An all-2D MoS 2 phototransistor achieved a detectivity as high as 3.5 × 10 14 cm Hz 1/2 W −1 under a high bias voltage of 10 V, in which a MoS 2 P–N homojunction played the roles of charge separation and a sensitive layer 23. In a hybrid MoS 2/PbS quantum-dot photodetector, photogenerated electrons were transferred to a MoS 2 layer, while photogenerated holes stayed in the quantum-dots 22, and a leakage path was inevitably formed and resulted in a large dark current. For example, charges were transferred from the channel to the bound water molecules on the SiO 2 surface in pristine MoS 2 phototransistors 7, and the poor charge separation ability of water molecules leads to a relatively low detectivity. The photogating mechanism has been widely used to provide a photo gain to improve device performance 4, 5, 6, which is basically achieved either by a trap-assisted photoconductive effect 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27 or by a photovoltaic effect 28. Photodetectors based on two-dimensional (2D) materials usually have a low responsivity and detectivity 1, 2, 3 because atomically-thin 2D layered materials have weak light absorption. The proposed double heterojunction PIBL mechanism adds to the techniques available for the fabrication of 2D material-based phototransistors with an ultrahigh photosensitivity. As a result, a detectivity of 9.8 × 10 16 cm Hz 1/2 W −1 has been achieved. The device works in a photo-induced barrier-lowering (PIBL) mechanism and its double heterojunctions between the channel and the electrodes can provide positive feedback to each other. Here, we report a molybdenum-based phototransistor with MoS 2 channel and α-MoO 3-x contact electrodes. Photogating is widely used to improve the responsivity of devices, which usually generates large noise current, resulting in limited detectivity. However, the intrinsic detection ability of 2D material-based photodetectors is low due to their atomic thickness. Going one step further and looking that number up, shows possible replacements as: 2N2495, 2N3279, 2N3280, 81, 82, 83, and 84, PTC145, GE-51, TR-17, HEPG0008, SK3006, JR200, ECG160, WEP637, 276-2005.And to further complicate matters, if you look up some of those numbers, the subs for them are different.Two-dimensional (2D) materials are promising for next-generation photo detection because of their exceptional properties such as a strong interaction with light, electronic and optical properties that depend on the number of layers, and the ability to form hybrid structures. For instance, Sams Transistor Substitution Handbook 1978 shows both the 121-294 and 295 as being equal to the 2N2654. If you look original part numbers up in several vintage transistor cross-reference books (those printed in the 70's for instance) you are likely to come up with different subs from each book. This is going to make repairing early solid-state equipment that used germanium transistors more and more difficult as the years go by. They should be used only as a last choice if the original cannot be found. So-called substitute transistors may or may not work properly in the circuit without making adjustments to bias, etc. They can go bad with age, just like old wax paper and electrolytic caps do. You have to be really careful with "NOS" germanium transistors if that's what yours are.
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