Reliability Testing of AlGaN/GaN Power Switch Devices as Low and High Temperature

Kurt Smith, Elena Georgieva, Jeff Haller, Peter Smith

Rakesh Lal, Yifeng Wu

Agenda

• Why Gallium Nitride Power Devices

• Power switch basics

• RF and power comparison

• Reliability results

• HVOS basics

• HVOS results

• Process evaluation

Renewable Energy Solar inverters

Automotive EV and charging

Industrial Motor drives/servo

Telecom/Industrial Power supplies

3

Size & weight reduction translates to longer distance per charge and lower system cost

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Reduces energy losses by up to 40%-50%, makes PV inverter significantly smaller & lighter

Ability to double available power in standardized server and telecom form factors

Proprietary & Confidential

GaN Value in Target Applications

Off state On state

650V

~25V

0V

0V 0V

?V

~0V

Transphorm 650V D-mode Power Switches: The Basics

Depletion-mode GaN Switch

• Normally on GaN HEMT

• SiFET in series provides a normally off switch

• Advantages • Simple process takes advantage of lateral geometry

• Similar to established RF devices

• Utilizes a standard gate driver (SiFET)

• Gate reliability is well established

• SiFET has well established reliability

• GaN HEMT gate not subjected to voltage overstress

• Tied to ground

• Disadvantages • Packaging is more complicated to develop

• Cost competitive after development

• Reduced maximum switching speed

• Not a concern in power applications

In off state, GaN HEMT source voltage becomes more positive until G-S voltage reaches threshold

D-mode power switches utilize proven technology to provide market ready solutions

D-mode Power Switch

Comparing Power GaN and RF GaN

• Power and RF applications will share many aspects of device reliability, but significant differences need to be accounted for:

• Power switching has much higher voltage

• Fields are similar due to field plates/device design

• Power switches operate in linear region of IV plane

• Current does not reach saturation

• No negative VD or ID

• IG very low (<nA/mm)

• 3 types of power switching conditions

• Soft switching (blue)

• Resistive switching (purple)

• Hard switching (not shown)

• Fast transient (<10ns) where high voltage and high current both exist

0

0 Drain Voltage

Dra

in C

urr

en

t

Soft Switching

JEDEC style qualification has been extended to more stringent Q101

TPH3205WSQA (650V): Q101 Test Condition Summary

• Automotive qual testing (Q101) is more stringent than standard JEDEC style qualification • Higher voltages (100% of rated during test)

• Operated in hard-switched boost converters at Tj = 175oC • Degradation is not device related (reference device changed by same amount)

• Careful characterization of device post HTOL showed no degradation

7 7

t (hr)

Loss

(W

)

Reference Device

GaN HEMTs

7

High-Temperature Operation Life Test – Dynamic Reliability

Devices show good robustness to switching events

200:400V converter operation at 175oC/300kHz/410W

On State Reliability

• On-state conditions are low voltage & medium current

• HEMT only testing due to temperature limitations of plastic packaging

• Commonly used high temperature DC current test is used

• Multiple reports of GaN reliability (RF) are applicable

• HEMT Vds = 10V

• Failure defined at 20% increase in Ron

HTDC results

Ea = 1.84eV (similar to reported values for Arrhenius model)

β = 2.2 (small variation)

Failure analysis indicates a increase in trapped charge in gate to drain region resulting in higher Ron and lower Idss

Test provides credible lifetime projections

Lifetime at operating temperatures is excellent

15

0°C

17

5°C

125⁰C 175⁰C 150⁰C

HVOS Basics

• High Voltage Off State (HVOS) testing evaluates high field reliability

• Similar to HTRB, but taken to failure

• Evaluated on product parts

• 650V automotive qualified process

• Step stress used to determine HVOS conditions

• Stepped from 600V to 1800V

• 10min to 1 hour per step

• Increase in leakage current starts around 1500V

• Related to vertical leakage in device

• Not related to avalanche breakdown

• HVOS testing is below increased leakage to more closely match conditions during normal operation

• Failure mechanism will be same

1200V

HVOS

Increased leakage

region

Accelerating voltage limited below increased leakage region

HVOS : Voltage Acceleration Factor

• HVOS Details • Standard product • Tj = 82°C • Vd ranged from 1050 to 1300V • Device in off state

• Results • Failures seen in GaN HEMT only • >99% were catastrophic failures (no increase in leakage) • Acceleration factor (α) is 0.026V-1

𝑡𝑡𝑓 ∝ 𝑒−𝛼𝑉

• Reported extensively

• TDDB linear model (most conservative)

• Recent reports range from 0.026 to 0.034V-1

• Additional testing at higher temperature showed significantly longer lifetime • Suspended due to length of test

10

Suspended test 150°C @ 1150V

α = 0.026V-1

Voltage acceleration factor is 0.026V-1

Comparing Power GaN and RF GaN: Temperature Affects

Temperature dependence is significantly different between RF and power applications

• RF operation shows a reduced envelope at high temperature

• Reduce peak voltage and current

• Power operation shows very little change due to temperature

• On state voltage will increase to maintain current

• Off state voltage will remain constant

0

0 Drain Voltage

Dra

in C

urr

en

t

Increasing Temperature

Soft Switching

High field in power switches is independent of temperature

Low Temperature HVOS testing

Temperature acceleration has a negative activation energy

1250V

• HVOS Details • Standard packaged product

• Tj ranged from -20°C to 150°C

• Vd ranged from 1050 to 1300V

• Results • Ea = -0.32eV

• Arrhenius model

• Analysis using Alta Pro

• Failures seen in GaN HEMT only

• >99% were catastrophic failures (no increase in leakage)

• Acceleration factor (α) was consistent at 0.026V-1

-50 0 50 100 150

Understanding High Voltage

• GaN devices do not have Si-like breakdown at high voltage

• GaN does not avalanche under normal conditions

• Region of increased leakage is a vertical effect • Device geometry does not affect

• Epi thickness increases vertical leakage

Breakdown voltage does not explain negative activation energy

• Increasing temperature reduces breakdown voltage • Does not explain the negative activation energy

• Testing conditions chosen to avoid breakdown effects

OFF State Wearout and Failure Mechanism

• Failures are catastrophic with localized crater randomly located

• < 0.1% show increase in leakage prior to failure

• Failure mechanism combination of factors

• Defects are created in high field region • Temperature acceleration factor

Source Drain Gate

Defects form

OFF State Wearout and Failure Mechanism

• Failures are catastrophic with localized crater randomly located

• < 0.1% show increase in leakage prior to failure

• Failure mechanism combination of factors

• Defects are created in high field region • Temperature acceleration factor

• Defects build up

Source Drain Gate

Defects increase

OFF State Wearout and Failure Mechanism

• Failures are catastrophic with localized crater randomly located

• < 0.1% show increase in leakage prior to failure

• Failure mechanism combination of factors

• Defects are created in high field region • Temperature acceleration factor

• Defects build up

• Defects reach critical density and failure occurs (TDDB style phenomenon) • Field acceleration factor

Mechanism explains both temperature and voltage acceleration factors

Source Drain Gate

Drain Source Gate

FP

Failure occurs

Models for Negative Activation Energy

Three mechanisms have been identified/reported that can result in a negative activation energy

• Impact ionization (proposed model) • Arrhenius model over limited temperature range • Supports defect creation in high field region

• Hot carrier defect formation • Mean free path of hot carriers will decrease with temperature limiting defect creation • Hot carriers come from leakage current inherent in cascode • Reported by Meneghini, etal IRPS 2017 • Would result in electron trapping

• High temperature would increase de-trapping causing decrease in lifetime

• Temperature related trap phenomena • Lower temperature would result in fewer traps being available for charging • Higher temperature would increase de-trapping and decrease lifetime

Impact ionization offers best fit to available data

Effects of Off State Acceleration Factors: Process Evaluation

Process shows excellent extrapolated lifetime with good stability

480V • Process variability is as important as determining acceleration factors • Use plot with extrapolated lifetime for all devices tested

• ~300 devices from 9 different lots

• Test conditions ranged:

• VD from 1150V to 1300V

• Temperature from -20°C to 150°C

• Process evaluated at 480V (nominal 650V device operation) • Temperature range for automotive is -40°C to 150°C

• Nominal temperature expected to be 85°C

• Process shows > 20yrs life at <1% failure rate • Off state for 480V @ -40°C

• On state at 175°C (previous slides)

-40°C 85°C

150°C

Summary

• Transphorm 650V GaN d-mode switches offer significant advantages over Si

• High field reliability is important to power switches due to bias in the off state

• Transphorm’s normally off switches have demonstrated excellent reliability

• First automotive (Q101) qualified

• Good on-state and switching reliability

• High voltage off state (HVOS) testing has been used to determine high field life times

• Field acceleration factor of 0.026V-1

• Temperature acceleration factor of -0.32eV

• High field degradation mode is combination of factors

• TDDB style – field

• Impact ionization creating defects– temperature

• Full set of acceleration factors allows better evaluation of process reliability for customer needs