BREAKDOWN IN MOSFETS

 There will be breakdown that takes place in the MOSFETS under

certain conditions.
 There are four types of breakdown conditions in MOSFETS.

 Oxide breakdown
 Avalanche breakdown
 Snap breakdown
 Punch through Effects

Oxide breakdown :

⚠️What is Oxide Breakdown in MOSFET?

📌 Definition:
Oxide breakdown is the irreversible damage of the gate oxide
(typically SiO₂ or high-k dielectrics) due to excessive electric field,
causing it to lose its insulating property and leak current or short.

🔍 Where does it happen?

 It occurs across the gate oxide layer between the gate and
channel.
 This oxide is extremely thin (as low as ~1–2 nm in modern
MOSFETs), so it's vulnerable to very high electric fields.

💥 Why does it happen?

🧪 When the electric field (E = V/tox) becomes too large:
1. Electrons gain enough energy to break Si-O bonds in the oxide.
2. Traps and defects are created in the oxide.
3. Over time, these defects form a conductive path → resulting in
leakage current or short circuit between gate and channel.

📉 Types of Oxide Breakdown

Type Description Common in
Sudden, permanent short Thick oxides (>5
Hard Breakdown
through the oxide nm)
Gradual leakage due to Thin oxides (<3
Soft Breakdown defect build-up; less nm), modern
visible initially MOSFETs
Time-Dependent Breakdown after
Long-term
Dielectric Breakdown prolonged usage under
reliability test
(TDDB) stress

📈 Electric Field Threshold

Typical breakdown field for SiO₂:
 ~10 MV/cm
So, for 1 nm oxide, breakdown occurs at just ~1V!
That's why modern technologies moved to high-k dielectrics →
allows thicker physical oxides for same capacitance.

🧠 Consequences of Oxide Breakdown

 Gate loses insulation → High gate leakage
 Device fails permanently or shows erratic behavior
  Affects device lifetime and reliability
 May lead to electromigration and thermal damage

How to Prevent It
1. Use High-k Dielectrics
– e.g., HfO₂ allows physical thickness while maintaining same
capacitance
2. Limit V_GS to safe values
– Avoid over-voltage stress
3. Design with Oxide Reliability Models
– Use TDDB testing to estimate device lifetime
4. Avoid ESD (Electrostatic Discharge)
– Use clamping circuits to protect gate

🧪 Interview Example Q&A

🔹 Q: What is oxide breakdown and how does it differ from
avalanche breakdown?
Answer:
 Oxide breakdown happens across the gate oxide due to high
gate voltage, damaging insulation.
 Avalanche breakdown happens at the drain-body junction due
to high drain voltage, causing impact ionization.
 Oxide breakdown causes permanent gate failure; avalanche
causes drain current surge.

🔑 Key Formula
Electric field across oxide:
Eox=Vgs/tox.
If Eox>10 MV/cm, breakdown likely occurs.

Avalanche Breakdown :

⚡ What is Avalanche Breakdown in MOSFET?

🔹 Definition:
Avalanche breakdown occurs when the reverse-biased drain-body
(or drain-substrate) junction is subjected to a very high electric field,
causing impact ionization — where energetic carriers (electrons or
holes) collide with atoms in the silicon lattice and generate
additional electron-hole pairs.
🔁 This leads to a chain reaction, and the current increases rapidly
even without an increase in applied voltage → known as avalanche.

📍 Where in MOSFET does it occur?

Avalanche breakdown typically occurs at the drain-substrate (body)
junction, which is reverse biased when:
 For NMOS: Drain is at high positive voltage, substrate is at
ground
 For PMOS: Drain is at low (negative) voltage, substrate at VDD

⏳ When does Avalanche Breakdown occur?

Avalanche breakdown happens when:
✅ Conditions:
1. Drain-Body junction is reverse biased heavily
 2. Drain voltage (V_DS) is very high, beyond device design limits
3. High electric field near the drain causes electrons to accelerate
and collide with lattice atoms
4. This initiates impact ionization, creating excess carriers and
high current

🔬 Physical Mechanism (Impact Ionization)

1. Electron gains high energy due to strong electric field near the
drain.
2. It collides with the silicon lattice atom.
3. The collision knocks loose another electron, forming an
electron-hole pair.
4. These new carriers repeat the process — leading to carrier
multiplication.

📈 MOSFET Characteristics under Avalanche

 Sharp increase in drain current (ID) even if VGS = 0
 Can damage the device permanently if not protected
 Common in high-voltage MOSFETs, power devices, or during
ESD (Electrostatic Discharge)

🧠 Interview Question Example

Q: Why is avalanche breakdown undesirable in MOSFETs?
Answer:
Because it causes excessive current that may lead to thermal
damage, oxide breakdown, or junction destruction. It indicates that
the drain voltage has exceeded the breakdown voltage (BV_DSS).
How to Prevent Avalanche Breakdown:
1. Limit drain voltage below breakdown voltage
2. Use guard rings or junction termination in layout
3. Implement clamping diodes for ESD protection
4. Design using avalanche-rated MOSFETs for power circuits

⚡ Summary Table
Parameter Avalanche Breakdown
Region Drain-body junction
Cause High reverse voltage → strong electric field
Effect Impact ionization → carrier multiplication
Result Sudden large current → possible damage
Avoided by Limiting VDS, protection circuits

 The sharp increase in the drain current despite the gate voltsge
is zero is due to the high electric field at the drain junction. This
causes an increase in the acceleration of the electrons near
drain terminal which leads to the avalanche effect at drain. And
hence the current here is due to the avalanche effect. The drain
current ain’t flowing through the channel but through the
substrate and the drain junction.

NEAR AVALANCHE AND SNAPBACK

BREAKDOWN :
  When VDS is applied high it creates an avalanche breakdown in
the drain terminal and the carriers accumulate there. Here the
holes from those carriers at the drain terminal passes through
the substrate to the body terminal and this creates a potential
drop since the substrate is nonzero resistance.
 This creates a potential difference and makes the source-body
junction as a forward biased pn junction.
 This creates a parasitic BJT action and snapback breakdown
takes place.

WORKING MECHANISM :

 The holes from the drain terminal accumulate at the source end
and creates a potential difference as the source body as
forward biased. Because since most of the holes that is positive
charge accumulate at the body (p type) and negative charge at
the source (n type) its is forward biased.
 Since the source Is heavily doped (n+ type), thus a large number
of electrons are injected from the source to the substrate. In
that some of the electrons diffuses to the reverse biased drain
terminal and contribute to the drain current.
 As body-substrate junction is forward biased in turns on the
parasitic BJT which causes the snapback breakdown.
This is what causes snapback:
 Sudden current surge
 Sudden drop in VDSV_{DS}VDS
 Device may enter a latch-up-like state
Q: What is a parasitic BJT in a MOSFET?
A: It is an unintended NPN or PNP transistor formed by the source,
body, and drain regions in a MOSFET. In an NMOS, it's an NPN
structure: source = emitter, body = base, and drain = collector. Under
high voltage stress or avalanche, it can turn on, leading to snapback
or latch-up.

When Does the Parasitic BJT Turn On?

1. Under high VDSV_{DS}VDS → Avalanche occurs at drain-body
junction.
2. Holes accumulate in the body region, raising body potential.
3. This forward-biases the source-body (base-emitter) junction.
4. Parasitic BJT turns ON, leading to:
o Snapback (sudden current surge)
o Possible device failure
o In CMOS circuits: even latch-up

PREVENTION FROM SNAPBACK EFFECT :

The snapback effect can be minimized by using a heavily doped
substrate that will prevent any significant voltage drop from being
developed. A thin epitaxial p-type layer with the proper doping
concentration to produce the required threshold voltage can be
grown on a heavily doped substrate

NEAR PUNCH THROUGH BREAKDOWN:

NOTE : Near Punch through effect is merely a short channel effect.

Near Punch-Through Effects Punch-through is the condition at which
the drain to-substrate space charge region extends completely across
the channel region to the source-to-substrate space charge region. In
this situation, the barrier between the source and drain is completely
eliminated and a very large drain current would exist. However, the
drain current will begin to increase rapidly before the actual punch-
through condition is reached. This characteristic is referred to as the
near punch-through condition, also known as Drain-Induced Barrier
Lowering (DIBL). Figure 11.25a shows the ideal energy-band diagram
from source to drain for a long n-channel MOSFET for the case when
VGS VT and when the drain-to-source voltage is relatively small. The
large potential barriers prevent significant current between the drain
and source. Figure 11.25b shows the energy-band diagram when a
relatively large drain voltage VDS2 is applied. The space charge region
near the drain terminal is beginning to interact with the source space
charge region and the potential barrier is being lowered. Since the
current is an exponential function of barrier height, the current will
increase very rapidly with drain voltage once this near punch-through
condition has been reached. Figure 11.26 shows some typical
characteristics of a short-channel device with a near punch-through
condition.