Number 2197

Evaluating Hot Carrier Induced
Degradation of MOSFET Devices

Application NoteSeries
Introduction

With decreased MOSFET gate length, hot carrier induced
degradation has become one of the most important reliability
concerns. In the hot carrier effect, carriers are accelerated by thechannel electric fields and become trapped in the oxide. These
trapped charges cause time dependent shifts in measured device
parameters, such as the threshold voltage (VTH), transconductance (GM), and linear (IDLIN) and saturation (IDSAT) draincurrent. In time, substantial device parameter degradation canoccur, resulting in device failure. Instrumentation for measuring
HCI must provide these three key capabilities:• To extract device parameters automatically.
• To create a stress-measure sequence with a variety ofstress times.
• To export the measure data easily for advanced analysis.
This application note describes how to use these keycapabilities in the Keithley Model 4200-SCS (Semiconductor
Characterization System) to perform hot carrier degradation
testing.

Si–SiO2 barrier and become trapped in the gate oxide. Trappedcharges cause device degradation and enhanced substrate current(ISUB).

Monitoring Hot Carrier Device
Performance
Figure 2 illustrates the hot carrier test methodology. Hot carrier
device test times are typically very long, so the first step is todetermine an optimized set of stress conditions. Then, a new
device is selected and initial device parameters such as VTH, GM,IDLIN and IDSAT are determined. The optimized stress is thenapplied for a specified stress interval. After each stress interval,
parametric measurements are made again and the degradation
determined. This test-stress cycle continues until the target
degradation or test time criteria are met. The hot carrier lifetime
is determined from the degradation vs. stress time data.
StartVD

VG

Determine StressConditions

Stress Device

Select Device

Re-Measure
Parameters

Gate
MetalSource
n+

Oxide
––+p–

Increment
Stress Time

Drain
Initial ParametricMeasurementsn+Avalanche

ISUB
Substrate

No

Valid?

Record Data

Termination?
Yes

Yes
Record Data

No

Data Analysis

EndFigure 1. Drain Avalanche Hot Carrier Effect.
Figure 2. Hot Carrier Stress Test Flowchart.

The MOSFET Hot Carrier EffectToday’s ULSI MOSFET devices feature extremely short channel
lengths and high electric fields. In these high electric fields, carriers are accelerated to high velocities, reaching a maximum
kinetic energy (hot) near the device drain. If the carrier energy ishigh enough, impact ionization can occur (Drain Avalanche HotCarrier Effect), creating electron-hole pairs (Figure 1). Theseholes and electrons can attain enough energy to surmount theHot Carrier Lifetime Using theKeithley 4200
Determining Stress Conditions
Before hot carrier testing begins, the drain and gate stress bias
voltage must be established. To ensure realistic channel hotcarrier conditions, the recommended maximum drain stress bias
voltage is less than 90% of the device’s drain-to-source
breakdown voltage.

Fitline

6.0E–04

IDRAIN

5.0E–04

IDRAIN

4.0E–04
3.0E–04
2.0E–04
1.0E–04

Gmext = 5.46E–4A/V
Vtext = 0.754V

1.6E–04

2.0E–00

8.0E–00

6.0E–00

4.0E–00

2.0E–00

1.0E–00

8.0E–01

6.0E–01

4.0E–01

0.0E+00
2.0E–01

VGstress at ISUBMAX Characterization

VGATE+1.821E+000, +1.371E–004

1.4E–04

Figure 4. Typical 4200-SCS VTH and GM measurement results.

1.2E–04

ISUB

7.0E–04

0.0E+00

At a given drain stress bias voltage, the gate stress bias(VGstress) should be selected to maximize hot carrier
degradation. For NMOS devices, this typically occurs at a gatebias (VGATE) that maximizes the substrate or bulk current (ISUB).
Figure 3 represents typical 4200-SCS ISUB vs. VGATE data. In
this case, the optimal VGstress is 1.821V.

Vtext and Gmext Measurement Results
8.0E–04

1.0E–04
8.0E–05
6.0E–054.0E–05
2.0E–05
0.0E–00
01234VGATE

Figure 3. Typical 4200-SCS ISUB data to define VGstress.

Determining Device Parameters
Hot carrier monitored parameters include VTH, GM, IDLIN andsaturated IDSAT. These parameters are initially determined before
stress and redetermined at each cumulative stress time. The IDLIN
is the measured drain current with the device biased in the linear
region, while IDSAT is the measured drain current with the device
biased in the saturation region. VTH and GM can be determinedusing either constant current or extrapolation methods. In theextrapolation method, the VTH is determined from the maximum
slope of the IDS vs. VGS curve.
The 4200-SCS’s Formulator Tool greatly simplifies
extracting these parameters. The built-in functions include
Differentiate to obtain GM, a MAX function to obtain themaximum GM (Gmext), and a least squares line-fit function toextract VTH (Vtext). Figure 4 illustrates the Formulator’sautomated data analysis capability.

Stressing the Device
After measuring the device parameters, the device is stressed atthe drain and gate stress bias conditions for a specified time.
Cumulative stress times are typically logarithmic, e.g., 10, 30,100, 300, 1000, etc. At each cumulative stress time, the device
parameters are measured again and recorded.
The 4200-SCS’s built-in test sequencer and Interactive Test
Module (ITM) capability greatly simplify implementing the hotcarrier test procedure. Figure 5 depicts the hot carrier stress test
sequence in the Project Navigator window and the hot carrier
stress test module (hcistress10) DefinitionTab. The Project
Navigator window displays the test sequence, which begins bymaking initial measurements of the idlin, idsat and vtlin (thevtlin module extracts parameters Vtext and Gmext). The
hcistress10 test then applies a ten-second stress at the specified
drain and gate stress voltages. Once the stress is complete,device parameters are measured again. This stress-measuresequence continues until the maximum specified stress time isexceeded.
The hcistress10 test Definition Tab is used to specify thestress voltages and stress times. In this example, the gate anddrain stress conditions are specified as 2.2V and 5.5V,
respectively. The stress time is specified in the Timing dialog
box (not shown), which is activated using the Timing …
Command Button (see Figure 5). In the Timing dialog box, thehcistress10 test is set to Sampling Mode with an interval of one
second and ten samples. This will provide a ten-second stress. To
obtain longer stress times, simply increase the number ofsamples or the sampling interval.

Data Analysis
N-channel hot carrier degradation vs. time data typically exhibit
linear behavior when plotted on log-log plots. Least-squares fit
regression analysis is used to interpolate or extrapolate the hotcarrier time to target lifetime. Target device lifetime is defined as
a 10% parameter change or, in the case of VTH, a change of50mV from its unstressed value.
The 4200-SCS automatically saves raw sweep data andextracted test results to fully compatible Microsoft Excel®
spreadsheets (data file extension: .XLS), which greatly simplifies
hot carrier data analysis. Figure 6 shows 4200-SCS IDLIN
degradation data vs. stress time. In this case, the projected time
to target failure is 1004.7 seconds.

Conclusion
The 4200-SCS greatly simplifies the determination of hot carrier
induced degradation. Built-in 4200-SCS software tools, such as
the Project Navigator, Formulator, and a fully compatible Excel
data format, provide a flexible, fast, and easy-to-use test
environment. Furthermore, the 4200-SCS links these tools with

Figure 5. Hot carrier project test sequence and hcistress10 test.

advanced Source-Measure Unit technology to create an ideal
test environment for challenging device reliability andcharacterization requirements.

S4200 Hot Carrier Linear Drain CurrentDegradation Data
100Reference

IDLIN
Degradation (%)

EIA/JESD28. JC-14.2 Committee. August 1995. “A Procedurefor Measuring N-Channel MOSFET Hot-Carrier-Induced
Degradation at Maximum Substrate Current Under DC Stress.”

101
110100

1000

10000

Time (seconds)

Figure 6. 4200-SCS hot carrier linear drain current degradation data analyzed
in Microsoft Excel.

Specifications are subject to change without notice.
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No. 2197
010010KAP