How does an NPN transistor work?

In an NPN transistor, the emitter (heavily doped N-type) injects electrons into the thin, lightly doped P-type base. When the base-emitter junction is forward biased (~0.7V), electrons diffuse across the narrow base. The reverse-biased collector-base junction sweeps these electrons into the collector, creating a large collector current controlled by a small base current. The current gain beta = I_C/I_B is typically 50-300.

What is the difference between avalanche and Zener breakdown?

Avalanche breakdown occurs at higher voltages (>5V) in moderately doped junctions. High electric fields accelerate carriers to energies sufficient for impact ionisation, creating a cascade of electron-hole pairs. Zener breakdown occurs at lower voltages (<5V) in heavily doped junctions where the narrow depletion region allows quantum mechanical tunnelling of electrons directly from valence band to conduction band.

What are the operating regions of a BJT?

A BJT has four operating regions: Cutoff (both junctions reverse biased, transistor off), Active (BE forward biased, BC reverse biased, used for amplification), Saturation (both junctions forward biased, transistor fully on, V_CE ≈ 0.2V), and Reverse Active (BE reverse, BC forward, rarely used). The region depends on the bias voltages applied to the base-emitter and base-collector junctions.

BJT Transistor Simulator

NPN & PNP — Simulate · Explore · Practice · Quiz

Mode

📖 User Guide

Type

Bias

View

Adjust V_BE and V_CE to see how the transistor operates. Press Run to animate carrier flow.

Region Cutoff

I_C 0 mA

I_B 0 µA

β 100

V_BE 0 V

Active : I_C = β × I_B → —

V_BE0 V

V_CE5 V

I_B0 µA

I_C0 mA

I_E0 mA

RegionCutoff

V_BE (V)

0.00 V

V_CE (V)

5.0 V

β (h_FE)

100

Score: 0 / 0

Press "New Problem" to begin.

Question 1 / 5

Press "Start Quiz" to begin a 5-question assessment.

User Guide

1 Overview

The BJT Transistor Simulator visualises how bipolar junction transistors work at the semiconductor level. See electrons and holes flow through NPN and PNP structures, watch depletion regions respond to bias voltages, and observe breakdown mechanisms in real time.

Four modes: Simulate (interactive animation), Explore (educational cards), Practice (unlimited problems), and Quiz (5-question assessment).

2 Building the Circuit

Bjt Transistor simulator interface preview

Choose NPN or PNP transistor type. Adjust V_BE (base-emitter voltage) and V_CE (collector-emitter voltage) using the sliders. Press Run to see animated carrier flow. The description panel explains what is happening physically at each bias point.

3 Energising the Circuit

The cross-section view shows the three doped semiconductor regions (Emitter, Base, Collector) with their junctions. Blue dots represent electrons, red dots represent holes. The depletion regions (grey/hatched zones) shrink or grow based on bias.

Key observations:

Increase V_BE past 0.5V to see the BE junction turn on
At V_BE ≈ 0.7V with V_CE > 0.2V: Active mode — electrons flow E→B→C
Reduce V_CE below 0.2V: Saturation — both junctions forward biased
Use the Avalanche preset to see cascade impact ionisation
Use the Zener preset to see quantum tunnelling at the BE junction

Right-click the canvas to save as image, copy data, or reset.

4 Circuit Theory

Five educational categories: Basics (PN junctions and BJT structure), Operating Regions (cutoff, active, saturation), Breakdown (avalanche, Zener, punch-through, thermal runaway), Formulas (Ebers-Moll, I-V equations), and Applications (amplifiers, switches, biasing circuits).

5 Try a Problem

Practice: Solve problems about BJT regions, current calculations, and breakdown identification. Instant feedback with explanations.

Quiz: 5 randomly selected questions. Scored with a star rating.

6 Key Concepts

Current gain: β = I_C / I_B (typically 50–300)

KCL: I_E = I_C + I_B

Active mode: I_C = β × I_B, V_BE ≈ 0.7V

Saturation: V_CE(sat) ≈ 0.2V, both junctions forward biased

Breakdown: Avalanche (>5V, impact ionisation), Zener (<5V, tunnelling)

7 Tips & Best Practices

Use presets to jump to each operating region and observe the differences.
Watch the depletion region width change as you sweep V_BE from 0 to 0.8V.
Compare NPN and PNP — carrier types are reversed but principles are identical.
In Explore mode, read the Breakdown category before attempting breakdown questions.
Audio feedback: click on interaction, success/error tones in Practice and Quiz.

Understanding BJT Transistors: NPN and PNP

BJT transistor simulator showing a PN junction cross-section with the three regions emitter base and collector, doped regions colour-coded, and animated charge carriers crossing the junction under bias, plus operating-region selector switches — Default state showing the PN junction structure. Toggle NPN/PNP and active/saturation/cutoff to see how bias voltages move the carriers.

BJT transistor simulator showing NPN in active region with electrons being injected from the N emitter through the thin P base into the N collector with current arrows visible — NPN in active mode — electrons injected from emitter, swept into collector.

BJT transistor simulator showing PNP configuration with holes as majority carriers flowing from P emitter through N base to P collector with voltage polarities reversed — PNP equivalent. Same physics, holes as carriers, opposite voltage polarities.

What Is a BJT Transistor?

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that uses two PN junctions to control current flow. The three regions — Emitter, Base, and Collector — are alternately doped to form either an NPN or PNP configuration. A small current at the base terminal controls a much larger current between collector and emitter, making BJTs essential for amplification and switching.

NPN vs PNP: How They Differ

In an NPN transistor, the emitter injects electrons into the thin P-type base. Most electrons cross the base and are swept into the collector by the reverse-biased BC junction. In a PNP transistor, holes are the majority carriers — they flow from emitter through the N-type base to the collector. The physics is identical; only the carrier types and voltage polarities are reversed.

Operating Regions and Breakdown

BJTs operate in four regions: Cutoff (off, both junctions reverse biased), Active (amplification, I_C = β×I_B), Saturation (fully on, V_CE ≈ 0.2V), and Reverse Active (rarely used). Beyond normal operation, two breakdown mechanisms are critical: avalanche breakdown (impact ionisation cascade at high V_CB) and Zener breakdown (quantum tunnelling at heavily doped junctions).

The Three Amplifier Configurations — CE, CB, CC

A BJT can be wired in three fundamentally different amplifier topologies, depending on which terminal is shared between input and output.

Config	Voltage gain	Current gain	Input impedance	Where it lives
Common Emitter (CE)	High (50−200×)	High (β ≈ 100)	Medium (~1 kΩ)	General-purpose amplifiers; the workhorse
Common Base (CB)	High	~1 (α ≈ 0.99)	Very low (~50 Ω)	High-frequency RF amplifiers; cascode stages
Common Collector (CC) / Emitter Follower	~1 (less than)	High	Very high (~100 kΩ)	Buffers; impedance matching; output drivers

Most analogue circuits use a mix: a CE stage for voltage gain, then a CC (emitter follower) at the output to drive a low-impedance load without losing the gain you just built. CB is reserved for specific high-frequency applications where its low input impedance is an advantage.

A BJT Worked Example — Simple Common-Emitter Amplifier

Design a CE amplifier: V_CC = 12 V, β = 100, want I_C = 1 mA. Pick R_C and find R_B.

Step	Working	Result
Want V_CE at mid-supply	V_RC = V_CC/2 = 6 V	—
Collector resistor	R_C = V_RC/I_C = 6/0.001	6 kΩ
Base current required	I_B = I_C/β = 0.001/100	10 µA
Base resistor (V_BE = 0.7 V)	R_B = (V_CC − V_BE)/I_B = (12−0.7)/10×10⁻⁶	1.13 MΩ
Voltage gain (small-signal)	A_v = −g_mR_C ≈ −(I_C/V_T)·R_C = −(40×10⁻³)·6000	−240

That last row is the magic: a 1 mV input swing on the base produces a 240 mV swing at the collector, inverted. With careful biasing and proper capacitor coupling, this is the basis of every audio preamp and signal-conditioning circuit since 1950.

References for BJT Analysis

Sedra & Smith — Microelectronic Circuits, 8th ed., Chapter 6 (BJT Transistors).
Boylestad & Nashelsky — Electronic Devices and Circuit Theory, 11th ed.
Horowitz & Hill — The Art of Electronics, 3rd ed. Chapter 2 covers BJT amplifier design with great practical insight.

Explore Related Simulators

Continue your electronics learning with our Ohm's Law Simulator for circuit fundamentals, Capacitor Bank Simulator for energy storage and RC circuits, Ray Optics Simulator for physics exploration, or the Litmus Test Virtual Lab for chemistry.