تجارت کردن بین کیفیت فیلم پلی سیلیکون و افزایش DC تقویت کننده محاسباتی ترانزیستور فیلم نازک

We have fabricated thin film transistor operational amplifiers on a variety of polycrystalline silicon films. We have examined the open loop DC gain of these modules, and have observed that higher quality polycrystalline silicon films usually cause a negative impact on the DC gain of the amplifier. In this paper we have attempted to quantify this relationship, presenting the gain as a function of the transistor mobility, threshold voltage and channel length modulation parameter, which collectively can describe the quality of the active film. We have found that primarily the saturation characteristics of the transistor, as represented by the channel length modulation parameter, and the device threshold voltage have the biggest impact on amplifier gain.

Thin-film-transistor (TFT) technology presents an exceptionally versatile platform for a variety of applications; some of them are display systems, sensors/actuators and their support electronics, micro-electromechanical systems and others. All of these implementations are characterized primarily by their low cost and high level of integration of devices and systems. TFT circuits can be implemented with either amorphous silicon (as the active layer) or polycrystalline silicon MOS transistors. Polycrystalline silicon TFT circuits possess many advantages over those fabricated on amorphous silicon films. This is due to the superior characteristics of the transistors themselves, as indicated by their higher mobility and lower threshold voltage for similar fabrication conditions, and also due to increased design flexibility, which spurs from the ability to fabricate not exclusively n-channel, but p-channel devices as well. Polysilicon TFTs therefore, represent excellent cost effective alternatives to amorphous silicon TFTs in display applications, as pixel transistors and integrated display driver circuits. Polysilicon films as the active layer for transistors are obtained in a variety of ways, the most common of which are furnace annealing, rapid thermal processing (RTP) or excimer laser annealing of amorphous silicon films. The quality of the resulting polysilicon film is revealed by (among other parameters) the average film grain size and the film trapping density. A number of different experimental measurements for estimating these parameters exist [1]. The quality of the film is also indirectly revealed by the characteristics of the transistors that are fabricated on it; effective carrier mobility and threshold voltage are commonly used to this aim. Because of the wide field of applications for polysilicon CMOS circuitry, the operational amplifier represents a basic building block of systems that use some form of signal conditioning (amplification, active filtering etc.). This is especially true for sensor/actuator systems that integrate transducer elements with data processing and telecommunication electronics. We have fabricated simple operational amplifiers as well as other analog and digital circuits for display driver applications, on polysilicon films that have been crystallized with a variety of different processes, and their characteristics have been described elsewhere [2]. This work discusses how one of the basic performance measures of the operational amplifier, its DC open loop gain, is affected by the polycrystalline silicon film quality. Indeed, it is of interest that the open loop gain was found to decrease as the quality of the active polycrystalline film improved. It will also be shown that this phenomenon is not confined within the limits of this specific design, but inherent to most amplifier designs.

We have theoretically analyzed the impact of the quality of the polysilicon film in the DC open loop gain of a specific TFT operational amplifier. We have verified that the channel length modulation parameter and the transistor threshold voltage have a primary impact on the gain, while the carrier mobility of the film by itself does not affect gain considerably. Overall, assuming similar fabrication conditions and DC biasing conditions, we observe that lower polysilicon film quality results in a higher open loop gain. It is also noteworthy that the value of this conclusion extends well beyond this specific amplifier design. Most operational amplifiers include an amplifying stage at their output, to boost their driving capabilities. Depending on the configuration of the final stage amplifier, the output will be taken across the output impedance of one (common source, source follower configuration) or more (push–pull, cascode configuration) transistors. Therefore, the dependence of gain on lambda will be present in all these cases, in a form at least as A∝λ−1. The largest Lambda will dominate, as the output impedances of the transistors will be in parallel. It is therefore important to achieve good saturation characteristics for the TFTs of the amplifier.