A short-wavelength selective reach-through avalanche photodiode

A new reach-through avalanche photodiode, designed for use with sources of short-wavelength light such as scintillators, is described. The device has a double junction p⁺-p-n-p^--n⁺ structure in which the central three layers, which comprise about 99% of the device thickness, are fully depleted. The p⁺ light-entry surface extends across the whole device and can be placed in contact with a scintillator. The multiplying p-n junction is buried and is located about 4 μm below the p⁺-layer so that only primary photo-electrons generated by short-wavelength (i.e., strongly absorbed) light are fully multiplied. The p^--n⁺ junction, or array of junctions, is located at the back of the wafer and is surrounded by a guard-ring. Typical characteristics for a device 120 μm thick and having a 25 mm² sensitive area, are a quantum efficiency (Q.E.) of 80% at 480 nm, a capacitance of 30 pF, operating voltage of <500 V, a speed of response of ~3 ns, a noise current of less than 1 pA/Hz^1/2 at a gain of 100, and an effective k value of .030