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TOF at 10ps with SiGe BJT Amplifiers

Lorenzo PaolozziUniversité de Genève

Time resolution of solid state detectors and design of fast, low noise, charge amplifier

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Time resolution of solid state detectors

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Solid state detectors with planar geometries

Why planar geometry with uniform field:

• Uniformity of electric field. Necessary to have same charge collection time disregarding particle hit position.

• Saturation of carrier drift velocity. Can be achieved more easily in the whole sensor with uniform field.

• Uniformity of Ramo Field. Necessary to have uniform output pulse rise time and to reduce time fluctuation due to charge collection noise.

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Intrinsic crystals used as particle detectors should have high band gap:Diamond Detectors

Excellent carriers velocity. Excellent radiation

hardness.High energy required to

produce an electron-hole pair (13 eV).

Detectors based on PN junction can have a lower band gap, since the free carriers concentration is reduced in the depletion region.

Silicon Detectors

Good carriers velocity. Good radiation

hardness.Low energy required to

produce an electron-hole pair (3.6 eV).

Solid state detectors with planar geometries

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Timing properties of the diamond and silicon crystals:

For solid state detectors with planar geometries based on direct collection of the ionization charge time resolution is typically limited by the signal to noise ratio of the output pulse from the amplifier.

Comparing the expected time resolution with an integrating front end electronics for silicon and diamond the result is:

𝜎 𝑡≅𝑅𝑖𝑠𝑒𝑇𝑖𝑚𝑒

𝑆𝑖𝑔𝑛𝑎𝑙𝑡𝑜𝑁𝑜𝑖𝑠𝑒𝑟𝑎𝑡𝑖𝑜

𝜎 𝑡 , 𝑠𝑖𝑙𝑖𝑐𝑜𝑛≅ 0 .83𝜎 𝑡 ,𝑑𝑖𝑎𝑚𝑜𝑛𝑑

Solid state detectors with planar geometries

Note: at this level time resolution does not depend on detector thickness for penetrating particles.

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• Time resolution for solid state detectors depends on signal to noise ratio.• The noise introduced by the source is typically considered negligible with respect to the

one from the amplifier itself.

For a time resolution below 100 ps, another source of noise should be introduced, that is charge collection noise1.When the ionizing particle traverses the detector, ionization occurs following Landau statistics.

Most of the produced clusters have a small charge. Few events with very large transferred energy are possible.

1 L. Paolozzi, Development of particle detectors and related Front End electronics for sub-nanosecond time measurement in high radiation environment, PhD Thesis

Charge collection noise

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Being the induced current, from Shockley-Ramo’s theorem

when the large clusters are absorbed at the electrodes, their contribution is removed from the induced current.The statistical origin of this variability of the induced current makes this effect irreducible, so that it can be considered as an equivalent noise current.

Time jitter introduced by the charge collection noise for a silicon detector traversed by a Minimum Ionizing Particle (MIP).50% Constant fraction threshold. Weightfield simulation2.

Charge collection noise

2 F. Cenna, N. Cartiglia, Weightfield2: A Simulation Program for silicon detectors, REMDD14, Firenze 2014. https://indico.cern.ch/event/313925/.

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Sensor readout for a TOF PET scanner

Strip readout to limit the number of readout channels:

• Sensor thickness .

• Strip pitch .

• Strip width to have uniform Ramo field.

• Double side readout with mean time measurement to obtain orthogonal coordinate and get rid of signal propagation time on the strip.

Line impedance

Preamplifier must be adapted to the transmission line.

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Design of fast, low noise, charge amplifier.

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Target performance of the new preamplifier

Target time resolution is 30 ps for the MPV of the deposited charge in the sensor.

If the sensor is operated at , it can be demonstrated that the expected time resolution with a charge amplifier operating as an ideal integrator is

𝜎 𝑡[𝑝𝑠 ]=5 .1

Δ𝐸 [𝑘𝑒𝑉 ]𝐸 .𝑁 .𝐶 .

• A target Equivalent Noise Charge of 250 electrons or better is required.

• The amplifier should operate as an ideal integrator for pulses with duration well below 10 ns.

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BJT technology has the best performance in terms of noise when a fast charge integration is operated on input pulses.

Studies on fast, low noise, charge amplifier.

3 E. Gatti, P. F. Manfredi, Processing the Signals from Solid-State Detectors in Elementary-Particle Physics, rivista del Nuovo Cimento Vol. 9, No. 1 (1986).

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The principle of custom charge amplifier in SiGe - BJT technology4:

Working principle and relative output pulse (below)

• Fast pulse integration• Possibility to match the impedance of a transmission line

Studies on fast, low noise, charge amplifier.

Example of Input pulse (above)

4 R. Cardarelli, A. Di Ciaccio, L. Paolozzi, Development of Multi-Layer Crystal Detector and related Front End electronics, Nuclear Instruments and Methods in Physics Research A 745 (2014) 82–87.

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The equivalent noise charge, for fast integration time, is dominated by the series noise:

In order to reduce noise, high value of current gain and small base spreading resistance are necessary.

Charge gain of the charge amplifier:

SiGe technology for very low noise fast amplifiers

𝐴𝑄=𝑑𝑉 𝑜𝑢𝑡

𝑑𝑄𝑠

=− 1

𝐶 𝑓+𝐶𝑑𝑒𝑡

|𝐴𝑣|

𝐸𝑁𝐶𝑠𝑒𝑟𝑖𝑒𝑠𝑛𝑜𝑖𝑠𝑒∝√2𝑘𝑇 ⟨𝑆𝑁𝐼 ⟩ [ (𝐶𝑑𝑒𝑡+𝐶𝑖𝑛)2h𝑖𝑒𝛽

+𝑅𝑏𝑏𝐶𝑑𝑒𝑡2 ]

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Amplifier current gain can be expressed as (NPN BJT)

Increase gain Reduce base width Reducing base doping

Spreading resistance increases!

𝛽=𝑖𝐶𝑖𝐵

=𝜏𝑝𝜏 𝑡

SiGe technology for very low noise fast amplifiers

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A possible approach: changing the charge transport mechanisms in the base from diffusion to drift.

SiGe heterojunction bipolar transistor technology.

Introduces an electric field in the base.

SiGe technology for very low noise fast amplifiers

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• The technology chosen for the Full Custom chip design was the SG25H3 SiGe HBT from IHP.

• Transition frequency of the order of with a 𝛽 value of 150 are available.

• The production consists of two multi-project test runs and is done via Europractice-ic service.

• Layout design and simulation for the first test run was carried out with Cadence.

Development of the new full custom Front End in SiGe technology

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Chip schematic layout

Development of the new full custom Front End in SiGe technology

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Input impedance for amplifier channel 1 (red) and 3 (blue) vs Supply Voltage

Development of the new full custom Front End in SiGe technology

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Output pulse rise time vs input pulse duration for amplifier channel 1 (red) and 3 (blue)

• Both channels operate as ideal integrators for pulses with nanosecond and sub- nanosecond duration.

Development of the new full custom Front End in SiGe technology

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Equivalent noise charge vs detector capacitance for amplifier channel 1 (red) and 3 (blue)

Development of the new full custom Front End in SiGe technology

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Calibration curve for amplifier channel 1 coupled to post-amplifier channel2. Ideal schematics (blue) and Extracted view (red)

Development of the new full custom Front End in SiGe technology

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Discriminator pulse duration vs input pulse charge.

• The compression permits to extend the dynamics of the discriminator up to a factor 200at constant relative error on the input charge.

Development of the new full custom Front End in SiGe technology

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The test preamplifier ASIC performance has been measured

10 100 1000 10000 100000 100000010.00

100.00

Bandwidth channel 3

Vcc = 5VVcc = 4VVcc = 3VVcc = 2V

Frequency (KHz)

Gai

n (d

B)

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100 1000 10000 100000 10000000.00

50.00

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Input Impedance channel 3

Vcc = 5V

Vcc = 4V

Vcc = 3V

Vcc = 2V

Frequency (kHz)

Impe

danc

e (O

hm)

The test preamplifier ASIC performance has been measured