Isothermal Current Instability in Silicon BJT and MOSFETs
Typical requirements of high middle time before failure (MTBF) ~ 106 hours result in application of FET devices under conditions of rather low operation temperatures < 120° C due to the activation energy ~ 0.7-1.2 eV for basic degradation mechanisms.
In state-of-the-art FETs, the temperature of the active region under operation (calculated assuming uniform dissipated power distribution) is significantly lower than the critical temperature of thermal instability. Even a few orders of magnitude overload in the dissipated power cannot result in the critical thermal instability temperature of ~ 300° C for Si FETs or ~ 500° C for GaAs FETs. However, at some critical pulse or DC voltage level a catastrophic failure is observed. The reason for failure is an electrical conductivity modulation followed by current instability. The instability results in the current-controlled negative differential resistance (NDR) followed by uncontrollable current increase and filamentation. In a very short time, this high Joule power level is generated in the filament region and further results in the local burnout. Since this type of current instability can be observed in a short pulsed regime (2-20 ns) at rather negligible level of Joule overheating, the basic physical mechanism of the electrical burnout is an isothermal instability phenomenon. In these conditions, the initial transistor temperature is just a passive parameter. As a rule, the typical time of isothermal instability evolution is a few orders shorter than the typical time of thermal instability. Thus, a simple identification of electrical or thermal nature of the instability can be provided by pulsed measurements. As will be demonstrated in Chapters 4 and 5, the basis of electrical mechanisms of catastrophic failures is the breakdown and instability in elementary semiconductor structures (n-i-n, p-i-n, and so on). However, as usual in real transistors the current instability may not converge down to just the instability in the elementary diode structure. In most practical cases, the avalanche-injection breakdown in one part of the transistor may initiate instability in another part of the device.
KeywordsBreakdown Voltage Space Charge Region Bipolar Transistor Negative Differential Resistance Negative Differential Conductivity
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