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| '''QBD''' is the term applied to the '''charge-to-breakdown''' measurement of a [[semiconductor]] device. It is a standard [[destructive testing|destructive test]] method used to determine the quality of [[gate oxide]]s in [[MOSFET|MOS]] devices. It is equal to the total accumulated [[electric charge|charge]] passing through the [[dielectric]] layer just before failure. Thus QBD is a measure of [[time-dependent gate oxide breakdown]]. As a measure of oxide quality, QBD can also be a useful predictor of product [[Reliability (semiconductor)|reliability]] under specified electrical stress conditions.
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| == Test method ==
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| [[Voltage]] is applied to the MOS structure to force a [[current source|controlled current]] through the oxide, i.e. to inject a controlled amount of charge into the dielectric layer. By measuring the time after which the measured voltage drops towards zero (when [[electrical breakdown]] occurs) and integrating the injected current over time, the charge needed to break the gate oxide is determined.
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| This gate charge [[integral]] is defined as:
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| <math>Q_{bd}=\int_{0}^{t_{bd}} i(t)\, dt</math>
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| where <math>t_{bd}</math> is the measurement time at the step just prior to destructive [[avalanche breakdown]].
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| === Variants ===
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| There are five common variants of the QBD test method:
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| # Linear voltage ramp (V-ramp test procedure)
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| # [[Constant current]] stress (CCS)
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| # Exponential current ramp (ECR) or (J-ramp test procedure)<ref>Dumin, Nels A., ''Transformation of Charge-to-Breakdown Obtained from Ramped Current Stresses Into Charge-to-Breakdown and Time-to-Breakdown Domains for Constant Current Stress'', [http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=00660307]</ref>
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| # Bounded J-ramp (a variant of the J-ramp procedure, in which the current ramp stops at a defined stress level, and continues as a constant current stress).
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| # Linear current ramp (LCR)
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| For the V-ramp test procedure, the [[ammeter|measured current]] is integrated to obtain QBD. The measured current is also used as a detection criterion for terminating the voltage ramp. One of the defined criteria is the change of logarithmic current slope between successive voltage steps.
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| === Analysis ===
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| The [[cumulative distribution function|cumulative distribution]] of measured QBD is commonly analysed using a [[Weibull chart]].
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| == Standards ==
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| === JEDEC Standard ===
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| * JESD35-A – Procedure for the Wafer-Level Testing of Thin Dielectrics, April 2001
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| == References ==
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| {{reflist}}
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| == See also ==
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| * [[Capacitor#Breakdown_voltage|Capacitor - breakdown section]]
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| * [[Field electron emission]]
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| {{DEFAULTSORT:Qbd (Electronics)}}
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| [[Category:Semiconductor device defects]]
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| {{Electronics-stub}}
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