1011 cm−2. The lowered barrier does not appear to reduce the electron collection in the charge coupled device operation under low light level illumination.

Very low dark current heterojunction CCD's

A model has been formulated which accounts for the major sources of dark current (J<inf>D</inf>) associated with a single pixel of a heterojunction, Schottky gate charge-coupled device (CCD). This model predicts the temperature dependence of J<inf>D</inf>and shows that for properly fabricated gates, bulk generation in the channel is the primary source of dark current. To verify the model, the dark current of Al<inf>0.3</inf>Ga<inf>0.7</inf>As/ GaAs n-p⁺heterostructure CCDs was measured over the temperature range 23-55°C. At room temperature,<tex>J_{D} \approx 83</tex>pA/cm², typically, and some pixels have J<inf>D</inf>as low as 43 pA/cm². These are the lowest dark currents reported to date for a CCD structure. The data at 55°C show that, typically, J<inf>D</inf>increases to ∼ 1 nA/cm². Furthermore, the data confirm the temperature dependence of J<inf>D</inf>predicted by the model.

A backside‐illuminated imaging AlGaAs/GaAs charge‐coupled device

A glass‐supported, backside‐illuminated AlGaAs/GaAs heterojunction charge‐coupled device (CCD) imager is reported. The CCD structure was grown by liquid phase epitaxy on a GaAs substrate. The top epi‐layer was bonded to glass and the GaAs substrate completely removed. A ten‐pixel three‐phase Schottky gate CCD was fabricated on the glass‐supported layer. The CCD was successfully operated as a line imager with the photosignal entering through the support glass.

Low dark current glass bonded AlGaAs/GaAs Schottky gate imaging CCD

The fabrication and characterization of a glass bonded, backside illuminated, AlGaAs/GaAs heterojunction CCD imager is reported. The dark current of these devices is found to be <1 nA/(cm²which indicates that the glass bonding procedure does not adversely effect the dark current device performance. To isolate process induced degradation of the CCD Schottky gates, diodes were fabricated on n-Al<inf>x</inf>Ga<inf>1-x</inf>As (0 ≤ × ≤ 0.55) by e-beam evaporation of Cr/AuGe, Ti/Pt/Au and Cr/Au. The data indicate that φ<inf>Bn</inf>(Cr) ≤ φ<inf>Bn</inf>(Ti) for all solid compositions examined. Low temperature annealing causes a degradation of the Cr/AuGe and Ti/Pt/Au devices; however, the characteristics of the Cr/Au diodes improve significantly

Heterojunction Phototransistors For Fiber-Optic Communications

The theory of operation of the heterojunction phototransistor is reviewed and the limitations on gain and speed-of-response are examined in the context of fiber-optic systems requirements. The response of the base potential is shown to depend on the input optical power yielding a power dependent gain-bandwidth product, fT. Model calculations assuming an optimized device structure with peak response in the 1. μm - 1.55 μm spectral region are used to demonstrate the consequences of this dependence. The results indicate that the HPT can have sufficient gain and speed-of-response if a dc bias current is used. A comparison of the S/N of the HPT and an APD is presented to clarify system applicability.

Effect of proton bombardment isolation on the DC and transient characteristics of Schottky barriers on Al x Ga 1-x As

The dark current of Schottky barrier diodes on n-AlGaAs/p-GaAs/n+GaAs structures isolated with a proton bombardment guard ring is measured as a function of proton dose. It was found that the optimum proton dose is near 1014cm-2which resulted in a dark current of 130 pA/cm2. The DC leakage currents are much greater than the dark currents under charge integration conditions and can be explained as a hole current from the p-GaAs to the Schottky metal.

AIGaAs/GaAs Heterojunction Charge-Coupled Devices (CCDs) For Visible/Near Infrared Imaging Applications

The use of AlGaAs/GaAs Schottky gate, heterojunction CCDs for visible/NIR imagers offers a number of distinct advantages over the use of silicon MOS homojunction CCDs, several of which are of particular importance for space applications. The use of the heteroepitaxial technology possible with the GaAs/AlAs alloy system allows the design of devices each of whose layers is optimized to a specific functions. A CCD imager, for example, has a wide bandgap electrically inactive AlGaAs layer for optical access to the device, a narrow bandgap p-GaAs layer as an optical absorber for good response to the visible-NIR spectrum, end an intermediate bandgap n-AlGaAs CCD channel for charge transfer and low dark current. adapting glass bonding technology originally developed for III-V photocathodes, a backside illuminated CCD can be realized which eliminates optical obscuration from the CCD gates. Finally, the fundamental properties of the AlGaAs/GaAs system together with the use of Schottky barrier gates eliminates cross-talk and blooming and results in a device which is intrinsically radiation hard. During this presentation, the above considerations will be discussed in detail and the status of the development program at Rockwell International will be summarized. Important demonstrations including glass bonded, backside illuminated imaging and the observation of room temperature dark currents <100 pA/cm2 will be descirbed.

Observation of two-dimensional charge transfer in GaAs

The first observation of two-dimensional (x-y) charge tranfer in a GaAs charge coupled device (CCD) is reported. A 32×32 bit 4-phase CCD array with serial read-in and read-out linear multiplexer CCDs was fabricated using Schottky barrier gate technology. Two dimensional charge transfer was verified by optically generating a charge packet in the array and transferring it to the read-out multiplexer. This result demonstrates the feasibility of designing GaAs CCD arrays for signal processing, memory and imaging applications.