KAHIBARO
Table of Contents
Current and Voltage Behavior
A diode is a two terminal semiconductor device that allows current to flow much more easily in one direction than in the other. The main idea of diode characteristics is to describe how the current through the diode depends on the voltage across it. This relationship is not linear, so a diode does not behave like an ordinary resistor.
If we call the diode voltage $V_D$ and the diode current $I_D$, then the most important feature is that small changes in voltage can produce very large changes in current when the diode is forward biased. In reverse bias, the current is usually extremely small until breakdown is reached.
Forward Bias Characteristic
When the diode is connected so that the p side is at a higher potential than the n side, it is forward biased. In this case, the barrier at the junction is reduced, and charge carriers can cross the junction more easily. For small forward voltages, the current is tiny. As the forward voltage increases, the current rises rapidly.
A more realistic mathematical model for the diode is the diode equation,
$$
I_D = I_S \left(e^{\frac{V_D}{nV_T}} - 1\right)
$$
where $I_S$ is the saturation current, $n$ is the ideality factor, and $V_T$ is the thermal voltage. At room temperature, $V_T$ is about
$$
V_T \approx 0.026 \text{ V}
$$
This equation shows why the current increases exponentially in forward bias.
Important diode law:
$$
I_D = I_S \left(e^{\frac{V_D}{nV_T}} - 1\right)
$$
A diode is a nonlinear device. Its current is not proportional to voltage.
For silicon diodes, noticeable conduction often begins around $0.6 \text{ V}$ to $0.7 \text{ V}$. For germanium diodes, this begins at a lower voltage, often around $0.2 \text{ V}$ to $0.3 \text{ V}$. These are practical values, not sharp physical thresholds.
Reverse Bias Characteristic
When the diode is connected so that the p side is at a lower potential than the n side, it is reverse biased. In this case, the junction barrier increases, and current is strongly suppressed. Only a very small reverse current flows, usually called leakage current or reverse saturation current.
For ordinary operating conditions, reverse current is so small that the diode can be treated as an open circuit. However, if the reverse voltage becomes large enough, breakdown occurs and the reverse current rises sharply.
In reverse bias, a normal diode carries only a very small leakage current until breakdown is reached.
After breakdown, current can become very large and may damage the diode unless it is limited by the circuit.
Breakdown Region
The breakdown region is the part of the reverse characteristic where the diode suddenly begins to conduct strongly in reverse. This happens at a specific reverse voltage called the breakdown voltage.
Some diodes are designed to operate safely in breakdown, such as Zener diodes. Ordinary signal diodes usually are not meant to stay in this region. If current is not limited, overheating can occur.
The two main physical mechanisms are Zener breakdown and avalanche breakdown, but the detailed physics belongs more to specialized diode topics. For beginners, the key point is that reverse conduction changes dramatically once the breakdown voltage is exceeded.
The I V Curve
The current voltage characteristic of a diode is usually shown as an $I$ versus $V$ graph. The curve has three important regions.
| Region | Voltage condition | Current behavior |
|---|---|---|
| Forward bias | $V_D > 0$ | Current rises rapidly after practical turn on |
| Reverse bias | $V_D < 0$ and below breakdown | Very small leakage current |
| Breakdown | Large negative $V_D$ | Large reverse current |
A typical qualitative graph looks like this.
Practical Turn On Voltage
In circuit work, people often speak of a turn on voltage or knee voltage. This is the approximate forward voltage where current starts increasing strongly. It is useful in simple calculations, even though the real curve is smooth and exponential.
Typical practical values are shown below.
| Diode material | Approximate forward voltage |
|---|---|
| Germanium | $0.2$ to $0.3 \text{ V}$ |
| Silicon | $0.6$ to $0.7 \text{ V}$ |
| Light emitting diode, LED | often $1.8 \text{ V}$ or more |
These values depend on current and temperature, so they are only approximate.
Static and Dynamic Resistance
Because a diode is nonlinear, its resistance is not a single fixed number. Two useful ideas are static resistance and dynamic resistance.
The static resistance at one operating point is
$$
R_{\text{static}} = \frac{V_D}{I_D}
$$
The dynamic resistance describes how the voltage changes for a small change in current near a chosen point,
$$
r_d = \frac{dV_D}{dI_D}
$$
Dynamic resistance is especially important when a diode carries a steady current and a small signal is superimposed on it.
For a diode, resistance is not constant.
$$
R_{\text{static}} = \frac{V_D}{I_D}, \qquad r_d = \frac{dV_D}{dI_D}
$$
Always specify the operating point when discussing diode resistance.
Temperature Dependence
Diode characteristics depend strongly on temperature. As temperature rises, the forward voltage needed for a given current usually decreases, while the reverse leakage current usually increases.
This means a diode circuit can behave differently in hot and cold conditions. In many practical cases, the forward voltage of a silicon diode decreases by roughly a few millivolts per degree Celsius.
Ideal and Real Diode Models
In simple circuit analysis, an ideal diode is often used. In that model, the diode acts like a perfect conductor in forward bias and a perfect insulator in reverse bias. Real diodes are not so perfect.
A more practical approximation is the constant voltage drop model. In this model, a silicon diode is treated as having a nearly fixed forward drop of about $0.7 \text{ V}$ when conducting.
| Model | Forward bias behavior | Reverse bias behavior |
|---|---|---|
| Ideal diode | Zero voltage drop when on | Zero current when off |
| Constant drop model | About $0.7 \text{ V}$ for silicon | Zero current until breakdown |
| Exponential model | Given by diode equation | Small leakage current |
These models are chosen depending on how accurate the analysis needs to be.
Summary of the Characteristic Behavior
The defining characteristic of a diode is its strongly one way conduction. In forward bias, current grows very rapidly with voltage. In reverse bias, current remains tiny until breakdown. Because of this nonlinear behavior, the diode is useful for directing current, protecting circuits, and shaping electrical signals.
Key facts about diode characteristics:
A diode conducts easily in forward bias.
A diode blocks current in reverse bias, except for small leakage.
At sufficiently large reverse voltage, breakdown occurs.
The diode $I$-$V$ relation is nonlinear and is approximately exponential in forward bias.