本論文我們製作了雙波段光偵測器(dual-band photodetector, DBPD)並以雙光源驗證其可行性。此光偵測器是以一般商用的 GaN-based LED磊晶片為基底，並搭配我們實驗室有專利的矽擴散製程，將原先的p-i-n junction結構反轉成n-p-i-n結構，此結構可視為兩個背對背連接的二極體，並透過厚的電子阻擋層(EBL)防止二極體之間電子的穿越，由於PN接面材料與能隙的不同，這兩個二極體具有不同的吸收光譜，實現了具有雙波段偵測特性的光偵測器。兩光偵測器分別為針對短波段入射光吸收的PDF及可吸收長波段入射光和未被PDF吸收的短波段入射光的PDS，在維持低暗電流密度的同時，可選擇性地驅動不同的二極體來實現雙波段的量測，以區分較有害的紫外線光強度與近紫外線/藍光的光強度。
該元件具有良好的暗電流特性，在0 V到4 V的偏壓下PDF及PDS量測到的最大暗電流分別為61.62 pA、80.76 pA，換算暗電流密度分別為34.93 nA/cm2、25.57 nA/cm2，並且透過驅動不同的光偵測器可以測量320 nm到375 nm或320 nm到455 nm的波段。在3 V偏壓下，PDF在入射光波長334 nm具有38.65%的峰值外部量子效率及104.1 mA/W的峰值響應率；PDS在入射光波長338 nm具有34.55%的峰值外部量子效率及94.18 mA/W的峰值響應率。透過我們的驗證實驗也證明了如果已知一寬頻光源，波長在320-455 nm偵測範圍，我們能透過比較PDF及PDS所偵測到的光電流來計算出此光源在320~375 nm與375~450 nm兩波段分別的平均光強度，當然若此光源頻寬很窄又靠近截止波長則較不準，此時若已知此光源的放射波長，可直接用此波長響應率來計算其光強度。

In this work, a GaN-based dual-band photodetector (DBPD) was fabricated and its dual-band detecting function was verified experimentally. Based on a commercial GaN LED wafer, the photodetector was fabricated with our patented silicon diffusion process to convert the original structure (p-i-n) into an n-p-i-n structure which can be considered as a structure of an n-p diode back-to-back connected to a p-i-n diode. The thick electron blocking layer in the LED epitaxial structure that prevents electron spilling from the multiple quantum wells, also prevents electron transport between the two diodes. When connecting the top n-p diode with a reverse bias, it served as a photodiode detecting ultraviolet (UV) light with a cut-off wavelength of approximately 375 nm. When connecting the bottom p-i-n diode with a reverse bias, it served as a photodiode detecting both UV and blue light with a cut-off wavelength of approximately 450 nm which can be changed by changing the bandgap of the active layer used in the LED structure.
The DBPD has excellent electrical characteristics and dual-band spectral responses. The dark current density of the top and bottom photodiodes at 4 V reverse bias were 34.93 nA/cm¬2 and 25.57 nA/cm2, respectively. When reverse biased at 3 V, the top photodiode has a peak external quantum efficiency (EQE) of 38.65% and responsivity of 104.1 mA/W at wavelength 334 nm; in turn, the bottom photodiode has a peak EQE of 34.55% and responsivity of 94.18 mA/W at wavelength 338 nm.
From the results of our verification experiment, we proved that if a board-band light source with wavelengths within the range of 320~450nm, we can distinguish and obtain the average light intensities in the two ranges (320~375nm and 375~450nm), respectively, by comparing the photocurrents of the top and bottom photodiodes. Note that the result may be inaccurate if the source’s spectrum is narrow and close to the cutoff wavelengths. Besides, if the source’s emission spectrum is well known, we can directly use the responsivity to calculate its intensity.

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