The catalytic activities are generally believed to be relevant to the electronic states of their active center, but understanding this relationship is usually difficult. Here, we design two types of catalysts for electrocatalytic urea via a coordination strategy in a metal–organic frameworks: CuIII-HHTP and CuII-HHTP. CuIII-HHTP exhibits an improved urea production rate of 7.78 mmol h−1 g−1 and an enhanced Faradaic efficiency of 23.09% at − 0.6 V vs. reversible hydrogen electrode, in sharp contrast to CuII-HHTP. Isolated CuIII species with S = 0 spin ground state are demonstrated as the active center in CuIII-HHTP, different from CuII with S = 1/2 in CuII-HHTP. We further demonstrate that isolated CuIII with an empty $${d}_{{{\text{x}}^{2}\text{-y}}^{2}}^{0}$$ d x 2 -y 2 0 orbital in CuIII-HHTP experiences a single-electron migration path with a lower energy barrier in the C–N coupling process, while CuII with a single-spin state ($${d}_{{{\text{x}}^{2}\text{-y}}^{2}}^{1}$$ d x 2 -y 2 1 ) in CuII-HHTP undergoes a two-electron migration pathway.

In this study, an accurate temperature prediction model is proposed for GaAs HBT, which considers both the bias voltage and current rather than power consumption only. The increase in temperature is closely related to the heat source property, which leads to a complex interaction between the lattice vibration and the uneven distribution of the electric field and current density. To improve the accuracy and stability of the temperature prediction model, a machine learning method of Extreme Learning Machine (ELM) optimized with an Atomic Search Algorithm (ASO) is introduced. The validity of the model is verified by comparing it with experimental observations by the QFI InfraScope TM temperature mapping system. The predicted temperatures for an 8 × 8 HBT power cell fabricated with 2 μm GaAs technology show good agreement with the measurement results, with a ±2 °C error and a relative error deviation below 3%. This demonstrates the superior performance of the proposed model in accurately predicting the temperature of GaAs HBT.

Here, we describe a heterojunction bipolar transistor (HBT) voltage-controlled oscillator (VCO) showing extremely low frequency drift with temperature using a compensation capacitor CCO across the base-collector electrodes. CCO has a 5~10 times value of junction capacitor and it efficiently shields resonance frequency stability from junction capacitance fluctuation with temperature. Taking advantage of the added capacitor CCO, we can use smaller capacitors to implement the Clapp-like structure oscillator with a compact layout. Fabricated with 2μm GaAs HBT technology, the oscillator achieves a frequency drift rate < 0.6MHz/℃ and >8dBm output power in 2.5-7.2GHz output frequency band. Excellent 2nd harmonics suppression <-30dBc and superior phase noise of -87dBc/Hz@10KHz and -105dBc/Hz@100KHz are observed at room temperature.