TABLE OF CONTENTS In practice, MOSFETs operate below the ballistic limit because carriers scatter within the device. Some carriers injected from the source into the channel backscatter and return to the source. To determine how far below the ballistic limit a device operates, we can plot the ballistic I-V characteristic for the device as described in Chapter 3 (knowing the device's oxide thickness and power supply and threshold voltages) and then compare the results to the measured data. Typical data for circa 2000 technology with L eff ? 100 nm is shown in Fig. 4.1. These results show that n-channel MOSFETs deliver roughly one-half of the ballistic on-current while p-channel MOSFET deliver roughly one-third of the ballistic current. On the other hand, the measured channel conductance at low drain bias is only one tenth of the ballistic conductance. Other studies of production devices find similar results, [4.1, 4.2] although research devices that operate much closer to the ballistic limit have also been reported [4.3]. It is interesting to note that device scaling is not bringing MOSFETs closer to the ballistic limit. For the past decade or more, n-MOSFETs have operated at about or slightly below one-half of the ballistic limit. Our goal in this chapter is to develop an understanding of how scattering affects transistors and to develop a simple quantitative model that extends the ballistic model developed in the previous chapter.
