Tham khảo tài liệu 'microengineering mems and interfacing - danny banks part 17', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Output Drivers 303 C B E FIGURE Darlington pair npn . The voltage dropped across the load will not be exactly 10 V because even when the transistor is fully turned on saturated some voltage is still dropped across it VCESAT . Additionally should the load be capacitive as is the case when driving an electrostatic actuator for example then a large amount of current will need to be available to ensure that the capacitor charges rapidly enough. Additional current amplification can be achieved by employing two transistors together as in a Darlington configuration Figure . Darlington transistors are available in a variety of formats and packages. The MOSFET The name metal-oxide-semiconductor field effect transistor MOSFET is derived from its structure. It has three terminals drain source and gate. Current flows between the drain and source through a semiconducting channel in a silicon substrate. This channel is controlled by the metal gate electrode which is separated from the channel by a thin oxide layer. MOSFETs can be found in one of two modes enhancement or depletion and may be either n-channel or p-channel. Enhancement-mode MOSFETs are more commonly employed than the depletionmode and for simplicity these will be the focus of this section. Figure shows the structure and symbols for n-channel Figure and Figure and p-channel Figure and Figure enhancement-mode mOsfets. Note the substrate connections in Figure . For correct operation this has to be connected to the most negative voltage for n-type and to the most positive voltage for p-type transistors. This is required to ensure that the pn junctions between substrate and the active parts of the device channel and source or drain implants are reverse biased. Although the structure of the device looks symmetrical discretely packaged transistors are usually supplied with the substrate internally connected to source which will normally be the more negative n-channel or .
