# Threshold voltage

The threshold voltage , also called flow , lock , forward , forward or knee voltage and rarely buckling voltage , is in electronics${\ displaystyle U _ {\ text {F}}}$

1. the voltage drop of a diode operated in the forward direction .
2. the base-emitter voltage of a bipolar transistor or the gate-source voltage of a field effect transistor at which a significant current flows in relation to the maximum collector or drain current .

Possible reference values for the threshold voltage:

• The voltage that can be read from the characteristic diagram of a diode when the apparently straight part of the characteristic is extended to the x-axis.
• In measuring devices with built-in diode tester, a current of 1 m is often A used as the measuring current; this corresponds generally to the 1-k Ω -Widerstandsmessbereich of the device.
• With self-blocking field effect transistors (type "e") the threshold voltage.

## description

Current-voltage characteristic curve of a silicon diode with a threshold voltage U F of approx. 0.7 V.

The current-voltage characteristic curve of a junction has no kink in the characteristic curve and also no natural threshold voltage. This is a misunderstanding that arises from the coarse scale used when displaying the characteristic curve. In reality, the characteristic curve is an exponential function (see below), and the differential internal resistance becomes higher and higher towards the origin of the coordinates , i. H. the characteristic curve steadily flatter.

The threshold voltages determined in this way for germanium signal diodes and Schottky diodes typically start at 0.3  V , those of silicon diodes typically at 0.6–0.7 V (see diffusion voltage ). For example, a bridge rectifier made of silicon diodes at a supply voltage of 5 V eff will only provide an output voltage of approx. 3.6  VDC under load because two diodes are always flowed through.

The Shockley equation describes the current flow through the diode in the pass band and is the special case of an Arrhenius equation : ${\ displaystyle I _ {\ text {D}}}$

${\ displaystyle I _ {\ text {D}} = I _ {\ text {S}} \, \ left (e ^ {\ frac {U _ {\ text {F}}} {n \, U _ {\ text {T }}}} - 1 \ right)}$

With

• Saturation reverse current (reverse current for short)${\ displaystyle I _ {\ text {S}} \ approx {10 ^ {- 12} \ dots 10 ^ {- 6} {\ text {A}}}}$
• Emission coefficient ${\ displaystyle n \ approx 1 \ dots 2}$
• Temperature stress ${\ displaystyle U _ {\ text {T}} = {\ frac {k \ cdot T} {q}} \ approx 25 \, {\ text {mV at room temperature}}}$

## Different meaning

In contrast, the term forward voltage is usually used in electrical engineering for the voltage drop across a diode or similar component under operating conditions. For example, if a rectifier is approved for one ampere , the manufacturer specifies the voltage for this current and the specified ambient temperature.

## literature

• Simon M. Sze, Kwok K. Ng: Physics of Semiconductor Devices . 3. Edition. John Wiley & Sons, 2006, ISBN 978-0-471-14323-9 .
• Ulrich Tietze, Christoph Schenk, Eberhard Gamm: Semiconductor circuit technology . 11., completely reworked. and exp. Edition. Springer, Berlin / Heidelberg 1999, ISBN 3-540-64192-0 .