Editing QBD (electronics)

{{Technical|date=September 2010}}

'''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 [[electric charge|charge]] passing through the [[dielectric]] layer (i.e. electron or hole [[fluence]] multiplied by the elementary charge) 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.

== Test method ==

[[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. 

This gate charge [[integral]] is defined as:
<math display="block">Q_\text{bd} = \int_{0}^{t_\text{bd}} i(t)\, dt</math>
where <math>t_\text{bd}</math> is the measurement time at the step just prior to destructive [[avalanche breakdown]].

=== Variants ===

There are five common variants of the QBD test method:

# Linear voltage ramp (V-ramp test procedure involving the [[current–voltage characteristic|current-voltage characteristic curve]]  (I–V curve) using a linearly increasing and/or decreasing voltage as in a [[sawtooth wave]] or [[triangle wave]])
# [[Constant current]] stress (CCS)
# Exponential current ramp (ECR) (involving the I–V curve using an exponentially increasing and/or decreasing voltage as in a [[RC_time_constant|RC time constant charging/discharging-based waveform]]) 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'', [https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=660307]</ref>
# 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).
# Linear current ramp (LCR) (involving the I–V curve using a linearly increasing and/or decreasing current as in a [[sawtooth wave]] or [[triangle wave]])

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.

=== Analysis ===

The [[cumulative distribution function|cumulative distribution]] of measured QBD is commonly analysed using a [[Weibull chart]].

== Standards ==
=== JEDEC standard ===
* JESD35-A &ndash; Procedure for the Wafer-Level Testing of Thin Dielectrics, April 2001

== See also ==
* {{section link|Capacitor|Breakdown voltage}}
* [[Field electron emission]]

== References ==

{{reflist}}

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[[Category:Semiconductor device defects]]

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