Diffused-Junction Detectors

PIPS detectors have been designed to replace SSB (silicon surface barrier) and DJ (diffused junction) detectors in alpha and beta/(conversion-)electron spectroscopies. Passivation normally means a Si02 layer on the surface, a key to low noise and relative ruggedness. Due to ion-implanted junctions, PIPS s have a very thin (<50 nm) and well-defined dead... 

Summary. We discuss how threshold detectors can be used for a direct measurement of the full counting statistics (FCS) of current fluctuations and how to implement Josephson junctions in this respect. We propose a scheme to characterize the full counting statistics from the current dependence of the escape rate measured. We illustrate the scheme with explicit results for tunnel, diffusive and quasi-ballistic mesoscopic conductors. 

From the measured I-V characteristics of the test element group, presented in GB-A-2246662 above, only the presence or absence of the pn-junction in the light responsive region is detected Even if a photo-detector has insufficient spatial resolution caused by the excessive diffusion length of the minority charge carrier in the semiconductor layer in which the pn-junctions are formed, the insufficient space resolution is not detected from the I-V characteristics of the test element group, and this photo-detector is selected as a non-defective. 

Silicon of high purity, normally p-type, is the basic material for this detector type. As with surface-barrier detectors, the silicon piece has the shape of a thin wafer. A thin layer of n-type silicon is formed on the front face of the wafer by applying a phosphorus compound to the surface and then heating the assembly to temperatures as high as 800-1000° C for less than an hour. The phosphorus diffuses into the silicon and dopes it with donors (Fig. 7.20). The n-type silicon in front and the p-type behind it form the p-n junction. 

A lilhium-drifted detector is formed by vapor-depositing lithium on the surface of a p-doped silicon crystal. When the crystal is heated to 4(X) C to 5(X) C the lithium diffuses into the crystal. Because lithium easily loses electrons, its presence converts the p-lype region to an n-type region. While still at an elevated temperature, a dc voltage applied across the crystal causes withdrawal of Ihe electrons from the lithium layer and holes from Ihe p-type layer. Current across thepn junction causes migration, or drifting, of lithium ions into the p layer and formation of the intrinsic layer, where the lithium ions replace the holes lost by conduction. When the crystal cools, this central layer has a high resistance relative to the other layers because the lithium ions in this medium are less mobile than the holes they displaced. 

Silicon semiconductor detectors for nuclear radiation monitors of neutron rays have been developed by Kitaguchi et al. (1995,1996). These are diffused p-n junction-type devices with low leakage current coated on the surface of the B-containing sensor element. Neutrons were detected as recoil protons by interaction of the proton radiator and a-particles generated by the nuclear reaction °B (n, a) Li. The energy response of this radiation detector meets the standard recommendations and is suited as an area monitor and a personal dosimeter as well. 

Operating Temperature. The calculations in the previous sections assumed that only fluctuations in the rate of arrival of photons from the forward hemisphere were important. This is evidenced by the employment of M(v, 7 ), which applies to a hemisphere. If the sensitive element of the detector is at the same temperature as the background, it will receive radiation not only from the forward hemisphere but from the reverse as well. Even though the back side of the element is mounted on a substrate, radiation will enter either through or from the substrate. Whether or not this is important is determined by whether the detector responds only to radiation incident on the front surface. In most photovoltaic detectors the back surface is much farther from the junction than the sum of the optical absorption depth and the carrier diffusion length. Thus most photovoltaic detectors have a preferred surface, and the background limit does not depend upon the mode of operation. 

The time constant of the photoconductive diode is determined by r > RC, where C is the capacitance of the diode plus the input capacitance of the circuit. Its lower limit is set by the diffusion time of the electrons on their way from the p-n junction where they are generated to the electrodes. Detectors from PbS, for example, have typical time constants of 0.1-1 ms, while InSb... 

When the detector is illuminated, electron-hole pairs are created by photon absorption within the p-n junction. The electrons are driven by the diffusion voltage into the n-region, the holes into the p-region. This leads to a decrease A Vd of the diffusion voltage, which appears as the photovoltage Vph = AFd across the open electrodes of the photodiode. If these electrodes are connected through an Ampere-meter, the photoinduced current... 

The short range of alpha particles (25-30 pm) and their significant absorption by detector windows make the measurement of such particles more difficult. The use of samples with a thickness greater than 6mg/cm is a well-established technique but gives poor energy resolution. Thin window proportional counters and pulse ionization chambers have also been used, but the methods of choice today are the surface-barrier and p-n junction detectors. These detectors usually require vacuum and very thin electrodepo-sited samples for spectroscopy measurements. Surface-barrier detectors usually exhibit better energy resolution than the diffused p-n junction devices of similar size. The surface-barrier detectors are more sensitive to the ambient atmosphere and they need an atmosphere free of chemical fumes and water vapor. 

Surface-barrier and diffused p-n junction detectors are the best detectors available for low-energy and heavy-charged particles. Typical detector energy resolutions are in the order of 10-20 keV with 100% detector efficiency. Practical limitations in the construction of these detectors restrict the depletion depths to less than 2 mm. The cost of these detectors is low. 

If an exclusion isojunction layer is situated near the extraction junction at a distance larger than L , the two junctions will decrease the carrier concentration in the detector independently on each other. The simation dramatically changes if these two junctions are situated at a distance smaller than the minority carrier diffusion length. In that case their effects add up and they jointly decrease minority carrier concentration by several orders of magnitude, down to levels beyond reach of each of these mechanisms separately. The majority concentration at that tends to the impurity level. 

The whole concept of extraction photodiodes poses a practical problem, since the requirement exists that the active region thickness must be smaller than the diffusion length. Such a requirement poses design restrictions regarding the absorption path and the total detector active area. One of the approaches utilized to solve the above problem was not to use a single detector but rather a densely packed matrix of separate elements with lateral contacts on a joint substrate and with a joint exclusion junction (Fig. 3.28a) [376]. 

---