Analog IC performance: concert of process, IC designer and system needs

Electronic News, August 12, 1996 by Martin Giles

Complementary bipolar processes, such as VIP, now allow the creation of vertical PNPs, thereby bringing their surface base width down to the size of the NPNs and allowing for greatly increased design density and overall device performance. In addition to taking up less space, the reduction of base width also allows for much higher device speeds. For instance, the use of VIP to reduce base widths from 3.0 microns to 0.5 micron, can boost transistor bandwidths from less than 50MHz to more than 400MHz (Fig. 1).

The designer

A major factor in achieving analog device performance is the creative role of the individual analog designer. Today's digital devices are almost invariably designed by large multi-faceted teams, with specialists working separately on cell design, system design, overall architecture, etc. In contrast, the analog designer is often responsible for the entire range of issues, encompassing the device architecture, circuit design and even how individual transistor design is done. Analog designers must work at a fundamental level, understanding the semiconductor physics involved, while simultaneously maintaining a consistent view of the overall objectives for the device.

An effective digital design team must function in concert, with all the precision and discipline of a large symphony orchestra; every note from each instrument must mesh perfectly with all the others according to a well-defined score.

In contrast, the analog design process can often be likened to a jazz jam session. Analog device designers must constantly extend their existing skills to improvise new creative solutions in concert with key support staff, such as mask designers.

The system requirements

The third key factor in successfully achieving analog performance is optimizing communications between the designer and the customer. While it is very important to start with a clearly defined specification from the customer, the analog designer often must go further in understanding overall system objectives. Because of the complexity of analog processes, the greater range of viable design approaches, and the need for analog devices to mesh with real-world phenomena, the more that the designer knows about the ultimate system objectives, the better they can help achieve them.

From a customer standpoint, it is very important to understand the capabilities of the your analog device vendor and to select design partners with enough breadth to optimally meet your objectives. For instance, if you narrowly select a vendor that has only the process capabilities you defined in your initial specification, you can easily become locked into a narrow design channel that does not allow for migration to more optimal processes as the project unfolds.

With any design project, the keys to success are clearly defining the overall objectives and then deploying the best available processes and skills to achieve those goals. In analog device design it is equally important to understand the complexity of the processes and to be aware of how design alternatives can greatly impact final system performance.