O. Koybasi

Graphene field effect transistor as a radiation and photodetector

We exploit the dependence of the electrical conductivity of graphene on a local electric field, which can be abruptly changed by charge carriers generated by ionizing radiation in an absorber material, to develop novel highperformance radiation sensors for detection of photons and other kinds of ionizing radiation. This new detection concept is implemented by configuring graphene as a field effect transistor (FET) on a radiation-absorbing undoped semiconductor substrate and applying a gate voltage across the sensor to drift charge carriers created by incident photons to the neighborhood of graphene, which gives rise to local electric field perturbations that change graphene resistance. Promising results have been obtained with CVD graphene FETs fabricated on various semiconductor substrates that have different bandgaps and stopping powers to address different application regimes. In particular, graphene FETs made on SiC have exhibited a ~200% increase in graphene resistance at a gate voltage of 50 V when exposed to room light at room temperature. Systematic studies have proven that the observed response is a field effect.

Graphene field effect transistor as radiation sensor

A novel radiation sensor based on a graphene field effect transistor (GFET) is experimentally demonstrated. The detection relies on the high sensitivity of the resistivity of graphene to the local change of electric field that can result from ionized charges produced in the underlying semiconductor substrate. We present the experimental results of our study on the response of graphene-based radiation detectors to X-rays, gamma-rays, and light photons. We observed increasing resistance change of graphene with increasing X-ray flux in an electrically biased GFET based on Si, SiC, and GaAs substrates. We have measured the temporal characteristics of our detector, along with the sensitivity of the device at high (40 keV, 80 µA) and low (15 keV, 15 µA) X-ray fluxes. Furthermore, we demonstrate room-temperature operation of a GFET based on a SiC absorber and explore new architecture for a faster response.

Guard Ring Simulations for n-on-p Silicon Particle Detectors

We propose a new guard ring geometry for n-on-p silicon particle detectors for high luminosity applications. The performance of the guard ring structure is evaluated with simulations up to a radiation fluence of 1 x 1015 neq/cm2 using an existing three level trap model for p-type FZ silicon. The post-irradiation performance improvement of guard rings with floating field plates pointing towards the sensitive region is demonstrated. The breakdown behavior of the guard ring structure is studied as a function of oxide charge, field plate length, and oxide thickness.

Electrical Characterization and Preliminary Beam Test Results of 3D Silicon CMS Pixel Detectors

The fabrication of 3D detectors which requires bulk micromachining of columnar electrodes has been realized with advancements in MEMS technology. Since the fabrication of the first 3D prototype in Stanford Nanofabrication Facility in 1997, a significant effort has been put forth to transfer the 3D detector technology to large scale manufacturing for future high luminosity collider experiments, in which the radiation hardness will be the primary concern, and other applications such as medical imaging and X-ray imaging for molecular biology. First, alternative 3D structures, single type column (STC) and double-side double type column (DDTC) 3D detectors, were produced at FBK-irst (Trento, Italy) and CNM-Barcelona (Spain), and assessed thoroughly to improve the production technology towards the standard full-3D detectors. The 3D collaboration has been extended to include SINTEF (Norway), which is committed to small to medium scale production of active edge full-3D silicon sensors. This paper focuses on p-type 3D detectors compatible with the CMS pixel front end electronics from the second run of fabrication at SINTEF clean room facilities. The sensors that passed the wafer level electrical characterization have been bump-bonded at IZM (Germany), assembled into modules and wire-bonded for functional characterization at Purdue University. We report the leakage current characteristics, bump-bond quality, threshold, noise, and gain measurement results of these 3D modules as well as the preliminary beam test data taken at Fermi National Accelerator Laboratory.

Graphene field effect transistor-based detectors for detection of ionizing radiation

We present the results of our recent efforts to develop novel ionizing radiation sensors based on the nanomaterial graphene. Graphene used in the field effect transistor architecture could be employed to detect the radiation-induced charge carriers produced in undoped semiconductor absorber substrates, even without the need for charge collection. The detection principle is based on the high sensitivity of graphene to ionization-induced local electric field perturbations in the electrically biased substrate. We experimentally demonstrated promising performance of graphene field effect transistors for detection of visible light, X-rays, gamma-rays, and alpha particles. We propose improved detector architectures which could result in a significant improvement of speed necessary for pulsed mode operation.

Design and simulation of a graphene DEPFET detector

Graphene field effect transistors (GFETs) fabricated on undoped semiconductor substrates have shown promise for sensing ionizing radiation with a potential of high sensitivity, low noise, low power, and room temperature operation. Radiation detection with GFET is based on the high sensitivity of graphene resistivity on local electric field perturbations caused by ionized charges generated in an electrically biased radiation absorbing semiconductor substrate. Those charges are drifted to the neighborhood of graphene by the gate voltage applied across the detector. GFET radiation sensors can be fabricated on a variety of substrates, exploiting their distinct material properties, to address different application regimes. The current simple GFETs lack the functionality of efficiently removing ionized charges accumulated underneath graphene after signal readout, which results in a slow response to irradiation cut-off and therefore compromises the ability to operate in the pulse mode. In order to overcome this limitation, we propose here a more advanced device architecture, namely, graphene DEPFET.

Design, Simulation, Fabrication, and Preliminary Tests of 3D CMS Pixel Detectors for the Super-LHC

The Super-LHC upgrade puts strong demands on the radiation hardness of the innermost tracking detectors of the CMS, which cannot be fulfilled with any conventional planar detector design. The so-called 3D detector architectures, which feature columnar electrodes passing through the substrate thickness, are under investigation as a potential solution for the closest operation points to the beams, where the radiation fluence is estimated to reach 1016 neq/cm2. Two different 3D detector designs with CMS pixel readout electronics are being developed and evaluated for their advantages and drawbacks. The fabrication of full-3D active edge CMS pixel devices with p-type substrate has been successfully completed at SINTEF. In this paper, we study the expected post-irradiation behaviors of these devices with simulations and, after a brief description of their fabrication, we report the first leakage current measurement results as performed on wafer.

Detection of light, X-rays, and gamma rays using graphene field effect transistors fabricated on SiC, CdTe, and AlGaAs/GaAs substrates

Our work demonstrates the potential of gated graphene field effect transistors (GFETs) fabricated on a variety of undoped semiconductor substrates such as SiC, CdTe, and GaAs to sense ionizing radiation with promise of high sensitivity, low noise, low power, and room temperature operation. We exploit distinct material properties of different substrates to address different application regimes. Radiation detection with GFET is based on the high sensitivity of graphene resistivity on local electric field perturbations caused by ionized charges generated in the radiation absorbing semiconductor substrate. Light, X-rays, and gamma rays have been detected in our experiments.