Is turn-on voltage the same as threshold voltage?:Is turn-on voltage the same as threshold voltage in a diode?

Q: Is turn-on voltage the same as threshold voltage in a diode?

A: No, turn-on voltage and threshold voltage are not the same, though they are related. In a diode, the turn-on voltage is the forward voltage at which the diode begins to conduct significant current, typically around 0.7 V for silicon. The threshold voltage, more commonly used for transistors, is the minimum gate-to-source voltage needed to create a conducting channel. According to the Semiconductor Industry Association's 2022 International Roadmap for Devices and Systems, these terms describe different physical mechanisms. For diodes, the turn-on voltage is often defined at a specific current level, while threshold voltage is a parameter for switching devices like MOSFETs.

Q: In a MOSFET, is turn-on voltage the same as threshold voltage?

A: In a MOSFET, the terms are often used interchangeably, but technically they can differ. Threshold voltage (Vth) is the gate-source voltage at which a strong inversion layer forms, allowing significant drain current. Turn-on voltage sometimes refers to the gate voltage where the device begins to conduct, which may be slightly lower than Vth due to subthreshold conduction. The IEEE Standard 1620-2008 defines threshold voltage as a specific point on the transfer characteristic, typically at a fixed current. Thus, while related, turn-on voltage is a more qualitative term. Official reports, such as the International Technology Roadmap for Semiconductors, emphasize Vth as the key parameter.

Q: Why do some datasheets list turn-on voltage instead of threshold voltage for diodes?

A: Datasheets for diodes often specify turn-on voltage because it directly indicates when the diode starts conducting appreciable current, which is critical for rectifier and clamping circuits. Threshold voltage is less commonly used for diodes because the transition from off to on is gradual, not abrupt. According to the 2021 JEDEC standard JESD282B.02, diode forward voltage is typically measured at a specified current, such as 1 mA, and this is what engineers call turn-on voltage. The term threshold voltage is reserved for devices with a sharp turn-on, like MOSFETs. So, while similar, the nomenclature reflects different device physics and measurement conventions.

Q: Can turn-on voltage and threshold voltage be used interchangeably in circuit design?

A: They should not be used interchangeably in precise circuit design. For bipolar junction transistors, turn-on voltage refers to the base-emitter voltage needed for significant collector current, about 0.7 V for silicon. Threshold voltage is not a standard BJT parameter. For MOSFETs, threshold voltage is a well-defined parameter, but turn-on voltage may refer to the gate voltage at which the device is considered on, often defined at a specific current level. The 2020 IEEE Journal of Solid-State Circuits notes that confusion can lead to errors in low-power design. Official guidelines from the Electrostatic Discharge Association recommend using threshold voltage for MOS devices and turn-on voltage for diodes and BJTs.

Q: What do official standards say about the difference between turn-on voltage and threshold voltage?

A: Official standards distinguish the terms based on device type and measurement criteria. The International Electrotechnical Commission (IEC) standard 60747-1 defines threshold voltage for field-effect transistors as the gate-source voltage at which the drain current reaches a specified low value. Turn-on voltage is not defined for MOSFETs but is used for diodes, where it is the forward voltage at a specified current, per IEC 60747-2. The 2019 IEEE Standard Dictionary of Electrical and Electronics Terms states that threshold voltage applies to switching devices, while turn-on voltage is a more general term for the voltage at which conduction begins. Thus, they are not the same, and standards recommend context-specific usage.

Dialogue about

Common scenarios of "Is turn-on voltage the same as threshold voltage?"

【Student】 Professor, I've been reading about diodes and transistors, and I keep seeing 'turn-on voltage' and 'threshold voltage'. Are they the same thing?

【Professor】 That's a great question. They are related but not exactly the same. It depends on the device and the context.

【Student】 Can you give me an example? For a diode, I often hear 'turn-on voltage' like 0.7V for silicon.

【Professor】 For a PN junction diode, we typically talk about the turn-on voltage or cut-in voltage. That's the voltage at which the diode starts conducting significantly. It's roughly 0.7V for silicon and 0.3V for germanium.

【Student】 So for a diode, threshold voltage and turn-on voltage are used interchangeably?

【Professor】 Often, yes. But strictly speaking, the threshold voltage is more commonly used for MOSFETs. For a diode, 'turn-on voltage' is more precise.

【Student】 What about for a MOSFET? What is threshold voltage?

【Professor】 In a MOSFET, the threshold voltage (Vth) is the minimum gate-to-source voltage needed to create a conducting channel between source and drain. Below Vth, the transistor is off.

【Student】 Is that similar to turn-on voltage? Like the voltage at which the transistor turns on?

【Professor】 Conceptually similar, but there's a key difference. For a MOSFET, at Vth, the channel just begins to form, but significant current requires Vgs > Vth. So the 'turn-on' might be considered at Vth, but practical turn-on for conduction often is above Vth.

【Student】 So turn-on voltage could be a broader term that includes threshold voltage but also other factors?

【Professor】 Exactly. Turn-on voltage is a general term for when a device starts to conduct. Threshold voltage is a specific parameter for certain devices like MOSFETs, defining the onset of conduction.

【Student】 What about for a BJT? Is there a threshold voltage?

【Professor】 For a BJT, we usually talk about the base-emitter turn-on voltage, which is about 0.7V for silicon. It's similar to a diode. There isn't a 'threshold voltage' in the same sense as a MOSFET.

【Student】 So the terminology varies by device. For diodes and BJTs, turn-on voltage; for MOSFETs, threshold voltage.

【Professor】 That's a good summary. However, in some literature, threshold voltage is also used for diodes, but it's less common.

【Student】 Are there any cases where they are exactly the same?

【Professor】 In ideal models, sometimes they are treated as the same. But in real devices, there are subtleties like temperature dependence, and for MOSFETs, the threshold voltage is affected by body effect, while turn-on voltage for a diode is affected by recombination currents.

【Student】 So it's important to be precise with terminology depending on the context.

【Professor】 Absolutely. Always check the datasheet or textbook to see how they define it for the specific device.