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# Phototransistors Based on III-V Semiconductors and Low-Dimensional Materials for High-Speed Imaging at High Operating Temperatures

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In this thesis, the optical gain mechanism in low-light conditions of phototransistor detectors (PTDs) is explored. An analytical formula is derived for the physical limit on the minimum number of detectable photons for PTDs. This formulation shows that the sensitivity of the PTD, regardless of its material composition, is related to the square root of the normalized total capacitance at the base layer. Since the base total capacitance is directly proportional to the size of the PTD, the formulation reflects the scaling effect on the sensitivity of the PTD. The model indicates that nanoscale PTDs (NPTDs) can reach single-photon sensitivity if their internal gain is high enough to cancel out the noise of the electronic readout circuit (ROIC). Our proposed model can be used to explore a wide range of PTDs, including nanowires, two-dimensional, and bulk devices.\\ Low-dimensional (LD) materials including quantum dots, nanowires, and two-dimensional materials, have been extensively employed to fabricate photodetectors with detection wavelengths ranging from the infrared to ultraviolet region of the spectrum. Literature reports often highlight extremely high photo-responses stemming from large internal gain mechanisms, but the best reported low-dimensional photodetectors have not yet surpassed the performance of bulk photodetectors on the key metric of sensitivity. In chapter \ref{ch:LD}, we introduce a unified picture to study and extract the sensitivity of low-dimensional detectors. We show that the geometry and design of these detectors can be optimized to achieve significantly higher photon detection sensitivity that can in principle exceed that of the best existing photodetectors.\\ InGaAs is the main material that is used for short wavelength infrared (SWIR) detection. InGaAs-based heterojunction phototransistors (HPTs) have been around for more than five decades. We show that 3D-engineered HPTs based on an InGaAs absorbing layer, characterized by nanoscale electronic area and a large optical area, can be used for the fabrication of sensitive SWIR imagers. These devices can reach single-photon noise equivalent power even at room temperature. To the best of our knowledge, this is the first comprehensive study on the sensitivity of the HPTs for imaging applications.\\ Several focal plane array (FPA) based on InGaAs NHPTs were fabricated and tested. Arrays of $320 \times 256$ 3D-egnineered HPT sensors with 2$\mu$m electronic area diameter and 30$\mu$m pixel pitch were used to make the sensor array. Despite the fact that the noise of the read-out circuity (ROIC) is almost 800 $e$-, our demonstrated FPA has only produced around 30 $e$- read noise. The reason for this noise reduction resides in the high internal gain ( $>$1000 $e$-/ph) of the HPT sensors, which practically cancels out the ROIC noise. The measured data show a very good agreement with the model predictions. We also studied the nonlinear relation between the gain and optical power in phototransistors. We have used a simple model based on Schottky-Read-Hall recombination of minority carriers at the base-layer to study this nonlinearity. We then explore the implications of this nonlinear relationship on the design of high-performance phototransistors. Three different HPT structures based on InGaAs collector layer, InP emitter layer, and base layers of different composition were grown, processed and measured. Gain and speed measurements show a large variation between these structures. The one with low-doped InGaAs base-layer has the highest gain at low optical illumination power, which makes it suitable for highly sensitive photodetection. The one with GaAsSb base-layer showed a high optical gain at the higher power of illumination, which makes it suitable for applications that require higher speed of operation. Our study shows that by controlling the carrier lifetime at the base layer, the phototransistor can be tuned to have the maximum gain at the desired optical power. Our modeling can be extended to a variety of detectors with internal gain such as nanowire detectors and detectors based on the 2D material.

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