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Diode Forward Voltage versus Forward Current

01 February 2008

In electronics, as in life, it's often not what you don't know that gets you in trouble, but rather the things you think you know but in fact are wrong. I ran into that yesterday when I add a 1N5711 Schottky diode to clamp the reverse base voltage across a 2SC1945 transistor 10 watt RF power amplifier.

When I measured the reverse  voltage clamping level, I expected it to be around 0.5 volts or less. After all, a Schottky diode has a drop around 0.4 to 0.5 volts, compared with a standard silicon diode's 0.7 volts, right? When I measured around 1.2 volts across the 1N5711, I first thought I installed a 1N4148 silicon diode instead. The diode was clearly (well, clearly after using a magnifying glass) marked 1N5711.

That lead me to look at data sheets for several diodes and to measure the forward current versus forward voltage of four signal diode types:

- 1N5711, Schottky diode
- 1N4148 Silicon diode
- 1N270 Germanium point contact diode
- 1N914 Silicon diode

This data is taken with an automated measurement system, employing an HP6038A system power supply, an HP3456A digital multimeter and an Agilent 34410A digital multimeter, all controlled via a GPIB bus with a Prologix controller card. The software is home made, running in Liberty Basic. I've inserted a 1KΩ 2 watt series resistor to make the HP6038A into a quasi constant current power supply. (The HP6038A will function in constant current mode, but its resolution and minimum step size is inadequate for our purposes.)

I've also looked at three of these diode types when used as RF detectors as might be the case in a simple RF probe. See  the page at Diodes for RF Probes

Because the DC power is continuously applied to the diode under test, at higher current levels the diode junction's temperature will be elevated. Since a diode's forward voltage drops with increasing temperature, it's reasonable to expect the measured data to show increasing error with increasing current. The error will be to measure less forward voltage than would be found than if measured using a brief current pulse (300 μs seems to be the commonly used value) where the forward voltage drop can be measure before thermal effects appreciably warm the junction. I don't have suitable equipment to make these tests in an automated environment and it would be very time consuming to collect data manually. Accordingly, we'll accept some error in these measurements, recognizing that in fact this constant current measured data is more useful for purposes where the diodes are use in an environment in which the junction is heated by the forward current.

As a comparison point, I also simulated these four diodes using LTspice. I don't know where the various diode models I use in LTspice came from�some may have been provide with the original LTspice installation but I've added many models to my component library over the years. Since the accuracy of a SPICE run depends upon the accuracy of the model, one should not ascribe divergence between prediction and measurement to the simulator. Rather, the model and the measurement techniques should both be questioned and further studied.

The plot below shows measured and LTspice predictions for these four diodes. I do not have a SPICE model for the 1N270 diode, so I instead used a similar 1N34A Germanium point-contact diode in the simulation. Also, the 1N4148 and 1N914 SPICE models are essentially identical, so their plots overlay each other and only one is visible.

The solid lines are measured data and dotted lines are the SPICE simulations.|

Measured and SPICE Predicted
Datasheet Extracts
1N5711 Low current Vf/If
1N5711 High Current Vf/If
1N4148 and 1N914 (Same datasheet for Fairchild Semiconductor manufactured diodes)
1N4148 & 1N914 Low Current Vf/If
1N4148 & 1N914 High Current Vf/If
1N270 Low Current
1N270 High Current


1N914/1N4148�It's easy to dispose of the 1N914/4148 diodes. The agreement between SPICE, measurement and the datasheet is quite good. We can look at a three data points, 0.1, 1 and 10 mA, for example. Except at 10 mA, the measured data matches the datasheet as close as I can read it. The SPICE simulation runs 4-7% lower than the measure values, which is likely close enough for almost all simulations.

Forward Voltage (millivolts)

Forward Current Measured SPICE Datasheet
0.1mA 500 480 500
1mA 620 585 620
10mA 746 694 730
1N270�There moderate agreement amongst measure, SPICE and the datasheet at low current levels, but in general, there's considerable divergence amongst the three sources.

Forward Voltage (millivolts)

Forward Current Measured SPICE* Datasheet
0.1mA 170 206 150
1mA 310 297 250
10mA 485 404 400


1N5711�The original purpose of these measurements was to understand the 1N5711 Schottky diode better. To that end, a careful look at the datasheet shows a distinct breakpoint around 0.4V and just over 1mA. The measured data shows the same breakpoint, albeit in a more curvilinear fashion, at 0.4V and 1mA.

The 1N5711's datasheet shows a second breakpoint around 10mA, also shown in the measured data, again in a more curvilinear fashion.

The SPICE simulation, however, shows little sign of the 1 mA breakpoint and a greatly attenuated 10 mA breakpoint.

So, returning to my original question, it seems that a 1N5711 Schottky diode will exhibit a forward voltage of 1.2 or 1.3 volts where the forward current are well over 100 mA. Accurately measuring Vf/If at high currents requires a pulse approach as the junction heats quite rapidly at 100 mW and greater dissipation.

The takeaway point from this exercise is that a 1N5711 Schottky signal diode does not always have 0.4 V forward voltage and in fact the forward voltage drop across a 1N5711 and a 1N4148 may be quite similar as the forward current exceeds 10-20 mA.