Hot Carriers

The term 'hot carriers' refers to either holes or electrons (also referred to as 'hot electrons' ) that have gained very high kinetic energy after being accelerated by a strong electric field in areas of high field intensities within a semiconductor (especially MOS) device. Because of their high kinetic energy, hot carriers can get injected and trapped in areas of the device where they shouldn't be, forming a space charge that causes the device to degrade or become unstable. The term 'hot carrier effects' , therefore, refers to device degradation or instability caused by hot carrier injection. According to the 5th Edition Hitachi Semiconductor Device Reliability Handbook, there are four (4) commonly encountered hot carrier injection mechanisms. These are 1) the drain avalanche hot carrier injection; 2) the channel hot electron injection; 3) the substrate hot electron injection; and 4) the secondary generated hot electron injection.



The drain avalanche hot carrier (DAHC) injection is said to produce the worst device degradation under normal operating temperature range. This occurs when a high voltage applied at the drain under non-saturated conditions (VD>VG) results in very high electric fields near the drain, which accelerate channel carriers into the drain's depletion region. Studies have shown that the worst effects occur when VD = 2 VG . The acceleration of the channel carriers causes them to collide with Si lattice atoms, creating dislodged electron-hole pairs in the process. This phenomenon is known as impact ionization , with some of the displaced e-h pairs also gaining enough energy to overcome the electric potential barrier between the silicon substrate and the gate oxide. Under the influence of drain-to-gate field, hot carriers that surmount the substrate-gate oxide barrier get injected into the gate oxide layer where they are sometimes trapped. This hot carrier injection process occurs mainly in a narrow injection zone at the drain end of the device where the lateral field is at its maximum. Hot carriers can be trapped at the Si-SiO2 interface (hence referred to as 'interface states') or within the oxide itself, forming a space charge (volume charge) that increases over time as more charges are trapped. These trapped charges shift some of the characteristics of the device, such as its threshold voltage (Vth) and its conveyed conductance (gm). Figure 1. DAHC injection involves impact ionization of carriers near the drain area; source: Hitachi Semiconductor Reliability Handbook Injected carriers that do not get trapped in the gate oxide become gate current . On the other hand, majority of the holes from the e-h pairs generated by impact ionization flow back to the substrate, comprising a large portion of the substrate's drift current . Excessive substrate current may therefore be an indication of hot carrier degradation. In gross cases, abnormally high substrate current can upset the balance of carrier flow and facilitate latch-up.



Channel hot electron (CHE) injection occurs when both the gate voltage and the drain voltage are significantly higher than the source voltage, with VG≈VD. Channel carriers that travel from the source to the drain are sometimes driven towards the gate oxide even before they reach the drain because of the high gate voltage. Figure 2. CHE injection involves propelling of carriers in the channel toward the oxide even before they reach the drain area; source: Hitachi Semiconductor Reliability Handbook Substrate hot electron (SHE) injection occurs when the substrate back bias is very positive or very negative, i.e., |VB|>> 0. Under this condition, carriers of one type in the substrate are driven by the substrate field toward the Si-SiO2 interface. As they move toward the substrate-oxide interface, they further gain kinetic energy from the high field in surface depletion region. They eventually overcome the surface energy barrier and get injected into the gate oxide, where some of them are trapped. Figure 3. SHE injection involves trapping of carriers from the substrate; source: Hitachi Semiconductor Reliability Handbook 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 AF = e ([Ea/k] [1/T1-1/T2] + C [V2-V1]) where: AF = acceleration factor of the mechanism; R1 = rate at which the hot carrier effects occur under conditions V1 and T1; R2 = rate at which the hot carrier effects occur under conditions V2 and T2; V1 and V2 = applied voltages for R1 and R2, respectively; T1 and T2 = applied temperatures (deg K) for R1 and R2, respectively; Ea = -0.2 eV to -0.06 eV; and C = a constant.



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