Transistor
A transistor is a semiconductor device with three terminals that can amplify electrical signals or act as an electronic switch by controlling current flow between two terminals using a small input signal at the third. The two main types are the Bipolar Junction Transistor (BJT), which is current-controlled, and the Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET), which is voltage-controlled. Transistors are the fundamental building blocks of modern electronics, with billions packed into a single integrated circuit chip.
Key Formula
Collector current = β × Base current (for BJT in active region)
LaTeX: I_C = \beta \cdot I_B
| Symbol | Meaning | Unit |
|---|---|---|
| I_C | Collector current | Ampere (A) |
| I_B | Base current (small control current) | Ampere (A) |
| \beta | DC current gain (hFE), typically 50–500 for BJT | dimensionless |
Worked Example
Problem
An NPN BJT has β = 150 and a base current I_B = 40 µA. Find the collector current and determine if the transistor is in saturation given V_CC = 5 V and R_C = 500 Ω.
Solution
Step 1: Collector current: I_C = β × I_B = 150 × 40 × 10⁻⁶ = 6 mA. Step 2: Check saturation: V_CE(sat) = V_CC − I_C × R_C = 5 − 0.006 × 500 = 5 − 3 = 2 V. Step 3: Since V_CE = 2 V > V_CE(sat) ≈ 0.2 V, transistor is in active region (not saturated). Step 4: Power dissipated: P = I_C × V_CE = 6 mA × 2 V = 12 mW.
Answer
I_C = 6 mA; transistor is in active region, V_CE = 2 V
BJT vs MOSFET Transistor Comparison
| Property | BJT (NPN/PNP) | MOSFET (N-channel/P-channel) | Preferred Use |
|---|---|---|---|
| Control mechanism | Current (base current) | Voltage (gate voltage) | MOSFET for digital, BJT for analogue |
| Input impedance | Low–moderate (kΩ) | Very high (GΩ) | MOSFET draws negligible gate current |
| Switching speed | Moderate (ns to µs) | Very fast (ps to ns) | MOSFET preferred for high-frequency |
| Gain parameter | β (current gain, 50–500) | g_m (transconductance, mA/V) | Different amplifier design |
| Power consumption | Higher (always has I_B) | Lower (no gate current) | MOSFET for battery-powered devices |
| Application | Audio amps, linear circuits | Digital ICs, power electronics | Context-dependent |
Interactive Tools
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Related Terms
Diode
A diode is a two-terminal semiconductor device that allows current to flow primarily in one direction (forward bias) and blocks it in the reverse direction (reverse bias), acting as an electrical one-way valve. It is formed by a p-n junction — a contact between p-type (positive hole-rich) and n-type (negative electron-rich) semiconductor material. Diodes are used in rectifiers to convert AC to DC, in signal clipping and clamping, as protection devices, and in light-emitting diodes (LEDs) and photodiodes for optoelectronics.
Operational Amplifier
An operational amplifier (op-amp) is a high-gain, direct-coupled differential amplifier with very high input impedance and very low output impedance, designed to perform mathematical operations such as addition, subtraction, integration, and differentiation on electrical signals. The ideal op-amp has infinite open-loop gain, infinite input impedance, zero output impedance, and infinite bandwidth. Op-amps are fundamental building blocks in analogue electronics, used in signal conditioning, filters, oscillators, comparators, and instrumentation.
Logic Gate
A logic gate is a fundamental electronic circuit that performs a basic Boolean logic operation on one or more binary inputs (0 or 1) to produce a single binary output, forming the building blocks of all digital systems. The primary logic gates are AND, OR, NOT, NAND, NOR, XOR, and XNOR, each implementing a specific logical function defined by a truth table. Logic gates are implemented using transistors and are combined in large numbers to create complex digital circuits such as processors, memory units, and controllers.
The word "transistor" is a portmanteau of "trans-resistance" or "transfer resistor", coined by John R. Pierce at Bell Labs in 1948. The device was invented by William Shockley, John Bardeen, and Walter Brattain at Bell Labs on 23 December 1947, earning them the 1956 Nobel Prize in Physics.