GaAs MESFET photodetectors for imaging arrays

338 Pages·2015·15.54 MB·English

by  Derek Abbott

r-l- \o "-
GaAs MtrSFET Photodetectors for
Imaging Arrays
by
Derek Abbott, B.Sc. (Hons.)
Thesis submitted for the degree of
Doctor of Philosophy
The lJniversity of Adelaide
Adelaide, South Australia
Centre for Gallium Arsenide VLSI Technology
Department of Electrical and Electronic Engineering
Faculty of Engineering
December 1995

Contents
Abstract
vilt
Declaration
X
Preface
xi
Acknowledgement
xiii
Resume xviii
Publications
xix
List of Figures
xxi
List of Tables xxvii
Glossary
xxix
1 Introduction
1
1.1 Aims and Significance
2
1.1.1 Long Term Aim
. 2
1.I.2
Immediate Aim of this Work
. 2
1.1.3 State of the Art and Present Knowledge in the Area of GaAs Arrays 3
7.L.4 Signiflcance of GaAs I
J
r.2 Overview of XY Array Strategy
6
I.2.I
Photocollection Modes
6
1.2.2 Pixel Array
7
1.3
Research Philosophy
8
I.4
Field of this Research
. 10
1.5 Achievements from this Research
1.6 GaAs Background and Future Vision
1.7 Summary and Chapter Overview
2 GaAs versus Silicon
2.r Introduction
2.2 History
2.2.1 Gallium
2.2.2 Arsenic
2.2.3 Gallium Arsenide
2.3 Competing Technologies
2.3.I
Diamond
2.3.2 Silicon Carbide (SiC)
.
2.3.3 Gallium Nitride (GaN)
2.3.4 PolymerElectronics
2.3.5 Porous Silicon
2.3.6 Silicon-on-Insulator (SOI)
2.3.7 Silicon-Germanium (SiGe) Heterostructures
2.4 The Semi-insulating (SI) GaAs Substrate
2.5 GaAs Characteristics
2.5.I
Bandgap
2.5.2
Speed
2.5.3
Radiation Hardness
2.5.4 Dark Current
2.5.5 Absorption Coefficients
2.5.6 AtmosphericBands
2.5.7
Quantum Efficiency Formulae
2.5.8 Quantum Efficiency & Responsivity
.
ii
2.5.9 Spatial Degradation by Diffusion
58
2.6
Summary
60
3 MESFET Photodetection
67
3.1 GaAs MESFET Optical Model
68
3.1.1 Background
68
3.I.2
Photoresponse Model
69
3.2 GaAs MESFET Photoresponse Measurements
73
3.2.7 Monochromator Measurements
73
3.2.2 Laser Flood Measurements
. 78
3.2.3
Focussed Laser Measurements
81
3.2.3.1 Two Dimensional Laser Scan Set Up 86
3.2.3.2 Two Dimensional Scan Results 86
3.2.3.3 Discussion
90
3.2.3.4 Experimental Refinement 94
3.3
Summary
94
4
Noise
103
4.I
Introduction i04
4.2 A Brief Early History of Noise 104
4.3 A New Derivation of the Thermal Noise Formula 109
4.3.1 Overview of Methods in the Literature 109
4.3.2 The Lumped Model
110
4.3.3 The Windowed Fourier Transform (WFT)
110
4.3.4 The Thermal Noise Formula TT4
4.4 The Classical Energy Catastrophe 118
4.5 The Quantum Enelgy Catastrophe 1i8
4.6 The Steak Grilling Debate
119
nl
4.7
Quantum Cut-Off Experimental Status 720
4.8
MacDonald's Objection . t22
4.9 The Case of Limiting R & C . .
. 122
4.10 kTC Noise
125
4.11 Power in a Matched Load t26
4.12 Distributed RC . r27
4.13 The Capacitor Paradox . t29
4.14 Forward's Photoamplifier
129
4.15 Measurement of Noise
130
4.16 Thermal Noise in MOSFETs
. 134
4.17 Flicker Noise in MOSFETs
. 134
4.18 Thermal Noise in MtrSFETs
135
4.19 Flicker Noise in MESFETs .
136
4.20 Shot Noise in MESFETs .
136
4.21 Noise Analysis of Imager Output Circuit .
137
4.2I.1 Silicon output circuit
139
4.2I.2 GaAs output circuit
i40
4.22 Summary t42
5 Imagers
153
5.1 Introduction
t54
5.2 Pixel Design .
154
5.2.1 Illumination Modes 154
5.2.2 Photocollection Modes 154
5.2.3 Channel to Substrate Mode
156
5.2.4 Gate to Channel Mode
158
5.2.5 Pixel Active Area
159
5.2.6 Pixel Design
159
iv
5.3 Array Organisation r62
5.4 Address Circuitry
166
5.4.I
Case 1: The Capacitively Bootstrapped Shift Register 169
5.4.2 Case 2: Conventional D Flip-Flop Shift Register 17r
5.4.3 Case 3: Edge Triggered D Flip-Flop Shift Register . 174
5.4.4 Register Comparison
174
5.5 Column-to-video line multiplexer
176
5.6 Summary . t77
6 Design Methodology
181
6.1 Introduction
r82
6.1.1 Technology
183
6.1.2 Parasitic Oscillations
184
6.1.3 Layout Approaches
186
6.1.3.1 Symbolic mask level layout 189
6.1.3.2 Practical constraints
190
6.1.3.3 CAD tools
191
6.1.3.4 Modelling of gallium arsenide circuits t92
6.1.3.5 Interface to fabrication
. 193
6.7.4 Performance Study 194
6.1.4.1 Adders . . r94
6.1.4.2 Multipliers .
198
6.2
Summary 20r
7 Visions & Applications
207
7.1 Introduction
208
7.2
Case 1: Aerospace paradigm
208
7.2.I
Image Processing
208
7.2.Ll
Imageregistration . 209
7.2.I.2
Image enhancement . 209
7.2.2 Polynomial Evaluator . . 22t
7.2.2.I Behavioural Model . 222
7.2.2.2 Algorithmic Solution . 223
7.2.2.3 Functional Specification . 224
7.2.2.4 Structural Description (Floor PIan) . 225
7.2.2.5 Symbolic Physical Description . . 225
7.2.2.6 PhysicalLayout . 228
7.3 Case 2: The Mobile Multimedia Paradigm . 235
7.3.1 Complementary Gallium Arsenide . 236
7.3.2 Objectives .'l t F7
. L¿I
7.3.3 Current State-of-the-Art .
237
7.3.4
Sub-systems . 237
7.3.4.1 High-density high-speed RAM . 239
7.3.4.2 Image compression . 239
7.3.4.3 Modulo Multiplier for Data Encryption/Decryption . 240
7.3.4.4 Solid-state imager . 240
7.3.4.5 RF/digital interface . 24r
7.4 Case 3: The Motion Detector Paradigm . 242
7.4.1 Introduction . 243
7.4.2 Comparison with other approaches . 247
7.4.3 Motion Detector Chip . 248
7.4.4 Range Estimation . 253
7.4.4.1 Gallium Arsenide Implementation . . 255
f .i) Case 4: The Spatial Light Modulator paradigm . 257
7.5.I
Significance . 259
vi
7.5.1.1 FLC/Si Technology .
. 259
7.5.1.2 GaAs Technology . . 260
7.5.1.3 Devices . 26r
7.5.I.4 Optical Neural Network . 263
7.6 Techniques and Methods .
264
7.6.1 FLC/Si Devices . 264
7.6.2 LED/GaAs and FLC/GaAs Devices . 266
7.6.3 Smart Pixel Circuitry . 266
7.6.4 SLM and ONN Testing . 266
7.7 Summary .
267
I Conclusions and Recommendations
277
A Finite Difference Method
283
4.1 FDM Voltage Potential Solution . 284
4.2 FDM Electric Field Solution . 285
B Quantum Efficiency 289
B.1 Quantum Efficiency in a Semi-Infinite Slab . 290
8.2 Quantum Efficiency in a Finite Slab . . 29r
C Heat Equations
296
D Complex fntegration
299
D.1 General Complex Integral for Capacitor Problem .
300
D.2 Noise Analysis of 2-Stage RC Ladder .
301
E Polynomial Equations
303
vll
ABSTRACT
Is it possible for an XY addressable imaging array to be realised in standard or near-
standard gallium arsenide digital IC technology? This was the original question at the
heart of this work. The motivation was the ability to embed such an imager with digital
GaAs processing on one IC chip. Integration of an imager with a processor offers the
promise of smart sensors that exploit the excellent power-delay product of GaAs, useful
for mobile applications - where, for instance, image compression may need to take place
in real-time on the same chip. Compact mobile systems of the future can also exploit the
mixed digital/RF capability of GaAs.
Measurements of the photoresponse of GaAs MESFtrTs are used to understand the pho-
todetection mechanisms to aid future study and design. A significant new internal gain
effect, in planar MEStrETs has been discovered which we call the "photovoltaic self-
biasing edge-effect." The edge-effect can be exploited to attain up to a factor of ten
improvement in photosensitivity. In order to project further the advantages of GaAs and
fully understand the challenges, the key differences in device physics between silicon and
GaAs are critically examined.
Theoretical quantum efficiency calculations are carried out and compared against the pho-
toresponse measurements. A new result is obtained showing that the quantum efficiency
in a finite slab (which models an epitaxial layer) is equal to the quantum efficiency in
a semi-infinite slab (which models a bulk substrate) plus a simple add,itiue second order
term.
A new optical technique for extracting semi-insulating (SI) substrate parameters, includ-
ing the carrier diffusion length, is demonstrated showing diffusion lengths of an order of
magnitude lower than in silicon. This is significant for improved spatial resolution in high
definition television (HDTV) imagers. From the perspective of silicon device physics this
is a surprising result, but is explained in terms of the deep levels in the GaAs SI substrate.
Various new pixel schemes and layouts are presented for the realisation of a GaAs XY
addressable imaging array. A number of shift register designs for readout of the imager are
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