UV-Enhanced Si Photodiode

Special UV-enhanced Si photodiodes can be made by positioning the p-n junction close to the surface. Then, quantum efficiencies of 50% can be achieved for A 

Visible- or solar-blind UV sensors can be made from a Si photodiode by additionally using an optical filter that transmits UV radiation only, see below. A more detailed explanation of the physics of UV photodiodes (made from Si as well as from other semiconductor materials) can be found in Ref. [1]. 

The most convincing argument for using Si photodiodes in UV detection is the availability of strong expertise in electronic Si devices. Processing and performance of opto-electronic Si devices have been optimized for decades, and the UV-enhanced photodiode is a high-performance niche product that can be produced at a reasonable price, thanks to these efforts. Its probably most serious drawback is the necessity of using filters for visible-blind applications, which considerably increases the cost of sensors and reduces their otherwise optimum sensitivity. 

Semiconductor photodiodes can be made visible-blind by using a semiconductor with a sufficiently high band gap. Promising materials are SiC (Eg=3.1 eV), GaN (Eg= 3.3 eV) and its related compound AlGaN (Eg=3.3-5 eV, depending on the Al/Ga ratio) and diamond (Eg=5.5 eV) [1], Sometimes, GaP (Eg=2.3 eV) is also used, but due to the low Eg GaP remains sensitive in the blue and green range. 

GaN as a semi-conducting material for electronics is about to be launched on the market, especially for the use in blue- and UV-emitting LEDs and laser diodes [2]. The material is deposited on crystalline substrates like sapphire using thin-film epitactical techniques. Often, metal-organic chemical vapor deposition (MOCVD) is used. The necessity for such technologies limits the production rate and pushes up costs. 

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A popular UV sensor designed for flame monitoring is the SFH 530 sensor by Infinion. It is a UV-enhanced Si photodiode with optical filter, concentrator lens and operational amplifier in a TO-39 package. Flame-sensing in gas ovens is another potential application of the described technique. 

Another problem is degradation of the sensor due to the high UV dose. The radiation resistance of most photodiodes decreases with wavelengths. UV-enhanced Si photodiodes show a loss of 10% in sensitivity already after an accumulated dose of some hundred J/cm2 at X = 254 nm. This is the dose a sensor will have received over the lifetime of an Hg lamp. Special silicon nitride-protected photodiodes are stable up to 105 J/cm2. A filter combined with an attenuator may help to achieve the required selectivity and reduce the exposure of the detector. However, the radiation stability of the filter has to be guaranteed. 

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