Introduction to radiation-resistant semiconductor devices and circuits

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OUTPUT NOISE VOLTAGE DENSITY [μV/√Hz] 0.15 0.10 0.05 0.00 post-rad pre-rad 1E+5 1E+6 1E+7 1E+8 FREQUENCY [Hz] FIGURE 5. Noise of a bipolar transistor preamplifier before and after irradiation to a fluence of 1.2⋅10 14 cm -2 (800 MeV protons). Figure 4 shows measured DC current gain for npn and pnp bipolar transistors irradiated to a fluence of 1.2⋅10 14 cm -2 (800 MeV protons). (26) These devices, fabricated in AT&T’s CBIC-V2 high-density complementary npn-pnp IC process, exhibit fT = 10 GHz for the npn and 4.5 GHz for the pnp transistors. In the CAFE chip designed for the ATLAS silicon tracker (29) the npn input device is operated at a current density of about 2 μA/(μm) 2 , where the post-rad current gain decreases to about 60% of its initial value. Although a smaller transistor would deteriorate less, the thermal noise contribution of the parasitic base resistance would be excessive, so a compromise is necessary. No measurable changes in transconductance were measured, as expected. The output resistance of these devices decreased by
Junction Field Effect Transistors (JFETs) JFETs (either silicon or GaAs) can be quite insensitive to both ionization and displacement effects. In these devices a conducting channel from the source to the drain is formed by appropriate doping, typically n type. The gate electrode is doped p type so that applying a reverse bias voltage relative to the channel will form a depletion region that changes the cross section of the conducting channel. (31) At low values of gate and drain voltages the channel is contiguous and resistive. At higher voltage levels the channel becomes fully depleted near the drain, but the current flow is still determined by the conducting channel near the source. Since the gate voltage now controls both the geometry and potential distribution, voltage-current characteristics become more complex and the device acts much like a controlled current source, i.e. it exhibits a high output resistance. With respect to radiation effects, the important fact is that device characteristics are determined essentially by the geometry and doping level of the channel. Typical doping levels are 10 15 to 10 18 cm -3 , so the effect of radiation-induced acceptor states is small. Silicon JFETs exhibit very good radiation resistance. Measurements on both standard commercial devices and custom designed integrated circuits have shown minimal changes in gain at fluences >10 14 neutrons/cm 2 and ionization doses up to 100 Mrad. (32,33,34). Low frequency noise (f < 100 kHz) may increase by an order of magnitude, but at high frequencies very little change in noise is observed. Measurements of Si JFETs at 90K also exhibit excellent radiation characteristics. (33) In some applications, analog storage circuitry for example, gate leakage current is important. Generation current in the gate depletion region due to displacement damage can affect the gate current strongly. Measurements on commercial JFETs irradiated by high-energy electrons to 100 Mrad (Φ≈ 10 15 cm -2 ) show the gate reverse current increasing 100 fold from an initial value of 70 pA. (35) Here one should choose the smallest geometry device commensurate with other requirements. At this point it is worth noting that the superior radiation resistance claimed for GaAs ICs has more to do with the use of JFETs or MESFETs (a Schottky barrier JFET) than the properties of the semiconductor. These devices are more radiation resistant than silicon MOSFETs (discussed below), but suffer from a much lower circuit density. 15
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Page 31: 32. Citterio, M. et al., “A Study

Introduction to radiation-resistant semiconductor devices and circuits

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