---
title: Inverse Low-Gain Avalanche Detector (iLGAD)
url: https://www.emergentmind.com/topics/inverse-low-gain-avalanche-detector-ilgad
type: topic
---

# Inverse Low-Gain Avalanche Detector (iLGAD)

Inverse Low-Gain Avalanche Detector (iLGAD) denotes an LGAD-family silicon detector architecture in which the avalanche-multiplication region is kept continuous over the sensitive area while the readout segmentation is transferred away from the multiplication side. In the canonical formulation, iLGAD is a **p-in-p LGAD** with segmented **\(p^+\)** electrodes on the ohmic side and a **non-segmented deep p-well** on the multiplication side, so that gain is intended to remain laterally uniform rather than collapsing in the inter-pad or inter-strip regions characteristic of conventional segmented LGADs [1904.02061]. The concept emerged from the broader LGAD program on moderate-gain avalanche sensors for timing and tracking, and it has since developed into a family of devices spanning strip trackers, hybrid pixels, and soft-X-ray sensors with optimized entrance windows [1511.07175].

## 1. Historical emergence and conceptual basis

The iLGAD concept was introduced as a response to a specific weakness of segmented LGADs. In a conventional LGAD, avalanche multiplication is produced by an optimized \(p^+\) layer under the shallow \(n^+\) diffusion, i.e. the basic APD-derived multiplication structure \(\mathrm{n^{++}\!-\!p^+\!-\!p}\), or, in an explicit sensor stack, \(n^{++}-p^+-p-n^{++}\) [1511.07175; 1312.1080]. The added \(p^+\) implant creates a localized high-field region at a depth of about \(1\text{–}5~\mu\text{m}\), with target gain typically in the range \(10\text{–}100\), far below Geiger-mode devices such as SiPMs [1312.1080]. In that conventional geometry, electron multiplication dominates: at \(270~kV/cm\), the ionization coefficients are approximately \(\alpha_e \sim 0.7\) pair/\(\mu\)m and \(\alpha_h \sim 0.1\) pair/\(\mu\)m, so hole multiplication is comparatively weak [1312.1080].

The timing motivation for the whole LGAD family preceded iLGAD-specific implementations. Early UFSD/LGAD modeling showed that internal gain could drive projected time resolution below \(20~\text{ps}\) for suitable thickness and capacitance conditions, establishing why moderate avalanche gain mattered for 4D tracking [1312.1080]. The 2015 introduction of iLGAD then reformulated the architectural question: instead of segmenting the multiplication junction itself, the multiplication side could remain continuous and the segmentation could be moved to the backside ohmic contact, thereby addressing the spatial non-uniformity observed in strip LGADs [1511.07175].

That shift became experimentally concrete in the first prototype studies from IMB-CNM. The 2019 strip-iLGAD work defined the device explicitly as a **p-in-p LGAD** in which the multiplication layer is continuous and the segmented electrodes lie on the opposite side, so that the device should ideally provide a **constant gain value over the entire sensitive region**, or **“100% fill-factor by design”** in the gain-response sense [1904.02061]. The same design logic later motivated pixel-scale hybrids for Timepix readout and optimized iLGAD peripheries for X-ray use [2409.20194; 2202.01552].

## 2. Device architecture and avalanche signal formation

In the standard iLGAD description, the multiplication layer remains on the junction side, but it is no longer patterned strip-by-strip or pad-by-pad. Instead, the segmented electrodes are implemented at the **\(p^+\)** ohmic side, while the multiplication side contains a **continuous** gain layer [1904.02061]. The 2015 IMB-CNM proposal described this as a “new approach to the charge collection” in which segmentation is implemented at the ohmic contact, thus ensuring uniform multiplication at the \(n^+p\) junction side [1511.07175].

The underlying avalanche law remains that of LGADs generally:
\[
N(x) = N_o * e^{\alpha*x} = G * N_o,
\]
with \(N_o\) the initial carrier population, \(\alpha\) the ionization coefficient, and \(G\) the gain [1312.1080]. What changes in iLGAD is not the basic multiplication mechanism but the spatial relationship between the high-field region and the segmented readout.

That geometric inversion changes signal formation. In conventional LGADs, the segmented junction side favors fast electron-side response. In iLGADs, the readout side is the ohmic side, and the collected signal is correspondingly more hole-dominated. The 2015 paper stated explicitly that the collected signal is due to **holes flowing back from the multiplication junction**, and that the consequence is increased detection time because hole mobility is lower than electron mobility [1511.07175]. The first beam-and-laser prototype study refined this picture: the measured iLGAD waveform contains contributions from **primary electrons** and from **secondary holes produced in the avalanche process**, yielding a slower and more structured pulse than in a reference PIN detector [1904.02061].

For soft X-rays, the architecture was specialized further. In the 2023 soft-X-ray photodiodes, the iLGAD was built on **p-type silicon** with the **gain layer placed on the backplane**, i.e. on the same side as the X-ray entrance window. Under reverse bias, **electrons drift toward the backplane** and **holes drift toward the front-side readout electrodes**. The gain layer is a shallow boron implant confined within roughly **\(1~\mu\text{m}\)** of the back surface, where the electric field exceeds about **\(300~kV/cm\)** [2310.14706]. In that configuration, the measured average gain was defined as
\[
g(E)=\frac{I^{\gamma}_\text{iLGAD}(E,\Phi)}{I^{\gamma}_\text{pin}(E,\Phi)},
\]
using a matched no-gain diode from the same wafer as reference [2310.14706].

A crucial refinement in the soft-X-ray regime is that multiplication depends on absorption depth. The 2023 study distinguished **electron-triggered avalanches** for photons absorbed beyond the gain layer from **hole-triggered avalanches** for photons absorbed before it, with fitted asymptotic multiplication factors satisfying \(M_e > M_h\) [2310.14706]. This depth dependence is not peculiar to photon science; it suggests a general iLGAD principle that pulse formation depends not only on lateral position but also on where the primary charge is generated relative to the continuous gain layer.

## 3. Fill factor, gain uniformity, and the inverse architecture

The central motivation for iLGAD is the **fill-factor problem** of segmented LGADs. In the timing-detector context of ATLAS and CMS, the baseline MTD concept used segmented LGAD pads of order \(1~\mathrm{mm}^2\), yielding timing around **20–30 ps** but poor position resolution; moreover, a minimum-ionizing particle crossing the inter-pad region encountered little or no multiplication, degrading timing severely [1904.02061]. More generally, conventional small-pitch LGADs require per-pixel or per-strip termination of the multiplication region, so the no-gain fraction grows as pitch shrinks [2409.20194].

The first direct experimental demonstration of the iLGAD solution came from strip devices fabricated at IMB-CNM on **\(285~\mu\mathrm{m}\)-thick high-resistivity float-zone wafers** [1904.02061]. In beam tests of unirradiated **45-strip**, **\(160~\mu\mathrm{m}\)-pitch** detectors at room temperature, the conventional LGAD strip showed a **bimodal** charge spectrum at **120 V**: a lower peak around **24 ke**, corresponding to interstrip hits with no gain, and a higher peak around **77 ke**, corresponding to hits under the multiplied strip region; the expected gain was about **3** [1904.02061]. By contrast, the iLGAD strip showed **one peak**, around **75 ke**, indicating that particles crossing anywhere in the sensitive region experienced multiplication [1904.02061]. In the terminology of that paper, this is what **“100% fill-factor by design”** meant: not generic geometric activity, but laterally uniform avalanche-gain response across the full sensitive region [1904.02061].

The earlier 2015 TCAD study had anticipated the same effect. For strip geometries, it showed that in a standard LGAD the high-field region is narrow and concentrated at the strip center, whereas in the iLGAD the maximum electric field is uniformly distributed at the multiplication side. In MIP simulations, gain appeared **wherever the MIP entered** the iLGAD structure, while in the LGAD it was strong mainly at the strip center [1511.07175]. That paper also stressed the trade-off: iLGAD solves gain non-uniformity but does so with hole collection and therefore slower response [1511.07175].

Later pixel-hybrid studies extended the same architectural claim to fine pitch. A **\(55~\mu\text{m}\)**-pitch, **\(300~\mu\text{m}\)**-thick Micron iLGAD bonded to **Timepix3** showed a gain of
\[
G = 4.86 \pm 0.08
\]
with **very good uniformity** across the whole gain area and efficiency exceeding **99.94\%** at **300 V** and **983 \(e^-\)** threshold [2409.20194]. A related **\(55~\mu\text{m}\)**-pitch, **\(250~\mu\text{m}\)**-thick large-area iLGAD on **Timepix4** showed an **almost uniform gain of approximately 4** and efficiency of **\(99.6 \pm 0.1\%\)** [2509.09308]. These results indicate that the inverse architecture remains effective when scaled to small-pitch hybrid pixels, even though timing outcomes depend strongly on thickness and readout conditions.

## 4. Tracking and timing performance

iLGAD timing performance has to be read against the conventional LGAD timing baseline. LGAD/UFSD studies established that internal gain improves timing primarily by increasing \(S/N\), thereby reducing jitter, and projected sub-\(20~\text{ps}\) performance for gain-\(10\) devices under favorable capacitance and thickness conditions [1312.1080]. iLGAD inherits the gain mechanism but not the same signal-formation geometry.

The first dedicated iLGAD strip-timing study therefore used a laser system rather than a beam-particle timing reference. A **1060 nm** picosecond pulsed IR laser was split into two optical paths separated by about **52 ns**; the device signal was amplified by a **Miteq 1660** current amplifier with **60 dB** gain and digitized by a **25 GSa/s** oscilloscope [1904.02061]. Timing extraction used a software CFD, and the time resolution was defined as
\[
\sigma_t = \frac{\sigma(\Delta t)}{\sqrt{2}}.
\]
In the best CFD region—dominated by the fast initial electron-induced part of the pulse—the iLGAD achieved **20 ps** at **700 V** and room temperature, with gain **4.8** [1904.02061]. The same work reported a preliminary spatial resolution of **\(72~\mu\mathrm{m}\)** at **300 V**, while explicitly noting that the beam-test electronics saturated above **100 V**, so the value was not optimal [1904.02061].

Hybrid-pixel measurements have so far yielded slower timing, consistent with thicker hole-collecting implementations. The **Timepix3**-bonded **\(55~\mu\text{m}\)** iLGAD reached hit time resolution down to **1.3 ns** **without correcting for the time-walk effects** [2409.20194]. The **Timepix4**-bonded **\(250~\mu\text{m}\)** iLGAD showed raw timing of about **750 ps**, improving from **746 ps** at **250 V** to **\(377 \pm 7\) ps** after per-pixel timewalk and clock corrections [2509.09308]. Grazing-angle measurements further showed a best depth-sliced timing of **359 ps** at **\(75~\mu\text{m}\)** depth and **900 \(e^-\)** threshold [2509.09308].

The Timepix4 study attributed this limitation primarily to the sensor rather than the ASIC. In that device, the amplified contribution arrives late relative to the weighting-field maximum near the readout implant, so the integrated signal can cross threshold before the multiplication-enhanced part contributes strongly [2509.09308]. A plausible implication is that iLGAD timing depends not only on gain magnitude but on the temporal ordering of charge multiplication and current induction, especially in thick backside-multiplication geometries.

## 5. X-ray and soft-X-ray implementations

iLGAD has also become a photon-science detector technology. The 2022 X-ray periphery study argued that iLGAD is attractive for X-rays because it combines internal multiplication, low readout noise, **100% fill factor**, and compatibility with thick silicon [2202.01552]. The emphasis there was not on the multiplication concept itself but on making the periphery robust under ionizing-dose-induced surface damage.

A parallel line of work optimized iLGADs for **soft X-rays** by combining the inverse geometry with a thin entrance window. In the 2023 SLS characterization, standalone **\(275~\mu\mathrm{m}\)**-thick FBK iLGAD diodes with **4 mm\(^2\)** active area were measured from **200 eV to 1 keV** [2310.14706]. For all sensor variations, the **QE above 250 eV was larger than 55\%**, specifically **55–67\% at 250 eV**, and the charge collection efficiency was found to be essentially **100\%**; the dominant residual loss was photon absorption in dielectric entrance-window layers with total thickness approximately **70–90 nm** [2310.14706]. The same study showed that the average gain increases with photon energy because deeper absorption favors electron-triggered multiplication [2310.14706].

For representative designs, the fitted asymptotic multiplication factors were markedly different for electron- and hole-triggered avalanches. For **W17** (standard gain layer, medium dose), the fit gave \(M_e = 8.88 \pm 0.03\) and \(M_h = 2.25 \pm 0.02\); for **W9** (shallow, high dose), \(M_e = 4.355 \pm 0.006\) and \(M_h = 1.64 \pm 0.01\); for **W13** (ultra-shallow, high dose), \(M_e = 3.12 \pm 0.01\) and \(M_h = 1.50 \pm 0.04\) [2310.14706]. Those values quantify a basic iLGAD feature in the soft-X-ray regime: the same detector can respond differently depending on whether photons are absorbed before or after the gain layer.

System-level applications followed quickly. A **JUNGFRAU** detector equipped with an iLGAD sensor was deployed at the EuXFEL hRIXS spectrometer and achieved **\(19.71 \pm 0.7~\mu\mathrm{m}\)** spatial resolution and a resolving power exceeding **10,000** at **47 kHz** frame rate [2511.12314]. Intra-train-resolved CuO data at **928.5 eV**, **1.1 MHz**, and **\(1.8~\mathrm{mJ/cm^2}\)** showed a decrease in emitted signal by about **10\%** over **\(340~\mu\mathrm{s}\)**, indicating FEL-induced effects that had to be monitored during high-repetition-rate operation [2511.12314]. In a related direction, the Timepix3-bonded iLGAD showed that gain enabled useful X-ray spectroscopy down to **4.5 keV**, with the Ti fluorescence line visible only in the multiplied region [2409.20194].

## 6. Periphery engineering, irradiation, and unresolved constraints

Beyond fill factor, iLGAD development has been shaped by edge termination and radiation response. The first-generation X-ray-oriented iLGAD periphery (**iLG1**) was not designed for oxide-charge buildup at the Si–SiO\(_2\) interface and suffered premature breakdown after irradiation [2202.01552]. TCAD-guided redesign first added floating rings and p-stops on the multiplication side, improving breakdown by about **50 V**, but this was insufficient because the ohmic side also developed high-field peaks under irradiation [2202.01552]. The decisive advance was **double-side optimization**, culminating in **iLG2**, which used multiplication-side floating rings and p-stops together with an ohmic-side field plate, high-doped p-stops, and channel-stopper engineering [2202.01552].

Fabricated on **\(285~\mu\mathrm{m}\)** high-resistivity p-type silicon \(>1~\text{k}\Omega\cdot\text{cm}\), iLG2 showed \(V_{GL}=38~\text{V}\), \(V_{FD}=70~\text{V}\), leakage current about **10 nA**, and breakdown around **450 V** for a **\(1.0 \times 1.0~\text{mm}^2\)** pad, with TCT-measured gain approximately **12 to 24** over **70–360 V** [2202.01552]. After **10 MRad** X-ray irradiation at **1.8 MRad/h**, iLG2 showed only a slight leakage-current increase and retained essentially the same voltage capability, whereas iLG1 degraded to breakdown near **75 V** [2202.01552]. That work established periphery engineering, rather than multiplication-layer redesign alone, as a prerequisite for ionizing-dose robustness in iLGAD X-ray sensors.

Radiation damage in the multiplication layer remains a broader LGAD-family problem. A compensated gain-layer proposal replaced the usual single \(p^+\) multiplication implant by overlapping \(p^+\) and \(n^+\) implants so that the effective doping \(N_A-N_D\) remained near the standard LGAD value while potentially becoming more stable under irradiation [2209.00494]. That concept was not developed specifically for iLGAD, and the paper provided no irradiated hardware validation; however, a plausible implication is that continuous iLGAD gain layers could use the same compensation principle if acceptor and donor removal proved sufficiently balanced [2209.00494].

A second indirect but important result concerns the **acceptor removal phenomenon**. A 2023 defect study argued that LGAD gain loss under irradiation is better explained by formation of a donor-like \(B_{Si}-Si_i\) complex than by older oxygen-centered assignments [2311.07280]. Since iLGAD relies on the same acceptor-based multiplication physics, that mechanism is likely to remain relevant. Likewise, a 2021 gain-suppression study in conventional LGADs showed that high local charge density can screen the gain-layer field and lower the measured gain [2107.10022]. That paper did not study iLGAD, but a plausible implication is that dense localized excitation, including focused X-ray absorption, can perturb multiplication even when the gain layer is laterally continuous.

The main open design trade-off therefore remains unchanged across the iLGAD literature: the inverse architecture cures segmentation-induced gain collapse, but it often shifts the detector toward hole-dominated, thickness-sensitive signal formation. This is why several papers identify **thinner sensors**, **SOI implementations**, higher field near the implant, and optimized electronics as the route to better timing while preserving the uniform-gain benefit of the inverse geometry [1511.07175; 1904.02061; 2509.09308].

Source: https://www.emergentmind.com/topics/inverse-low-gain-avalanche-detector-ilgad