When a positive gate voltage is applied to an amorphous-silicon thin-film transistor, electrons become trapped in states close to the silicon-dielectric interface. This is studied by a new technique involving the transient discharge current produced under illumination. It is suggested that the behavior may involve metastable dangling bonds generated within the amorphous silicon as a consequence of the field-effect-induced increase in electron concentration. This constitutes an important new instability mechanism for amorphous-silicon thin-film transistors.PACS numbers: 71.55.Fr, 73.40.Qv, 85.30.Tv Amorphous-silicon thin-film transistors (TFT's) are currently of great commercial interest. The best such devices presently available are fabricated with a hydrogenated-amorphous-silicon (a-Si:H) active layer and an amorphous-silicon-nitride gate dielectric, both grown by the glow-discharge technique. 1 The trapping of charge in the region of the semiconductor-dielectric interface is of considerable importance in determining device stability. A TFT subjected to a large positive gate voltage for a long period exhibits a large positive shift in threshold voltage. This was interpreted in terms of electron injection into the nitride. 2 Additionally, from studies of the silicon-nitride interface, Street and Thompson 3 have suggested that appreciable charge storage occurs in the silicon, involving states of depth 0.25 eV, plus deeper centers not resolved by their particular experimental technique. TFT's have also been shown to alter their characteristics as a result of prolonged illumination, 4 in line with the known creation of metastable dangling-bond states as observed in the Staebler-Wronski effect. 5 In this paper, we describe a new photoconductive technique for the study of excess trapped carriers. The devices used were prepared by the glow-discharge decomposition of silane in a capacitatively coupled reaction chamber, as described elsewhere. 6 The experimental process involves the photoinduced discharge of devices in which charge has been allowed to accumulate during a period of application of a gate voltage. To avoid complications associated with transient changes in source-drain current under illumination, these electrodes were linked [ Fig. 1(a)]. As indicated in Fig. 1 (b), a gate field was applied for a period t g and then removed. Under these conditions, the source and drain contacts are injecting, and excess electron majority carriers are drawn across the a-Si:H layer towards the silicon-nitride interface, in a time much shorter than the t g values used here. After a time delay of t d , the specimen was briefly illuminated, yielding a discharge current pulse which was detected across resistor R. Illumination for 150 msec at a flux of 10 16 cm"" 2 sec~l was sufficient to produce a complete discharge. The total charge Q, assessed by integration of the transient current through 7?, established the residual number of carriers trapped. Figure 2(a) displays the influence upon Q of delay time and temperature. To obtain...

Phys. Rev. Lett. 56, 919(1986)].The sentence beginning on line 11 of the right-hand column on page 921 which reads, "The deviation counting rate is very small and the combination of counting rates plotted in Fig. 2 is very sensitive to background" should be replaced by "The deviation for the highest-energy points occurs near the end point where the £-decay counting rate is very small and the combination of counting rates plotted in Fig. 2 is very sensitive to background."

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