Hot Carrier Effects (Page 3 of 3)

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Secondary generated hot electron (SGHE) injection involves the generation of hot carriers from impact ionization involving a secondary carrier that was likewise created by an earlier incident of impact ionization.  This occurs under conditions similar to DAHC, i.e., the applied voltage at the drain is high or VD>VG, which is the driving condition for impact ionization. The main difference, however, is the influence of the substrate's back bias in the hot carrier generation. This back bias results in a field that tends to drive the hot carriers generated by the secondary carriers toward the surface region, where they further gain kinetic energy to overcome the surface energy barrier.

Figure 4.  SGHE injection involves hot carriers generated by secondary

carriers; source: Hitachi Semiconductor Reliability Handbook 

Hot carrier effects are brought about or aggravated by reductions in device dimensions without corresponding reductions in operating voltages, resulting in higher electric fields internal to the device. Problems due to hot carrier injection therefore constitute a major obstacle towards higher circuit densities. Recent studies have even shown that voltage reduction alone will not eliminate hot carrier effects, which were observed to manifest even at reduced drain voltages, e.g., 1.8 V.  

Thus, optimum design of devices to minimize, if not prevent, hot carrier effects is the best solution for hot carrier problems. Common design techniques for preventing hot carrier effects include: 1) increase in channel lengths; 2) n+ / n- double diffusion of sources and drains; 3)  use of graded drain junctions; 4) introduction of self-aligned n- regions between the channel and the n+ junctions to create an offset gate; and 5) use of buried p+ channels.

Hot carrier phenomena are accelerated by low temperature, mainly because this condition reduces charge detrapping. A simple acceleration model for hot carrier effects is as follows:

AF = R2 / R1