---
title: "Triac vs diac; BJT vs MOSFET selection"
subject: "Power Electronics"
chapter: 1
key: "semester_7/power_electronics:triac-diac-bjt-mosfet-selection"
source: https://www.bctengineer.com/q/semester_7/power_electronics/triac-diac-bjt-mosfet-selection
site: BCT Engineering
---

# Triac vs diac; BJT vs MOSFET selection

Power Electronics · Chapter 1: Characteristics and specification of power electronic devices

- Asked in: 2082 Baishakh, 4+4 marks
- Video: [MOSFET BJT or IGBT - Brief comparison Basic components #004](https://www.youtube.com/watch?v=VlMdSCI29A0) — Electronoobs
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/power_electronics/triac-diac-bjt-mosfet-selection
- Whole chapter: https://www.bctengineer.com/md/semester_7/power_electronics/chapter-1

## Question

What are the differences between triac and diac? What are the key factors that engineers consider when selecting between power BJTs and power MOSFETs based on their conduction and switching loss characteristics?

## Short answer

A diac is a two-terminal bidirectional trigger diode without a gate; a triac is a three-terminal bidirectional thyristor with a gate. For BJT vs MOSFET, choose BJT for low-frequency high-voltage conduction loss, MOSFET for high-frequency low switching loss and easy paralleling; judge by total conduction plus switching loss at operating conditions.

## Answer

#### Triac versus diac

A **diac** is a two-terminal, bidirectional trigger diode that conducts in either direction once its breakover voltage (about 30 V) is reached. A **triac** is a three-terminal bidirectional thyristor (two SCRs in anti-parallel with one gate) used to control a.c. power.

| Point | Diac | Triac |
|---|---|---|
| Terminals | 2 (MT1, MT2) | 3 (MT1, MT2, gate) |
| Gate | No gate | Has a gate |
| Turn-on | Only by breakover voltage | By gate pulse in either half cycle |
| Layers | Typically 3/5-layer, no gate | 5-layer bidirectional thyristor |
| Current rating | Small (mA) | Large (A to tens of A) |
| Use | Trigger device for triac (light dimmer) | Main power switch in a.c. controllers |

Example: in a fan regulator, the diac fires the triac when the RC network voltage reaches about 30 V.

#### Choosing between power BJT and power MOSFET

| Factor | Power BJT | Power MOSFET |
|---|---|---|
| Conduction loss | Low: $V_{CE(sat)} \approx 0.5$-1 V nearly independent of rating | $I^2 R_{DS(on)}$; $R_{DS(on)}$ rises sharply with voltage rating |
| Switching loss | High: slow due to minority-carrier storage time | Very low: majority carrier, switches in tens of ns |
| Suitable frequency | Up to a few kHz | Tens to hundreds of kHz |
| Drive | Current-driven; large continuous base current | Voltage-driven; almost no steady gate current |
| Temperature | Negative coefficient; second breakdown and thermal runaway risk | Positive coefficient of $R_{DS(on)}$; easy paralleling, no second breakdown |

Key considerations for engineers:

1. **Voltage level:** at high voltage (above about 500 V), MOSFET $R_{DS(on)}$ is high, so conduction loss favours the BJT (or IGBT).
2. **Switching frequency:** at high frequency, switching loss dominates, so the MOSFET is chosen (SMPS, high-frequency converters).
3. **Total loss:** $P = P_{cond} + P_{sw}$, where $P_{sw} \propto f$; the device with lower total loss at the working current and frequency is chosen.
4. **Drive circuit cost**, ruggedness (safe operating area) and ease of paralleling.

Example: a 100 kHz, 48 V DC-DC converter uses a MOSFET; a low-frequency, high-voltage motor drive traditionally used BJTs (now usually IGBTs).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the main structural difference between a diac and a triac?
   A. A diac has a gate; a triac has none
   B. A diac has no gate (2 terminals); a triac has a gate (3 terminals)
   C. Both have three terminals
   D. A diac is unidirectional; a triac is bidirectional
   Answer: B. A diac is a two-terminal trigger diode turned on only by breakover voltage; a triac has a gate and can be fired in either half cycle. (Key point: Triac vs diac terminals)

2. In a fan regulator, what does the diac do?
   A. Carries the main load current
   B. Rectifies the supply
   C. Fires the triac when the RC network voltage reaches about 30 V
   D. Limits the triac di/dt
   Answer: C. The diac is a trigger device with breakover near 30 V and has a small current rating; it triggers the triac. (Key point: Diac use)

3. Why is a MOSFET preferred for high-frequency switching?
   A. Lower conduction loss at high voltage
   B. Majority-carrier device, so very low switching loss and fast switching
   C. Needs a large continuous gate current
   D. Has a negative temperature coefficient
   Answer: B. A MOSFET has no minority-carrier storage, switches in tens of ns, so switching loss (proportional to f) is low. (Key point: MOSFET vs BJT at high frequency)

4. Above about 500 V, why does conduction loss favour the BJT (or IGBT) over the MOSFET?
   A. MOSFET RDS(on) rises sharply with voltage rating
   B. BJT VCE(sat) rises sharply with voltage rating
   C. MOSFET needs more base current
   D. BJT switches faster
   Answer: A. MOSFET conduction loss is I^2 RDS(on), and RDS(on) grows sharply with voltage rating, whereas VCE(sat) of a BJT is nearly independent of rating. (Key point: High-voltage choice)

## Related in Power Electronics

- [BJT switching characteristics and base drive](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abjt-switching-gate-signal) — Power Electronics, chapter 1
- [Power MOSFET switching; threshold gate voltage](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Amosfet-switching-threshold) — Power Electronics, chapter 1
- [Battery charger bridge 30 V: conduction, power](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abattery-bridge-30v-conduction-power) — Power Electronics, chapter 2, asked 2×
- [Power transistor switching and base drive](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Apower-transistor-switching-base-drive) — Power Electronics, chapter 1
- [BJT as a switch in power circuit](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abjt-as-power-switch) — Power Electronics, chapter 1, asked 2×
- [Battery charger bridge 30 V: conduction period](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_electronics%3Abattery-bridge-30v-conduction) — Power Electronics, chapter 2, asked 4×

## Related in other subjects

- [TRIAC light dimmer conduction angles and power](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Atriac-light-dimmer) — Advanced Electronics, chapter 7
- [DIAC drawback; TRIAC four-quadrant operation](https://www.bctengineer.com/question.md?key=semester_4%2Fadvanced_electronics%3Adiac-drawback-triac-quadrants) — Advanced Electronics, chapter 7
- [BJT bipolar vs FET unipolar; MOSFET transconductance](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3Abjt-bipolar-fet-unipolar-gm) — Electronics Devices and Circuit, chapter 3
- [BJT as switch; MOSFET versus BJT](https://www.bctengineer.com/question.md?key=semester_2%2Fbasic_electronics_engineering%3Abjt-as-switch-mosfet-vs-bjt) — Basic Electronics Engineering, chapter 3