Analog electronics – general introduction

1. Analog electronics – general introduction • Analog – continuous in time • Digital – discrete in time • Design of amplifiers and filters • ADCs • Logic gates • Receivers, transmitters • Storage cells L 001 010 100 Sensor Filter ADC DSP Amplifier E S 1 1 0 1 0 Advanced analog circuit design

2. Analog electronics – general introduction • Digital design: compromise between power consumption and processing speed • Analog design: compromise between speed, power consumption, resolution, supply voltage, linearity… • Analog circuit are crosstalk and noise sensitive • Analog design can‘t be automatized • Different levels of abstraction G S D B PMOS A Transistor Verstärker System Advanced analog circuit design

3. Analog electronics in scientific applications • Particle detectors with high spatial resolution • Semiconductor detectors with spatial resolution are today widely used in consumer digital cameras, professional HDTV cameras, medical imaging and in science-grade instruments for particle physics, astronomy, material and biology studies (x-ray diffraction imaging, electron-microscopy) and many other fields. • Spatial resolution of semiconductor detectors is achieved by segmenting the sensor surface into many small picture elements ("pixels"). Every segment has its own signal collecting region that can be readout individually. • These detectors are distinguishable from the sensors for consumer electronics either by its low noise and single-particle detection capability or by other properties such as 100% fill-factor, high time resolution, high dynamic range, radiation tolerance, etc. • Multi-channel systems • Pixel electronics • Signal amplification, signal transmission, sampling, comparison, A/D conversion, time-measurements, amplitude measurement • Amplifiers, filters, switched-voltage/current circuits, comparators, A/D convertors, oscillators… • AC analysis, feedback • Transistor models • Noise, threshold dispersion • Semiconductors – solid state physics Advanced analog circuit design

4. Pixel electronics Amplifier P-”guard-ring” N-well Filter Comparator SRAM Hit memory DAC 55 μm Advanced analog circuit design

5. Pixel sensors for particle physics • Pixel sensors are used to detect high-energy charged particles, and to determine particle trajectories. • Since particles tracking requires many layers of planar detectors, tracking sensors should be as transparent for particles as possible. They should be very thin, otherwise the particles will be deflected from their initial trajectories. • Silicon is the best material for such detectors since silicon-based technologies offer the possibility to implement any possible semiconductor device (from PN junction to the completed signal processing electronics) on the sensor. Advanced analog circuit design

6. Pixel sensors for particle physics Advanced analog circuit design

7. Pixel sensors for particle physics Advanced analog circuit design

8. Pixel-sensors for medical imaging • In the case of high energy photon (x-ray or gamma) detection for medical imaging, the requirements are opposite. Photon sensors should be thick enough to absorb the largest part of the radiation. Due to its low absorption coefficient, silicon is not the best material for high-energy photon detection. • The most of practical pixel sensors for such radiation are based on indirect detection. Such sensors consist of a layer of scintillator material that converts the high-energy photons into visible light. The light detection is then performed by a silicon pixel sensor layer. Advanced analog circuit design

9. Pixel-sensors for medical imaging Advanced analog circuit design

10. Classification of pixel-sensors • Hybrid- and monolithic detectors • Monolithic pixel detectors: An n x m pixel matrix is placed on one chip and usually connected by means of signal multiplexing to n (or less) readout channels placed on the same or different chip. Pixels of a monolithic detector must be equipped with a certain readout electronics that at least perform the simplest tasks such as signal clearing, multiplexing and in most cases the amplification. (Some of monolithic detectors employ even more complex in-pixel signal- processing and data reduction. In this case we are talking about "intelligent" pixels that can e.g. detect particle hits, perform A/D conversion, transmit pixel addresses, perform time measurements, etc.) There are n or less connections between the pixel matrix and the block of readout channels. • Hybrid pixel detectors: Each pixel on the sensor chip has its own channel on the readout chip. There are n x m connection between two chips. • Technology – custom or specific • The development of such detectors is relatively low-cost since they use modern commercially available and well characterized CMOS technologies. • Pixel detectors in the technologies that are specially developed or adjusted for particle (or visible light) detection, like the technologies on high resistance substrate, thick epi-layer, etc. Advanced analog circuit design

11. Hybrid detectors with fully-depleted sensors n-type collecting region (n-diffusion) Pixel i Pixel i Potential enegry (e-) Signal collection Substrate P-type Si - depleted P-type Si - depleted P-type Si - undepleted P-type Si - undepleted Advanced analog circuit design

12. Hybrid detectors with fully-depleted sensors • Standard (bump-bonded) hybrid pixel detectors • The bump-bonded hybrid pixel detectors are used in high-energy physics for particle tracking, and in medicine and synchrotron experiments as direct detectors for x-rays. They are based on a relatively simple pixel sensor (ohmic or with pn junctions) without any pixel electronics and bump-connections between the pixel sensor and the readout pixel chip • The connection between the sensor and the readout chip is mechanically complex and expensive, especially in the case of small pixel sizes. Pixel Readout chip Min. pitch ~50 μm Bumps Fully-depleted sensor Signal charge Advanced analog circuit design

13. Hybrid-detector for cell imaging Power/signal supply for RO-chip Bonding matrix for one RO-chip Pixel matrix RO-chip (in a “gel”-pack) Advanced analog circuit design

14. Capacitive coupled hybrid detector Pixel Readout chip Glue Smart diode- or fully-depleted sensor Signal charge Advanced analog circuit design

15. Capacitive coupled hybrid detector Power supply and cont. signals for the readout chip 1.5 mm Power supply and cont. signals for the sensor Readout chip (CAPPIX) Sensor chip (CAPSENSE) Advanced analog circuit design

16. 3D hybrid-detector • 3D-integration is a technology that allows for both vertical and horizontal connection between electronic components placed on different chips (thinned dies) stacked vertically. Pixel Readout chip2 Wafer bond TSV Readout chip1 Wafer bond Fully-depleted sensor Signal charge Advanced analog circuit design

17. Standard monolithic detector - MAPS • In the case of a standard monolithic CMOS sensor ("Monolithic Active Pixel Sensor“) - the sensitive area is undepleted epitaxially-grown silicon layer and the charge is spread and separated by diffusion. Some part of the charge is finally attracted by the next well/diffusion. NMOS transistor in p-well N-well (collecting region) Pixel i P-type epi-layer P-type substrate Energy (e-) Charge collection (diffusion) MAPS Advanced analog circuit design

18. Standard monolithic detector - MAPS • Pixel rows are consecutively "selected" by connecting their outputs (usually single-transistor amplifier outputs) to column lines. The pixel signals are in this way transported to the readout channels. Such a multiplexing requires at least one electronic switch per pixel implemented with a transistor. Select(i) Select(i+1) Signal out P-type epi-layer P-type substrate Advanced analog circuit design

19. Standard monolithic detector - MAPS • MAPS are slower and not as radiation tolerant as the hybrid detectors. • standard MAPS do not allow implementation of complete set of CMOS electronics inside pixels (only n-channel FETs - NMOS transistors - can be used) N-well (collecting region) Pixel i NMOS transistor in p-well PMOS transistor in n-well P-type epi-layer P-type substrate Energy (e-) Signal loss Signal collection MAPS with a PMOS transistor in pixel Advanced analog circuit design

20. Enhanced MAPS Pixel PMOS in a shallow p-well NMOS shielded by a deep p-well N-well (collecting region) P-doped epi layer INMAPS Advanced analog circuit design

21. T-well detector and smart diode array P-well Pixel Deep n-well 2. n-well NMOS PMOS Diffusion Epi-layer Pixel T-well MAPS Potential energy (e-) “Smart” diode Deep n-well Drift Potential energy (e-) Depleted E-field region P-substrate “Smart diode” array Advanced analog circuit design

22. SOI monolithic detector • An SOI detector is based on a modified SOI process. SOI detectors use the electronics layer for the readout circuits and the high-resistivity support layer as a fully-depleted (drift-based) sensor. The sensor is typically 300um thick and has the conventional form of a matrix of pn junctions. A connection through the buried oxide is made to connect the readout electronics with the sensor. CMOS pixel electronics Connection Electronics layer Buried oxide Energy (e-) Support layer Advanced analog circuit design

23. DEPFET monolithic detector Pixel PMOS Ext. gate Clear Elect. Interact. Int. gate Int. gate Signal clearing Potential en. (e-) Signal collection N-substrate (depleted) P-type backside contact Advanced analog circuit design

24. SDD monolithic detector Drift “rings” N-doped collecting region Energy (e-) Depleted n-type substrate Undepleted p-type backside contact Advanced analog circuit design

25. Monolithic detector - SDA ADC channel Pixel matrix 2.7 mm Advanced analog circuit design

26. Amplification • In its simplest form, pixel signal amplification is performed using a single-transistor amplifier. In the case of Field Effect Transistors (FETs), a single-transistor amplifier is sensitive to the voltage change on its input (gate). The charge signal generated by ionization is first collected by the collecting region. The amplifier is coupled with the collecting region by means of DC-coupling (wire) or by use of AC-coupling (capacitance). The conversion factor between the charge signal and the voltage change is the capacitance of the collecting region, referred to as detector capacitance. Clearly the voltage signal will be higher if the collection region has smaller capacitance. • More efficient amplification is achieved by multi-transistor amplifiers. Such amplifiers are typical for hybrid detectors and advanced CMOS monolithic detectors. They are often equipped with feedback circuit which makes the amplification more linear. An example of an amplifier with feedback is the charge sensitive amplifier - CSA. CSA is sensitive only to the charge injected into its input, the capacitance of the input node does not influence the output signal amplitude. Advanced analog circuit design

27. Amplification Bias V Bias V Charge sensitive amplifier Bias R Bias R Out Out Isig Isig Cdet Cdet Simple voltage amplifier (source follower) Detector (equivalent circuit) Detector Advanced analog circuit design

28. Noise • An amplifier not only performs the amplification of the input signal; unfortunately it also introduces electronic noise. Let us explain this: Every amplifier needs to be biased in order to achieve the desired amplification, which means that the amplifier transistor(s) must conduct a certain bias- (DC) current. The signal on transistor's gate will then modulate the current. Thermal motion of the charge carriers inside the transistor active region (channel), leads to bias current fluctuations. These fluctuations are small compared to the bias current itself, but since the bias current is almost always much larger than the signal, its noise can in many cases exceed the signal. A way to decrease the noise is to extend the measurement time (or add a low-pass filter/shaper). Noise signals are random signals with expected value zero and if the measurement takes long time, the average of the noise during measurement interval will in fact approach zero. Most signals, however, have nonzero DC value and they are unaffected by the measurement time. • We could conclude that the detector capacitance does not play any role if we use CSA. This is, however, not true. The noise of a charge sensitive amplifier depends linearly on the detector capacitance. The reason for this is that the negative feedback which cancels the output noise becomes less efficient if the input amplifier node is loaded with a large capacitance. Advanced analog circuit design

29. Noise Advanced analog circuit design

30. „Time walk“ Advanced analog circuit design

31. KTC Noise • Almost every electronic circuit that employs transistors will be affected by their noise. This holds also for the transistor-based pulsed-reset circuit. During the pulsed reset, i.e. when the reset switch is closed, the potential of the collecting region will fluctuate around the desired reset value due to the thermal noise in the reset transistor. When the reset transistor is turned off, the instantaneous value of the reset voltage will be frozen. The instantaneous value is the sum of the desired reset-voltage and the reset error. The reset error superposes to the signal and leads to a measurement uncertainty. It is interesting to note that the reset noise only depends on the detector capacitance (not on the reset transistor resistance): • σ2v = kT/Cdet, • with σ2v variance of the voltage reset error, k Boltzmann's constant, T temperature and Cdet detector capacitance. Advanced analog circuit design

32. KTC Noise Advanced analog circuit design

33. Properties of pixel sensors • Properties • Pixel size • Detector capacitance • Noise • readout amplifier • reset- and bias-resistor noise • The leakage-current noise • σ2v = kT/(gm t). • The magnitude of the noise determines the smallest detectable signal. • Signal to noise ratio (SNR) • SNR is the ratio between a chosen reference signal and the noise. • SNR ~ (gm t)0.5/Cdet • Dynamic range • Dynamic range is the ratio between the greatest undistorted signal (the greatest signal for which the readout does not saturate) and the smallest detectable signal (determined by the noise). • Time resolution • Power consumption • FOM = P t / SNR2 • Radiation tolerance • Fixed pattern noise • FPN refers to a non-temporal spatial noise and is due to device mismatch in the pixels and/or readout channels. • Radiation length Advanced analog circuit design