What Is a Transistor?

What Is a Thyristor?

Key Differences

Structure and Electrical Properties

Applications and Limits

Pros and Cons

Frequently Asked Questions

A transistor is the foundation of modern electronics. Its main jobs are signal amplification, logic operation, and circuit switching. It is the core part of almost all digital and analog circuits, including CPUs, memory, and power converters.

Main Types

• BJT (Bipolar Junction Transistor): A current-controlled device (like NPN or PNP). It is good at small signal amplification.
• MOSFET: A voltage-controlled device. It switches fast and needs little drive power. Low RDS(on) (on-resistance) MOSFETs save energy.
• IGBT: A popular switch for high voltage and high current. It combines easy control (like a MOSFET) and low loss (like a BJT).

Key Features

Transistors need continuous drive signals (voltage or current) to stay on. They switch fast (in nanoseconds), are easy to put into chips (like CMOS), and can work in linear mode. This makes them good for high-speed and logic circuits.

A thyristor, also called an SCR (Silicon Controlled Rectifier), is a top choice for high-power AC control. It is great at rectifying, voltage adjusting, and switching big current. Its power level (thousands of amps or tens of kV) is much higher than a transistor.

Main Types

• SCR: Basic type that conducts in one direction. It is key for high-voltage DC.
• TRIAC: Works in both directions. It controls AC in dimmers and regulators. Industrial TRIACs are strong and work well in tough conditions.
• GTO/IGCT: Can turn off by gate signal. Used in very high-power converters.

Special Feature

Thyristors have a "latching effect". One pulse can turn them on and keep them on, even if the control signal stops. They turn off only when current stops or voltage reverses. They are simple and low-loss, but switch slower (in microseconds).

The inside structure decides the outside performance. Transistors and thyristors are built very differently.

Key Structure Differences

Transistor:

• BJT: Has three layers (NPN or PNP) and three terminals: Emitter (E) sends carriers, Base (B) controls them, and Collector (C) receives them. Current flows from E to C.
• MOSFET: Has three terminals: Gate (G), Source (S), and Drain (D). An insulation layer (SiO₂) separates the gate and the channel. Current flows sideways from S to D. Gate has high resistance, so needs very little drive power. It also has a body diode.
• IGBT: Has a MOSFET input and a BJT-like output. It adds a P+ layer to the MOSFET. It combines easy drive and low VCE(sat) (saturation voltage).

Thyristor:

• SCR: Has four layers (P-N-P-N) and three junctions (J1, J2, J3). Terminals are Anode (A), Cathode (K), and Gate (G).
• TRIAC: Acts like two SCRs in opposite directions. It has five layers and three terminals: T1, T2 (main terminals) and Gate (G). It can turn on in both AC half cycles.

Key Electrical Parameters

• Voltage Rating: SCRs can handle thousands of volts (VRRM, VDRM). BJTs/MOSFETs are lower. IGBTs can go up to 6.5kV. SCRs cost less for the same rating.

1. Common IGBT ratings: 600V–1700V (mainstream), 3300V–6500V (industrial).
2. SCRs: from hundreds of volts up to 10kV+.

• Current Capacity: SCRs can carry hundreds to thousands of amps. A single chip can do 100A–1000A+. MOSFETs and BJTs need parallel setup for high current. IGBTs can reach hundreds to thousands of amps in modules.
• Voltage Drop / Loss:

1. SCRs have very low on-state voltage (VTM), usually 1–2V, so loss is small.
2. BJTs have higher VCE(sat); MOSFET loss depends on RDS(on); IGBTs are in between (1.5–3V and rising with current).
3. Example: At 2000A, a 1V drop saves 2000W compared to IGBT with 2V.

• di/dt and dv/dt Tolerance:

1. di/dt = current rise rate.
2. dv/dt = voltage rise rate.
3. IGBTs and new MOSFETs handle these better than SCRs. SCRs need snubber circuits to prevent damage or false triggers.

• Switching Time:

1. MOSFET: very fast (tens of ns)
2. IGBT: 100ns to a few μs
3. SCR: slow (turn-off time tq = tens to hundreds of μs)

Typical Applications

Transistor (good at fast, flexible, and integrated control):

MOSFET:

• Switching Power Supplies (SMPS): phone chargers, PC/server power, telecom power.
• DC-DC Converters: motherboard power, car power, LED drivers.
• Motor Drives: BLDC motors, stepper motors, small inverters (H-bridge).
• High-Frequency Use: RF amplifiers, wireless charging, induction heating.
• Digital Logic Circuits: CPUs, DRAM/Flash memory, FPGA, ASIC (CMOS based).

BJT:

• Analog Signal Amplifier: audio preamps, sensor conditioning, linear power supplies.
• Low-Speed Switching: relay drivers, basic logic.

IGBT:

• Medium/High Power Motor Drives: industrial VFDs, EV/HEV inverters, train converters.
• Power Inverters: solar inverters, UPS, welding power.
• Induction Heating (mid-frequency): cooktops, industrial heaters.

Thyristor (best for very high power, low-frequency):

SCR:

• High Power Rectifier: electrolysis (aluminum/copper), DC arc furnace, big battery charging.
• HVDC Transmission: converts AC to DC at converter stations.
• Static Switches: big DC switching.
• Soft Starters: reduce startup current for AC motors.
• UPS Rectifiers: for data centers, industry.

TRIAC:

• AC Phase Control: home dimmers, heater power control, fan speed.
• Solid State Relay (SSR): contactless AC switch.
• Surge Protection: with SIDAC in protection circuits.
• GTO/IGCT: (used in older high-power systems, now partly replaced by high-end IGBT modules)
• Very Large Converters: big fans, metallurgy, marine drives.

Main Advantages

Transistor:

• High-Speed Master: MOSFETs switch in ns, great for GHz/MHz circuits.
• Full Control: You can easily turn on/off with signals. Good for PWM, precise control, logic.
• Highly Integrated: MOSFETs (CMOS) enable large chips like CPUs and memory.
• Easy to Turn Off: Just remove the drive signal.
• Good for Parallel Use: MOSFETs/IGBTs have positive temp coefficient, easy to combine for more current.

Thyristor:

• Power Giant: Handles very high voltage and current (kV/kA range).
• Low Loss: Very low VTM in high current, better efficiency than BJT/IGBT.
• Latch Saves Power: Needs no continuous drive when on. Handles surge current well.
• Simple and Reliable: Especially as SCR/TRIAC modules, mature and strong.

Main Disadvantages

Transistor:

• Voltage Drop Loss: BJT VCE(sat) is high; MOSFET RDS(on) causes loss in high current; IGBT also has drop.
• Drive Power Needed: Needs continuous control signal = energy used.
• Switching Loss: High speed causes switching loss. Needs soft-switching to reduce this.
• Delicate Gate: MOSFET gates are thin; must protect from static (ESD).

Thyristor:

• Hard to Turn Off: Gate can’t turn it off (except GTO/IGCT). Needs current stop or reverse voltage.
• Slow Switching: Long tq time, so low working frequency (<1kHz).
• dv/dt Problem: Fast voltage change may cause false turn-on. Needs snubber circuits.
• Can’t Amplify: Not suitable for linear signal amplification.

What is the difference between a thyristor and a transistor?

Unlike a transistor, which is a three-layer, two-junction semiconductor device (NPN or PNP) with three terminals, a thyristor features a four-layer, three-junction structure (P-N-P-N) also with three terminals. They differ significantly in their construction, operational principles, control mechanisms, and typical applications.

What are transistors?

A transistor is a semiconductor device constructed from three semiconductor layers or a channel with a gate. It functions to amplify signals, switch electrical currents, or regulate voltage within electronic circuits.

How many transistors in a cpu?​

The quantity of transistors integrated into a CPU differs considerably depending on its design, technological generation, and purpose, with modern examples incorporating anywhere from millions to billions.