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    - Simplifying Boolean Equation with Karnaugh Maps.

Are We Having a Midlife Crisis in Our 20s?

 

Sometimes I look at people in their 20s and wonder how we are all supposed to be okay.

We are young. We are supposed to be enjoying life, figuring things out, making memories, trying new things. And yet, so many of us are constantly worried about whether we’re doing enough, earning enough, achieving enough, or simply moving in the right direction.

Am I in the right career? Am I earning enough? Should I be doing more? Why is everyone else ahead of me? Will I ever have the life I imagined? Should I be in a relationship? Should I be getting married? Should I move to another city?

Somewhere between asking ourselves all these questions and pretending that we have everything under control, our 20s can start feeling strangely like a midlife crisis.

Except we’re nowhere near midlife.

We’re in our 20s.

We Thought Our 20s Would Be Different

When we were younger, being in your 20s looked exciting. You were finally going to be independent. You’d have a job, your own money, friends, travel, relationships, maybe even your dream career. Most importantly, you imagined that by then you would know what you were doing.

Nobody really tells you about the other side of it.

The confusion. The comparison. The uncertainty. The strange feeling that everyone else somehow received a map for adulthood and you didn’t.

You finish college, start working, and suddenly someone asks, “So, what’s your plan for the next five years?”

And you’re thinking, I don’t even know what I’m doing next Tuesday.

Everyone Seems to Be Moving Ahead

Social media makes this feeling even stronger.

You open Instagram or LinkedIn and within a few minutes you see someone getting promoted, someone buying a house, someone getting married, someone travelling, someone moving abroad, someone starting a business, or someone announcing another achievement.

Then you look at your own life.

Maybe you’re still trying to understand what you actually want. Maybe you’re in a job you don’t love. Maybe you’re saving money and can’t afford the lifestyle you see online. Maybe you’re single. Maybe you’re in a relationship but still unsure about the future.

Or maybe you’re doing perfectly fine, but somehow “fine” doesn’t feel good enough anymore.

The problem is that we don’t compare our lives with reality. We compare our ordinary days with everyone else’s best moments.

And then we wonder why we feel behind.

Suddenly, Time Feels Different

One of the strangest things about your 20s is realizing that time doesn’t feel the way it did when you were younger.

When you’re 15, 25 sounds incredibly old. Then you turn 25 and realize you still feel like the same person who was trying to figure things out at 18. The only difference is that now people expect you to have answers.

You start thinking about where you want to be at 30, then 35, then 40. Suddenly every decision feels important.

What if I choose the wrong career? What if I stay somewhere too long? What if I waste these years? What if I don’t meet the right person? What if I never become successful?

We’re young enough to have decades ahead of us, yet sometimes we feel like we’re already running out of time.

That’s a strange kind of pressure.

We Want Freedom and Stability at the Same Time

Our 20s are full of contradictions.

We want to travel and explore, but we also want financial stability. We want to take risks, but we’re afraid of failing. We want independence, but sometimes we wish someone would simply tell us what we’re supposed to do.

We want a successful career, but we don’t want our entire identity to become our job. We want relationships, but we’re afraid of settling down too early. We want to enjoy our youth, but we feel guilty whenever we’re not being productive.

It’s exhausting trying to want everything at once.

Maybe that’s part of why our 20s feel so confusing. We’re trying to build a future while still figuring out who we are.

The Career Question Never Really Ends

There is a particular kind of anxiety that comes with realizing you might not love what you studied for.

You spend years getting a degree, get a job, and then realize the job isn’t what you imagined. Suddenly you start wondering whether you’ve already made the wrong choice.

Should I change careers? Should I study again? Should I take a risk? Should I stay because the salary is good? Should I choose stability over happiness?

And every time you look around, someone seems to be doing exactly what they always wanted.

But we rarely see the uncertainty behind other people’s decisions. We see the promotion, not the doubts before it. We see the new job, not the fear of leaving the old one. We see the announcement, not the months of uncertainty that came before it.

Maybe everyone is questioning their choices too.

Maybe some people are just better at hiding it.

Friendships Change Too

Your 20s aren’t only about careers and money. Your relationships start changing as well.

People move to different cities. Some get married. Some become completely focused on work. Some start families. Some friendships simply become less frequent without any dramatic ending.

And suddenly, people you thought would always be around aren’t always available anymore.

You can have good friends and still feel lonely. Not because anyone did something wrong, but because everyone’s life starts moving in different directions.

Growing up sometimes means learning how to love people from a distance.

You don’t stop caring about them. You just stop sharing the same everyday life.

Then Come the Questions

At some point, the questions start coming from everywhere.

“When are you getting married?”

“Are you seeing someone?”

“When are you settling down?”

“What are you doing next?”

Sometimes these questions are asked casually, but when you’re already uncertain about your life, they can feel much heavier.

It’s easy to start believing that there is a correct timeline for adulthood and that you’re somehow falling behind it.

But who decided that by a certain age you should have a certain salary, relationship, house, career and lifestyle?

There isn’t one universal timeline.

And different doesn’t automatically mean behind.

Maybe We’re Not Having a Midlife Crisis

Maybe we’re experiencing something else.

Maybe our 20s are simply the first time we’re fully responsible for our own choices.

As children, many decisions are made for us. School, college, schedules, rules, expectations. Then suddenly you’re an adult and nobody tells you exactly what to do anymore.

You have freedom, but freedom comes with uncertainty.

You can change your career. Move cities. End a relationship. Start again. Study something completely different. Travel. Stay where you are.

There are so many possibilities that sometimes having too many choices feels almost as difficult as having none.

Maybe We’re Supposed to Feel Lost

We often talk about being lost as though it means something has gone wrong.

But maybe getting lost is part of figuring out where you actually want to go.

You don’t have to know your entire life at 24. You don’t have to have your career figured out at 25. You don’t have to know exactly who you’ll become at 27.

People change. What you want changes. Your priorities change. Things you once thought were incredibly important may stop mattering a few years later.

And that’s not failure.

That’s life.

Maybe the person you are at 22 isn’t supposed to be the person you are at 30.

What If You’re Not Behind?

This is probably the question we need to ask ourselves more often.

Not “Why am I behind?”

But “Behind whom?”

Someone can find their career at 22. Someone else can find it at 32. Someone can meet their partner at 21, while someone else may meet them at 38. Someone can travel the world in their 20s, while someone else spends those years building financial stability.

None of these lives are automatically better.

They’re simply different.

The problem is that we have become so used to seeing everyone’s timeline that we forget our own life is not supposed to look exactly like someone else’s.

Maybe We Don’t Need to Figure Everything Out

I think we put too much pressure on ourselves to have answers.

Maybe we need better questions instead.

What kind of life actually makes me happy?

What do I genuinely care about?

What am I doing because I want it, and what am I doing because everyone expects it?

What am I afraid to try?

What would I do if I stopped comparing myself?

What kind of person do I want to become?

Some of these questions won’t have immediate answers. Some may take years.

That’s okay.

You are allowed to change your mind.

You’re allowed to start over.

You’re allowed to take a different path from the one you imagined at 18.

Maybe Our 20s Aren’t a Crisis

Maybe our 20s are simply the first chapter where we realize that we are the ones holding the pen.

And that’s scary.

We’ll make wrong decisions. We’ll stay somewhere too long. We’ll leave too early. We’ll lose people. We’ll change careers. We’ll start again. We’ll make mistakes and hopefully learn from them.

Maybe the goal of your 20s isn’t to build a perfect life.

Maybe it’s to slowly build a life that actually feels like yours.

So if you’re in your 20s and you feel confused, behind, uncertain, or strangely exhausted by the pressure to figure everything out, maybe nothing is wrong with you.

Maybe you’re just growing.

And growing is rarely as neat as we imagine it will be.

Perhaps we’re not having a midlife crisis after all.

Perhaps we’re simply meeting adulthood for the first time.

What is a Zener Diode? Working, Breakdown and V–I Characteristics Explained

What is a Zener Diode?

A Zener diode is a special type of PN junction diode that is designed to operate in reverse bias — specifically in the breakdown region — without getting damaged.

Unlike a normal diode that gets destroyed when reverse breakdown happens, a Zener diode is made to safely conduct in that region.

---

⚡ Construction and Symbol

• It looks similar to a normal diode but has heavily doped P and N regions.
• Because of this heavy doping, the depletion layer is very thin, and breakdown occurs at a lower voltage (called the Zener breakdown voltage).
• Symbol: Like a normal diode, but with bent edges on the cathode line (representing the breakdown feature).

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🔹 Working Principle

A Zener diode works in two modes depending on how it is connected:

1. Forward Bias

• Behaves just like a normal diode.
• Current starts increasing rapidly after the cut-in voltage (~0.7 V for Si, 0.3 V for Ge).
• This region is not the main operating region of the Zener diode.

2. Reverse Bias (Main Operation)

• When reverse voltage is applied, initially a tiny reverse current (leakage) flows.
• Because of heavy doping, the depletion layer is extremely thin (only a few nanometers).
• Even a small reverse voltage creates a very strong electric field (on the order of 10⁶ V/m).
• This strong field is powerful enough to pull electrons directly out of their covalent bonds without needing collisions.
• This quantum mechanical process is called tunneling — electrons “tunnel” through the barrier.
• As a result, the diode conducts suddenly at a precise low voltage (Zener voltage).

⚡ So, Zener breakdown occurs due to “tunneling” of electrons through a strong electric field.

And Avalanche breakdown occurs due to “impact ionization” — collision-based multiplication of electrons.

• Zener Breakdown: Below 5–6 volts (e.g., 2.4V, 3.3V, 5.1V Zener diodes)
• Avalanche Breakdown: Above 6 volts (e.g., 12V, 24V, 100V diodes)

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📉 VI Characteristics of Zener Diode

1. Forward Bias Region

• Similar to a normal diode.
• Very small current till cut-in voltage (~0.7 V).
• After that, current increases rapidly with small voltage increase.

2. Reverse Bias Region

• At first, only a small leakage current flows.

When a Zener diode is reverse biased, it behaves like a normal diode at first — only a small reverse saturation current flows due to minority carriers.

As the reverse voltage increases, the electric field across its very thin depletion region becomes extremely strong because the diode is heavily doped.

When this field reaches a critical strength, it causes electrons in the valence band of the P-side to tunnel directly into the conduction band of the N-side, even without collisions.

This phenomenon is known as Zener breakdown, and it occurs at a specific voltage called the Zener breakdown voltage (V_Z).

At this point, a sharp increase in current is observed while the voltage across the diode remains nearly constant at V_Z.

This region of the V–I characteristic is nearly vertical, showing that even with a large change in current, the voltage does not rise much.

The ability of the Zener diode to maintain a steady voltage during breakdown makes it ideal for use as a voltage regulator in circuits.

---

Conclusion

A Zener diode is a specially designed PN junction diode that operates safely in the reverse breakdown region.

Unlike a normal diode, it is heavily doped so that breakdown occurs at a controlled voltage called the Zener voltage.

This allows the diode to maintain a nearly constant voltage across it, making it extremely useful in voltage regulation and protection circuits.

By understanding Zener breakdown, avalanche breakdown, and diode characteristics, we build a strong foundation for studying more advanced semiconductor and electronic circuits.

👉 In upcoming blogs, we will continue exploring more semiconductor devices and electronic circuit concepts.

Understanding Avalanche Breakdown in PN Junction Diodes

🧱 The Widening of the Depletion Region

As the depletion layer widens:

• More fixed positive ions appear on the N-side (because electrons leave that region).
• More fixed negative ions appear on the P-side (because holes leave that region).

This builds a stronger electric field across the junction that opposes any current flow.

Because of this strong electric field, it becomes very difficult for majority carriers to cross the junction.

Hence, almost no current flows — this is why a diode “blocks” current in reverse bias.

---

What Are “Thermally Generated Free Electrons”?

Even when no external voltage is applied to a semiconductor, atoms inside the crystal lattice are constantly vibrating due to temperature (heat energy).

This is because the atoms in a solid are not completely stationary — they always have some thermal energy that increases with temperature.

At any non-zero temperature (even at room temperature, ~300 K):

• Some of this thermal energy is large enough to break the covalent bonds between silicon atoms.
• When a covalent bond breaks, one electron becomes free to move in the lattice.
• The atom that lost the electron becomes a hole (a vacant bond that can accept another electron).

So, this process creates electron-hole pairs (EHPs) even without any external energy source like light or voltage.

These are called thermally generated charge carriers:

• The free electron → goes to the conduction band (can move freely).
• The hole → remains in the valence band (acts as a positive charge).

However, this happens only occasionally — because most electrons still stay bonded — so the number of thermally generated carriers is small.

Still, they exist everywhere in the diode.

---

⚡ Now, What Happens During Reverse Bias?

When a reverse voltage is applied:

• The electric field across the depletion region becomes very strong.
• This field pulls the thermally generated electrons (minority carriers) that appear near the junction.

Now, these few free electrons are accelerated by the electric field — meaning they gain kinetic energy as they move through the depletion region.

The stronger the electric field, the faster these electrons move.

At low reverse voltages, the field is not strong enough to cause anything major — these electrons just move across, creating a small, constant reverse saturation current (I₀).

But as the reverse voltage increases, the electric field strength becomes enormous — in the order of 10⁵ to 10⁶ V/cm.

Now the electrons gain so much energy that something dramatic happens.

---

💥 Impact Ionization — The Start of Avalanche Breakdown

A moving electron in a crystal normally bounces off atoms, but if it’s moving very fast (has high kinetic energy), each collision becomes violent enough to:

• Knock out an electron from the valence bond of a silicon atom.
• This atom loses one of its bonded electrons → creating a new hole.
• The knocked-out electron becomes a new free electron.

So from one energetic electron, now we have two free electrons and one hole.

This process is called impact ionization.

You can think of it like a cue ball in billiards — when a fast-moving ball hits others, it scatters them and creates more motion.

Similarly, one high-speed electron “hits” the atomic lattice and liberates more electrons.

---

⚙️ The Chain Reaction (Avalanche Effect)

Now, the newly freed electrons also experience the same strong electric field.

They too get accelerated to high speeds and collide with more atoms, knocking out more electrons.

This becomes a chain reaction:

1 electron → 2 → 4 → 8 → 16 → 32 …

Each step doubles the number of charge carriers.

Within a very short time, millions of new electrons and holes are created in the depletion region.

This rapid multiplication of carriers is called avalanche multiplication — because it grows like a snow avalanche, starting from a few carriers and exploding into many.

---

⚠️ What This Means Inside the Diode

Inside the diode during avalanche breakdown:

• The depletion region is flooded with newly created free electrons and holes.
• These carriers now contribute to a large current, even though the applied voltage hasn’t increased much.
• The crystal lattice experiences frequent collisions → which generate heat.
• If not controlled by a resistor or external circuit, this can overheat and permanently damage the junction.

That’s why normal diodes are not meant to operate in breakdown — except in special cases (like Zener diodes) where it’s designed to handle it.

---

Hence, in reverse bias, the diode shows almost no current because the widened depletion region blocks majority carriers. Only a very small reverse saturation current (I₀) flows due to thermally generated minority carriers, which appears as a nearly flat line on the V–I graph.

As the reverse voltage increases, this current remains almost constant until the breakdown voltage (V_BR) is reached. At this point, the electric field becomes strong enough to cause impact ionization, leading to avalanche breakdown. The current then rises sharply with only a small increase in voltage, producing the steep bend on the reverse side of the V–I characteristic.

---

What’s Next?

In this blog, we studied how a very strong reverse electric field can accelerate charge carriers and cause avalanche multiplication, leading to avalanche breakdown in a diode.

But avalanche breakdown is not the only type of reverse breakdown mechanism. In heavily doped PN junctions, another important phenomenon called Zener breakdown occurs due to quantum mechanical tunneling.

In the next blog, we will study:

• What is a Zener diode
• Zener breakdown mechanism
• Tunneling effect
• V–I characteristics of a Zener diode
• Voltage regulation using Zener diodes

👉 Click below to continue to the next part:

What is a Zener Diode? Working and Breakdown Explained

Understanding the V–I Characteristics of a PN Junction Diode

To truly understand how a diode behaves, we need to study how the current (I) through it changes as we vary the voltage (V) across it.

This relationship between voltage and current is called the V–I characteristic of the diode.

---

What are V–I Characteristics?

The V–I characteristic (Voltage–Current characteristic) is a graphical representation that shows how much current flows through the diode for a given applied voltage.

In simple words, if you slowly increase the voltage across a diode and note down the current at each step — then plot those values on a graph — the resulting curve is called the V–I characteristic curve of the diode.

This curve tells us how the diode behaves electrically under:

• Forward bias condition (conducting mode)
• Reverse bias condition (non-conducting mode)

---

How the V–I Characteristic is Obtained

To obtain the V–I characteristic practically, the diode is connected in a circuit with:

• A variable voltage source,
• A current-measuring device (like an ammeter), and
• A voltage-measuring device (like a voltmeter) across the diode.

We vary the applied voltage gradually — first in the forward direction and then in the reverse direction — and record the corresponding current at each step.

When the voltage and current values are plotted on a graph:

• The x-axis represents the voltage (V) applied across the diode.
• The y-axis represents the current (I) flowing through the diode.

---

V–I Characteristic in Forward Bias

When the diode is forward biased, the positive terminal of the battery is connected to the P-side and the negative terminal to the N-side.

Initially, as the voltage is applied, the diode does not conduct immediately. This is because the applied voltage must first overcome the potential barrier (V₀) created by the internal electric field of the depletion region.

For silicon diodes, this barrier potential is about 0.7 V, and for germanium, it is about 0.3 V.

At voltages below this barrier potential:

• The external voltage is not sufficient to reduce the potential barrier.
• Only a very small current flows because the applied voltage is not sufficient to overcome the barrier.
• The diode behaves almost like an open circuit.

Once the applied voltage equals the barrier potential:

• The depletion region becomes very narrow.
• The internal electric field is effectively neutralized.
• Majority carriers (electrons from N-side and holes from P-side) can now cross the junction freely.

At this point, the current starts increasing rapidly with even a small increase in voltage. This is because the diode now offers very little resistance in the forward direction.

The forward characteristic curve thus shows:

• A small, almost flat region near zero voltage (negligible current),
• Followed by a sharp exponential rise in current beyond the cut-in or threshold voltage.

---

V–I Characteristic in Reverse Bias

When a diode is reverse biased, the P-type side is connected to the negative terminal of the battery, and the N-type side is connected to the positive terminal.

So:

• The negative terminal pulls holes away from the junction in the P-side.
• The positive terminal pulls electrons away from the junction in the N-side.

As a result:

• Electrons near the junction on the N-side are pulled back toward the positive terminal.
• Holes near the junction on the P-side are pulled back toward the negative terminal.

This means that majority carriers (electrons in N and holes in P) move away from the junction, causing the depletion region to become wider.

---

What’s Next?

So far, we have studied how a diode behaves under forward and reverse bias conditions and how its current changes with applied voltage. But what happens when the reverse voltage becomes extremely large?

In the next blog, we will study:

• Reverse breakdown mechanism
• Thermally generated carriers
• Impact ionization
• Avalanche multiplication
• Avalanche breakdown in diodes

👉 Click below to continue to the next part:

Avalanche Breakdown in Diodes Explained

Forward Bias vs Reverse Bias in a Diode Explained

Let’s now see what happens when we apply an external voltage to the diode. This process is called biasing.

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1️⃣ Forward Bias

In forward bias, the positive terminal of the battery is connected to the P-side, and the negative terminal to the N-side.

Now, think of what’s happening internally:

• The positive terminal repels holes in the P-side towards the junction.
• The negative terminal pushes electrons in the N-side towards the junction.
• As a result, the depletion region becomes thinner, and the barrier potential decreases.

When the applied voltage becomes equal to or greater than the barrier potential (0.7 V for silicon), the junction becomes conductive.

Electrons now have enough energy to cross the barrier, recombine with holes, and current begins to flow through the diode.

This current is called Forward Current, and the diode is said to be in its conducting state.

So, in simple words — in forward bias, the diode allows current to flow.

---

2️⃣ Reverse Bias

Now let’s reverse the connections — connect the positive terminal to the N-side, and the negative terminal to the P-side.

What happens?

• The positive terminal pulls electrons away from the junction.
• The negative terminal pulls holes away from the junction.
• The depletion region widens, and the barrier potential increases.

As a result, no significant current flows. Only a tiny current (called leakage current) flows due to the minority charge carriers.

This current is extremely small and often negligible.

So, in reverse bias, the diode acts like an open switch — it blocks current.

---

What’s Next?

Now that we understand how a diode behaves under forward and reverse bias conditions, the next step is to study how the current through the diode changes with applied voltage.

In the next blog, we will study:

• V–I characteristics of a diode
• Cut-in (threshold) voltage
• Forward and reverse current behavior
• Reverse saturation current
• Breakdown region

👉 Click below to continue to the next part:

V–I Characteristics of a Diode Explained

What is a Diode? PN Junction, Depletion Region and Barrier Potential Explained

What is a Diode?

A diode is a two-terminal semiconductor device that allows current to flow in only one direction — from the Anode (P-side) to the Cathode (N-side) — and blocks it in the opposite direction. Diodes are one of the most fundamental semiconductor devices used in electronics. They are widely used in rectifiers, voltage regulators, signal clipping circuits, and digital logic.

Construction: How a Diode is Made

A diode is created by joining a P-type and an N-type semiconductor crystal together — forming a PN junction.

Let’s quickly recall what these materials are:

• P-type semiconductor: This is a material doped with trivalent atoms (like Boron). It has an abundance of holes (missing electrons) — which act as positive charge carriers.
• N-type semiconductor: This is doped with pentavalent atoms (like Phosphorus). It has extra free electrons, which are negative charge carriers.

When P type and N type are placed together:

1. Before contact
   • N-type region: has many free electrons (majority carriers).
   • P-type region: has many holes (majority carriers).
   • Both sides are neutral overall.
2. After contact (joining P and N)
   • There is a concentration gradient:
   • Electrons are high in N-side, low in P-side.
   • Holes are high in P-side, low in N-side.
3. Electron diffusion
   • Electrons from the N-side diffuse (move) across the junction into the P-side (because they move from high → low concentration).
   • When they cross and recombine with holes in the P-side, they leave behind positive donor ions (fixed) on the N-side.
4. Hole diffusion
   • Holes from the P-side also move slightly toward the junction and some cross into the N-side (though fewer, because electron mobility is higher).
   • Those holes that cross recombine with electrons near the junction, leaving negative acceptor ions (fixed) behind on the P-side.

As a result, a narrow region near the junction is formed that contains immobile positive ions on the N-side and immobile negative ions on the P-side.

This region is called the Depletion Region, as it is depleted of free charge carriers (electrons and holes).

Formation of the Electric Field

The presence of fixed positive and negative ions on either side of the junction produces an electric field (E) directed from the N-region (positive ions) toward the P-region (negative ions).

This electric field opposes further diffusion of electrons and holes across the junction.

As a result:

• Electrons experience a force opposite to this field (P → N).
• Holes experience a force along the field (P → N).
• The diffusion of majority carriers (electrons and holes) decreases.
• Diffusion current arises due to concentration gradients of charge carriers — basically, charges move from high concentration → low concentration.

Electrons naturally move from N → P (high electron concentration in N, low in P).

Holes naturally move from P → N (high hole concentration in P, low in N).

This movement of carriers creates a diffusion current.

• Drift current arises when charge carriers move due to an electric field.

In a PN junction, the depletion region has an electric field (from N → P). Drift current occurs because the electric field in the depletion region forces charge carriers to move.

Electrons move opposite to the electric field.

Holes move in the direction of the electric field.

This movement of carriers due to the electric field forms the drift current. Holes move along the field (N → P side). This movement due to the electric field is called drift current.

• An equilibrium condition is eventually reached where the diffusion current is exactly balanced by the drift current caused by this electric field.
• The potential difference across the junction is called the barrier potential (V₀).

Electrons that have crossed recombine immediately near the junction; they do not travel deep into the opposite side. The immobile ions and electric field maintain the depletion region.

Barrier Potential (V₀)

The barrier potential is the built-in potential difference across the depletion region created by the electric field of the immobile ions.

An external voltage must reduce or overcome this barrier sufficiently to significantly reduce the barrier and allow conduction.

Typical values are:

• For Silicon (Si) → around 0.7 volts
• For Germanium (Ge) → around 0.3 volts

This is why a silicon diode does not conduct until the voltage across it reaches roughly 0.7 V — that’s the point where the external voltage breaks down the built-in electric field and allows current to start flowing.

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What’s Next?

Now that we understand how a PN junction diode is formed, how the depletion region develops, and how the barrier potential controls current flow, the next step is to understand what happens when an external voltage is applied to the diode.

In the next blog, we will study:

• Forward Bias
• Reverse Bias
• Carrier movement inside the diode
• How the depletion region changes during biasing

👉 Click below to continue to the next part:

Understanding Forward Bias and Reverse Bias in a Diode