Performance analysis of a thermionic thermal detector at 400 K, 300 K, and 200 K

Abstract

The Thermionic Thermal Detector (TTD) senses infrared radiation by temperature modulation of thermionic emission current within a silicon Schottky diode. The thermionic emission current is the well known Richardson dark current. The TTD operates in the LWIR band. The physics of TTD operation is distinct from that of silicon Schottky barrier MWIR detectors, such as PtSi/Si which are based on internal photoemission. In fact, the TTD has high detection efficiency. The architecture of a TTD array is very similar to that of microbolometer arrays, except the detector elements are thermally isolated Schottky diodes, operating under reverse bias. When the TTD array is illuminated by an infrared image, the temperature of individual detector elements will vary with the local incident power of the image. Under small signal conditions, the dark current of individual detectors will vary as temperature, resulting in an electronic image of the infrared scene.The reverse bias dark current of a Schottky diode varies exponentially with temperature. For the small temperature variations observed on the focal plane of an uncooled sensor, this variation is approximately linear. The rate of temperature variation is determined by the Schottky barrier potential and, to a lesser extent by the applied bias potential. The operating temperature range of the detector can be designed into the device by selecting a metal with the appropriate Schottky barrier height. Experimental Schottky barrier heights were determined using Richardson dark current activation energy analysis. Devices optimized for operation at room ambient temperature have a 6%/K temperature coefficient. The use of Schottky diode thermionic emission for uncooled infrared imaging offers several advantages relative to current technology. TTD manufacture is 100% silicon processing compatible. Schottky barrier based thermionic emission arrays have the same uniformity characteristics as MWIR Schottky barrier photoemissive arrays. Operating TTDs in reverse bias provides a high impedance " current source" to the multiplexer, resulting in negligible Johnson noise. This mode of operation also results in negligible detector i/f-noise and drift. In addition, the TTD thermionic emission detection process has high efficiency, fully comparable with the best current thermal detectors. This paper describes the theoretical performance of a TTD based LWIR sensor. The focal plane architecture and multiplexer is similar to current microbolometer arrays, except the VO resistor is replaced with a TTD thermionic emission sensing element. The analysis models the detector using the appropriate optical radiation, thermal diffusion, and electrical conduction equations for 400K, 300K and 200K operation, using three different metal silicides. The effects of the optical radiation noise, thermal diffusion noise, Johnson noise, shot noise, and amplifier noise are considered in the analysis. Samples of the TTD designed for operation at 300 K and suitable for integration with a multiplexer, were fabricated and ...

Silicon-integrated uncooled infrared detectors: Perspectives on thin films and microstructures

“…Thermionic Schottky-barrier devices have performance advantages over thermoresistive devices (e.g., VO x films), owing to the absence of 1/f noise and low operating current. 29 The design in Fig. 2 has the further advantage in that it generates a thermodynamic output signal with a self-referencing zero offset.…”

New Type Far IR and THz Schottky Barrier Detectors for Scientific and Civil Application

“…However, the extra energy of these "hot" electrons gets transferred to all the free electrons in the metal silicide through inelastic electron-electron collisions within time τ e-e ≤ 10 −12 s. At the same time, at a low temperature the electron-phonon energy relaxation time appears to be much longer and lies within 10 −7 -10 −10 s. At this condition an electron gas temperature increase has been observed which results in a growth of the probability of the states where the energy levels greater than Ψ MS are occupied by holes (in the p-type silicon) and, therefore, an occurrence of an additional current of thermionic emission from metal into the semiconductor. The difference between the currents of the thermionic emission at absorption of the radiation and without the absorption creates a signal current as in [8].…”

“…To provide maximum relative increase in the detector's temperature (per unit of the absorbed energy) and prevent leakage of the heat flow to the substrate, the full thermal capacity of the detector has been reduced as much as possible. In particular, such SB thermionic detectors are considered in [8].…”