Lab guide
Where does the 0.7 V diode cut-in voltage come from?
What the 'turn-on voltage' really is, from simulated diode current and voltage data.
Silicon diodes are said to "turn on at 0.7 V". Plot the current against voltage yourself, though, and there is no sudden corner at 0.7 V. Fitting simulated data from a PSpice diode model to an exponential shows why.
1. Data and an exponential model
| Diode voltage | Current | Model |
|---|---|---|
| 300 mV | 4.91 µA | 4.92 µA |
| 400 mV | 34 µA | 34.4 µA |
| 450 mV | 91 µA | 91 µA |
| 520 mV | 357 µA | 355 µA |
| 580 mV | 1.14 mA | 1.14 mA |
| 620 mV | 2.52 mA | 2.49 mA |
| 650 mV | 4.48 mA | 4.46 mA |
| 680 mV | 8.05 mA | 7.99 mA |
| 700 mV | 11.8 mA | 11.8 mA |
| 720 mV | 17.4 mA | 17.4 mA |
| 740 mV | 25.5 mA | 25.7 mA |
| 760 mV | 37.3 mA | 37.9 mA |
All twelve points fit the single equation I = Is · e^(V / nVT) to within 1.6%. A log-linear fit gives nVT = 51.4 mV (ideality factor n ≈ 1.99) and Is = 14.4 nA.
2. Why there is no corner
Because it is exponential, every 118 mV multiplies the current by 10, at 300 mV just as at 700 mV. On a linear plot the curve seems to bend near 0.6–0.7 V only because that is where the current reaches milliamps and becomes visible.
3. The cut-in voltage depends on the current
| Current you call 'on' | Voltage at that current |
|---|---|
| 10 µA | 336 mV |
| 100 µA | 455 mV |
| 1 mA | 573 mV |
| 10 mA | 692 mV |
| 100 mA | 810 mV |
Call 1 mA "on" and the diode turns on at about 573 mV; call it 10 mA and it is about 692 mV. The textbook 0.6–0.7 V corresponds to the few to tens of milliamps typical circuits draw. Higher current or a colder diode pushes it a little higher.
4. Connecting it to the half-wave rectifier
In the same lab's half-wave rectifier simulation, a 5.00 V input peak gave a 4.35 V output peak, so 0.65 V was lost in the diode. With 4.35 mA peak through a 1 kΩ load, the model gives a diode voltage of 649 mV. That is why 0.65 V is a sensible diode drop for rectifier calculations. Adding a smoothing capacitor raises the charging current and the drop a little. Carry on in the rectifier ripple calculator.
5. In your lab report
- Quote a cut-in voltage together with its current, for example "0.57 V at I = 1 mA".
- Plot current on a log axis and the curve becomes a straight line whose slope gives the ideality factor n, which makes for a much more concrete discussion.
LEDs work the same way, which is why each color has a different forward voltage. Size an LED resistor with the LED resistor calculator, and see the zener diode calculator for diodes used in reverse breakdown.