Simulation · Physics · Class 12
A one-way valve for current
From the lesson Diode V-I characteristics in Semiconductor Electronics: Materials, Devices and Simple Circuits. Change the values and watch what happens.
The idea behind it
NCERT §14.6.2
- To trace the V-I curve, a battery feeds the diode through a potentiometer (or rheostat) so the voltage can be varied. A milliammeter reads the large forward current and a microammeter the tiny reverse current.
- Forward bias: the current stays almost zero until the voltage passes a threshold, then rises steeply (exponentially) for small further increases.
- This threshold or cut-in voltage is about 0.2 V for a germanium diode and about 0.7 V for a silicon diode.
- Reverse bias: the current is very small (µA) and almost constant as the voltage changes. This is the reverse saturation current.
- At a critical reverse voltage, the breakdown voltage Vbr, the reverse current suddenly shoots up. Unless an external circuit limits it below the rated value, overheating destroys the junction; the same happens in forward bias if the current exceeds its rating.
- General-purpose diodes are used only below breakdown, in the reverse saturation region.
- So a diode passes current essentially one way: its forward resistance is low and its reverse resistance is high. Rectifiers exploit this.
- Dynamic resistance: rd = ΔV/ΔI (Eq. 14.6), the ratio of a small change in voltage to the resulting small change in current.
- Example 14.4 (silicon diode): between 10 mA at 0.7 V and 20 mA at 0.8 V, the forward resistance is rfb = 0.1 V / 10 mA = 10 Ω.
- At V = −10 V the curve gives I = −1 µA, so the reverse resistance is rrb = 10 V / 1 µA = 1.0 × 10⁷ Ω, a million times the forward value.
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