Noise Characteristic Of AlGaN Avalanche Photodiodes: Measurement And Analysis

In the ultraviolet(UV) illumination and detection field, Ga N-Al Ga N based material has lots of advantages than other wide-band-gap materials, which includes the direct and changeable band gap, working under large power, high temperature and high speed conditions. 2014, Shuji Nakamura with other two Japanese scientists won the Nobel Prize for their breakthrough on Ga N films growing and Ga N p-type doping, which confirming the importance of Ga N based detectors and LEDs. In recent ten years, studies on Ga N-Al Ga N based avalanche photodiodes(APDs) have gained a large improvement. Chinese researchers also contributed a lot in this filed. On the other hand, noise characteristic and impact ionization factor, as two important characteristics of Al Ga N based APDs, has been reported by R. Mc Clintock, Turgut Tut and etcetera in recent years. In my department, although the Ga N- Al Ga N based APDs have been studied for years by Jintong Xu and his group, noise measurement and the study of noise characteristics are still less. This thesis paid attention to noise measurements of Al Ga N solar-blind APDs. The works in this thesis includes: 1, measurement system setting up and adjustment; 2, measuring Si based APDs to test the measurement system and comprehend the law of noise performance; 3, measuring the noise of Al Ga N APDs and try to simply analyzing the noise characteristics of Al Ga N APDs.First of all, a noise measurement system which can satisfy the requirements of high bias voltage and small signals was set up. In order to shield the system from interference of industry frequency and space electromagnetic fields, plural shielding boxes and effective connecting to ground were used. Following this the system was adjusted. The model SR570 preamplifier was found a non-standard frequency response function when lower than 3 d B band width. While the frequency response of model DL1211 preamplifier drops rapidly near the margin of 3 d B band width. To ensure the system can measure the noise accurately, the frequency response function of preamplifier was calibrated by measuring and calculating the noise of resistances.High quality Si APDs were used to test the noise measurement system. The noise spectral densities of Si APDs were measured and, then, fitted. Si APDs were dominated by excess shot noise. When Multiplication lower than 300, excess noise factor fulfilled the classical theory when the ratio of impact ionization factor was 0.1- 0.2. When multiplication higher than 300, the performance of excess noise factor was like the reported results with “dead space” effect. The noise equivalent power of Si APDs was calculated, and was contrasted with the counterpart in the guideline, which showed a good match. Broadband noise of Si APDs was measured, and signal-noise ratio was calculated. Results showed that the bias voltage at which largest ratio obtained was dominated by both noise of APD and noise of background environments. Only when the noise of APD and background were close, the signal-noise ratio got the largest point.Al Ga N APDs were produced, and the current- voltage relation and noise characteristics of these APDs were measured. Most of these devices appeared avalanche breakdown at a bias voltage from 90 V to 130 V. Noise spectral densities were measured. Till 12 k Hz, only low- frequency noise was observed. The noise spectrums were fitted. Results showed that main component of the noise accepted the Hooge’s 1/f noise, and there might be burst noise component mixed. Because the Hooge’s noise can be observed only in devices which have relatively good surface quality, the result reflected a good surface passivation. And on the other hand, the 1/f noise domination reflected reletive serious problems on material quality. The signalnoise ratio was calculated from noise spectral density. Results showed a largest ratio at about 60 V, which far from the breakdown voltage, and the corresponding multiplication is below 50. This result is agreed with the measurements on Si APDs, which be connected to the noise of environment background. And then the broadband noise was measured and signal- noise ratio is calculated. Although the calculated results showed that the largest ratio appeared at high a voltage close to the breakdown, the relationship between the ratio and bias voltage or background noise was not obvious. The signal- noise ratio was decided by the noise of Al Ga N APD itself at particular conditions.