Heinrich Dr. Schlötterer

Properties of ESFI MOS transistors due to the floating substrate and the finite volume

The specific current-voltage characteristics of epitaxial silicon films on insulator (ESFI®) SOS MOS transistors are shown, discussed in comparison to bulk silicon MOSTs, and explained by the differences in geometrical considerations, charge distribution, and operation mode, The ESFI MOSTs are produced on silicon islands, in most applications, the electrical substrate is at floating potential. This results in two effects. At first a threshold voltage change occurs with increasing drain voltage, producing a kink in the current curve; if the drain voltage further increases, a parasitic bipolar transistor begins to work and effects another kink or bend in the curve. On the other hand, the finite vo|ume effects a strong dependence of the base width of the parasitic bipolar transistor on the drain voltage and causes a rise of the current amplification with the drain voltage. The finite volume below the gate oxide also limits the bulk-charge magnitudes with subsequent increase in mobile carrier charge, thereby increasing the transconductance. All these effects are also described theoretically; the ID-VDcharacteristics could be simulated by computer model based on the physical effects.

Influence of the floating substrate potential on the characteristics of ESFI MOS transistors

Abstract A model is proposed to explain the anomalous current-voltage characteristics of ESFI MOS transistors. Due to the floating state the substrate potential of the ESFI transistor is increasing with increasing majority carrier current flowing through the substrate to source. In the region of multiplication by avalanche that effect will get quite pronounced. The change of substrate potential yields a change of the threshold voltage hereby increasing the drain current and resulting a bend in the I D (U D ) curves. The assumptions of the model have been justified by additional experiments as with illumination of light or increased temperatures. Based on the physical model a computer program was developed to simulate the I D (U D ) characteristics of ESFI MOS transistors of the enhancement type, resulting good agreement between measured and simulated characteristics.

Mechanical and electrical properties of epitaxial silicon films on spinel

Abstract Silicon films on single crystal magnesium-aluminium spinel slices and similarly on sapphire are strained under compressive stress. This is due to the difference in thermal expansion and to the high growth temperature, causing a curvature of the slices. The coefficients of thermal expansion have been measured by a dilatometer. From the curvature measured by a stylus tracing method a residual stress of about 8 × 10 3 kp/cm 2 has been determined. The expected change in resistivity, effective Hall mobility, and band gap has been calculated from the known influence of stress on the electrical properties of bulk semiconductors, and has been compared with the experiments. The measured mobility as a function of carrier concentration has a maximum between 10 16 and 10 17 carriers per cm 3 . The highest mobilities measured were 600 cm 2 /V-sec for n -type and 350 cm 2 /V-sec for p -type silicon films. The observed decrease of carrier mobility with decreasing carrier concentration (for n , p ≤ 10 16 cm −3 ) and the relatively low minority carrier lifetime are explained by the assumption of space-charges localized at crystal defect accumulations. In accordance with this assumption an anomalous dependence of mobility on temperature has also been found. The existence of localized space charge regions was further confirmed by the change of photo-voltage on scanning the film with a finely focused laser beam. First characteristics of MOSTs on relatively thick silicon films on spinel do not show any remarkable difference from that of transistors on bulk silicon, both having been produced by applying the usual MOS technology.