1. Introduction
A transistor is the most fundamental active electronic component in modern computing.
Every CPU, GPU, RAM chip, SSD, smartphone, and server is built from billions of transistors.
If you understand transistors deeply, you understand how computers actually work—below programming languages, below operating systems, at the hardware logic level.
Transistors are used to:
- Switch electrical signals (digital logic)
- Amplify signals (analog systems)
- Store bits (memory)
- Build logic gates (AND, OR, NOT)
- Construct CPUs, registers, caches, and controllers
2. Why Transistors Exist
Before transistors, computers used vacuum tubes:
- Large
- Fragile
- High power consumption
- Massive heat output
Transistors replaced them because they are:
- Extremely small
- Energy efficient
- Fast
- Reliable
- Cheap to mass-produce
This replacement is why modern computing exists.
3. Fundamental Concept (Mental Model)
At its core:
A transistor is an electrically controlled switch.
- A small signal controls a larger current
- One terminal controls whether current flows between two other terminals
This single idea enables binary computation.
4. Main Types of Transistors
There are two primary families:
4.1 Bipolar Junction Transistor (BJT)
- Current-controlled
- Older technology
- Used in analog amplification
4.2 Field Effect Transistor (FET)
- Voltage-controlled
- Dominant in digital electronics
- Includes MOSFET (most important)
Modern CPUs use MOSFETs exclusively.
5. Bipolar Junction Transistor (BJT)
5.1 BJT Structure (Architecture)
A BJT has three layers of semiconductor:
- Emitter (E)
- Base (B)
- Collector (C)
Two configurations:
- NPN
- PNP
NPN Example
N (Emitter) | P (Base) | N (Collector)
5.2 Internal Physics (How It Works)
- Base is very thin
- Small current into the Base
- Allows large current to flow from Collector to Emitter
This is current amplification.
5.3 BJT Operating Modes
| Mode | Base-Emitter | Base-Collector | Description |
|---|---|---|---|
| Cutoff | OFF | OFF | Transistor OFF |
| Active | ON | OFF | Amplification |
| Saturation | ON | ON | Fully ON (switch) |
| Reverse Active | OFF | ON | Rare |
5.4 BJT Equations (Electrical Syntax)
Collector current:
IC = β × IB
Where:
-
IC= Collector current -
IB= Base current -
β= Current gain (hFE)
5.5 BJT as a Digital Switch
if (base_current > threshold) {
collector_emitter = ON;
} else {
collector_emitter = OFF;
}
6. Field Effect Transistor (FET)
Unlike BJTs, FETs are voltage-controlled.
6.1 Why FETs Dominate Digital Systems
- Almost zero input current
- Low power consumption
- Scales extremely well (nanometers)
- Perfect for logic gates
7. MOSFET (Metal-Oxide-Semiconductor FET)
7.1 MOSFET Architecture
A MOSFET has four terminals:
- Gate (G)
- Source (S)
- Drain (D)
- Body (B) (often tied to source)
Two main types:
- NMOS
- PMOS
7.2 Physical Internal Structure
Gate
-----
Oxide (Insulator)
-----
Semiconductor Channel
-----
Source ---- Drain
Key idea:
- Gate voltage creates an electric field
- Field forms or destroys a conductive channel
7.3 NMOS vs PMOS
| Feature | NMOS | PMOS |
|---|---|---|
| Charge carriers | Electrons | Holes |
| Turns ON when | Gate HIGH | Gate LOW |
| Speed | Faster | Slower |
| Power | Lower | Higher |
8. MOSFET Operating Modes
8.1 Cutoff Mode (OFF)
VGS < Vth
No channel exists.
8.2 Linear (Triode) Mode
VGS > Vth
VDS small
Acts like a resistor.
8.3 Saturation Mode
VGS > Vth
VDS large
Used in amplification and switching.
9. MOSFET Equations (Formal Syntax)
Drain Current (Saturation)
ID = (1/2) × μ × Cox × (W/L) × (VGS − Vth)²
Where:
-
μ= Carrier mobility -
Cox= Oxide capacitance -
W/L= Channel width/length ratio -
VGS= Gate-Source voltage -
Vth= Threshold voltage
10. Transistor as a Logic Switch
NMOS Example
if (gate_voltage == HIGH) {
output = LOW;
} else {
output = HIGH;
}
This behavior creates a NOT gate.
11. Building Logic Gates from Transistors
11.1 NOT Gate (Inverter)
- 1 NMOS
- 1 PMOS
Input → Gate
Output → Drain junction
11.2 NAND Gate
- 2 PMOS (parallel)
- 2 NMOS (series)
NAND is critical because:
All digital logic can be built from NAND gates alone
11.3 AND, OR, XOR
Constructed by combining:
- NMOS pull-down networks
- PMOS pull-up networks
12. CMOS Technology
Modern chips use CMOS (Complementary MOS):
- NMOS pulls output LOW
- PMOS pulls output HIGH
- Never ON at the same time
Benefits:
- Near-zero static power
- High noise immunity
- Extreme scalability
13. Transistors in CPUs
Inside a CPU:
- Logic gates → Adders → ALUs
- Registers → Flip-flops → Latches
- Caches → SRAM cells (6 transistors per bit)
Example: SRAM Cell
6 Transistors = 1 Bit
14. Transistors and Binary
| Voltage | Meaning |
|---|---|
| 0V | Binary 0 |
| VDD | Binary 1 |
Transistors physically represent bits.
15. Moore’s Law and Scaling
- Transistor size now < 3 nm
- Billions per chip
- Quantum effects becoming problematic
16. Power, Heat, and Leakage
As transistors shrink:
- Leakage current increases
- Heat density increases
- Clock speeds plateau
This is why modern CPUs focus on:
- Parallelism
- Efficiency
- Specialized cores
17. Transistors vs Software Abstractions
| Layer | Built From |
|---|---|
| Software | Instructions |
| ISA | Micro-ops |
| Micro-ops | Logic gates |
| Logic gates | Transistors |
Every line of code eventually becomes transistor switching.
18. Real-World Developer Perspective
When you write:
if (a > b) {
c = 1;
}
It becomes:
- Comparators
- Logic gates
- Voltage changes
- Transistor switching
19. Summary
Transistors are:
- The foundation of digital computing
- Voltage-controlled or current-controlled switches
- The building blocks of CPUs, memory, and logic
- Responsible for all modern technology
Understanding transistors means understanding how reality executes code.
20. Final Thought
Software is just a high-level description of transistor behavior.
If you master transistors, nothing in computer science feels magical anymore.
Top comments (1)
Nice post, now I see why back in the day when programming begun programmers felt like "wizards" when the only way to talk to the computer was to code raw 0's and 1's