Chất cảm quang có nhiệm vụ là lớp bảo vệ có hình dạng cần thiết cho bề mặt khỏi bị tác dụng của các dung môi hoá học. Người ta phân loại cảm quang thành cảm quang dương và cảm quang âm dựa vào cơ chế phản ứng xẩy ra trong cảm quang khi bị chiếu sáng và sự thay đổi tính chất trong quá trình chiếu sáng. | provided that L is long enough so that the difference between L and Lỵ is negligible because L - Lẩ is fixed by the design rules . Since the L W ratio rather than either L or w determines the resistance it may seem to make little difference what the values of L and IV actually are but this is often not the case. In addition to the difficulty in determining the effective length L or L fur short resistors the edges will typically be somewhat rough due tu unevenness in processing. This unevenness will cause variations in resistance. Making IV and L larger reduces the relative effects of both the edge variations and the difference between L and Lỵ. In addition the power-handling capability will be increased with larger devices. These improvements with larger devices are obtained at the expense of increased area which will reduce the number of dies per wafer. For MOSFETs it will be shown in Chapter 3 that the length width ratio rather than the length and width themselves plays one of the major rules in the device model. Again increasing the area for a fixed length width ratio will reduce the effects of unevenness in the edges but this is obtained at the expense of increased area and increased gate capacitance. As mentioned earlier it is common practice when laying out capacitors to make one plate typically the upper a little smaller than the other so that the smaller plate effectively defines the plate area even if minor misalignments in the masks occur. This also makes the edge field effects easier to account for. Although the area of the smaller plate essentially determines the capacitance the geometry of this plate itself remains to be determined. If the relationship between this capacitor value and others in the circuit is not of major importance the shape of the capacitor will likely be selected to conform to available spaces around adjacent components thus minimizing circuit area. Many applications require that resistor or capacitor ratios be accurately .
