The article discusses the electrical characteristics of PN junction diode, focusing on their forward and reverse bias behavior, including how voltage and current affect diode performance. It also compares the differences between germanium and silicon diodes, highlighting their unique properties and applications in electronic circuits.
This article examines PN junction diode electrical characteristics, such as voltage, current, and temperature. Since these characteristics vary greatly in an operating circuit, it is best to look at them graphically. This makes it possible to see how the device responds under different operating conditions.
Forward Bias Characteristics
When a diode is connected in the forward-bias direction, Figure 1, it conducts forward current ($I_{F}$). The value of forward current is directly dependent on the amount of forward voltage ($V_{F}$). The relationship between forward voltage and forward current is called the ampere-volt, or I–V, characteristic of a diode, Figure 2. Note, in particular, that $V_{F}$ is measured across the diode and that $I_{F}$ is a measure of what flows through the diode. The value of the source voltage ($V_{s}$) does not necessarily compare in value with VF.
Figure 1. Forward Biased Circuit for PN Junction Diode.
When $V_{F}$ equals 0 V, $I_{F}$ equals 0 mA. This value starts at the origin (0) of the graph. If $V_{F}$ is gradually increased in 0.1-V steps, $I_{F}$ begins to increase. When the value of $V_{F}$ is great enough to overcome the barrier potential of the P–N junction, a substantial increase in $I_{F}$ occurs. The point at which this occurs is often called the knee voltage. Knee voltage ($V_{k}$) is approximately 0.3 V for germanium diodes and 0.7 V for silicon.
If the value of VF increases much beyond $V_{k}$, the forward current becomes quite large. This, in effect, causes heat to develop across the junction. Excessive junction heat can destroy a diode. To prevent this from happening, a protective resistor, called a limiting resistor, is connected in series with the diode. This resistor limits $I_{F}$ to some point below the maxi mum current value of the diode. Diodes should not be operated in the forward direction without a current-limiting resistor.
A forward-biased silicon diode can have its current limited by a limiting resistor ($R_{limit}$). Assume that the limiting resistor has a value of $100 \Omega$. When the diode is connected in this manner, the forward current depends on the source voltage and the value of the limiting resistor.
Figure 2. Forward I-V Characteristic of the PN Junction Diode
Ohm’s law can determine the resulting forward current. Forward current ($I_{F}$), therefore, is determined by the following expression:
$$I_{f}=\frac{V_{s}-V_{k}}{R_{limit}}$$
Note that this formula takes into account the voltage drop across the diode when it is forward biased. For a silicon diode, this is 0.7 V. The voltage across the limiting resistor is the source voltage (V S) minus the knee voltage (V K) across the diode. Therefore,
$$I_{f}=\frac{V_{s}-V_{k}}{R_{limit}}$$
$$I_{f}=\frac{10V-0.7V}{100\Omega}$$
$$I_{f}=\frac{9.3V}{100\Omega}$$
= 0.093A or 93mA
Example 1
What is the forward current of this circuit?
Solution
The source voltage ($V_{s}$) for this circuit is 10 V and the limiting resistor ($R_{limit}$) is $200 \Omega$. The knee voltage ($V_{k}$) for a germanium diode is 0.3 V. Therefore,
$$I_{f}=\frac{V_{s}-V_{k}}{R_{limit}}$$
$$I_{f}=\frac{10-0.3}{200 \Omega}$$
$$I_{f}=\frac{9.7V}{200 \Omega}$$
= 0.0485A or 48.5mA
Reverse Bias Characteristics
When a diode is connected in the reverse-bias direction, it has an $I_{R}V_{R}$ characteristic. Figure 3 shows the reverse biased circuit for a PN Junction Diode. Whereas, the reverse I–V characteristic of the diode is shown in Figure 4. This characteristic has different values of $I_{R}$ and $V_{R}$. Reverse current is usually quite small. The vertical I–V line in this graph has current values graduated in microamperes. The number of minority current carriers that take part is quite small. In general, this means that IR remains rather constant over a large part of $V_{R}$. Note also that $V_{R}$ is graduated in 100-V increments. Starting at zero when the reverse voltage of a diode is increased, there is only a slight change in $I_{R}$. At the voltage breakdown $V_{R}$ point, current increases rapidly. The voltage across the diode remains fairly constant at this time. This constant-voltage characteristic leads to a number of reverse-biased diode applications. Normally, diodes are used in applications where the $V_{R}$ is not reached.
Figure 3. Reverse Biased PN Junction Diode
Figure 4. Reverse I-V Characteristic of a PN Junction Diode
The physical processes responsible for current conduction in a reverse- biased diode are called Zener breakdown and avalanche breakdown. Zener breakdown occurs when electrons are pulled from their covalent bonds in a strong electric field. This occurs at a rather high value of $V_{R}$. When large numbers of covalent bonds are broken at the same time, there is a sudden increase in $I_{R}$.
Avalanche breakdown is an energy-related condition of reverse biasing. At high values of $V_{R}$, minority carriers gain a great deal of energy. This gain may be great enough to drive electrons out of their covalent bonding, which creates new electron hole pairs. These carriers then move across the junction and produce other ionizing collisions and additional electrons. The process continues to build until an avalanche of current carriers is produced, at which point the process is irreversible.
Combined I–V Characteristics
The forward and reverse I–V characteristics of a diode are generally combined on a single characteristic curve. Figure 5 shows a rather standard method of displaying this curve. Forward-bias and reverse-bias voltages, $V_{F}$ and $V_{R}$, are usually plotted on the horizontal axis of the graph. $V_{F}$ extends to the right and $V_{R}$ to the left. The point of origin, or zero value, is at the center of the horizontal line.
Forward and reverse current values are shown vertically on the graph. $I_{F}$ extends above the horizontal axis, with $I_{R}$ extending downward. The origin serves as a zero indication for all four values. This means that combined $V_{F}$ and $I_{F}$ values are located in the upper-right part of the graph, and $V_{R}$ and $I_{R}$ are located in the lower-left corner. Different scales are normally used to display forward and reverse values.
Germanium requires less forward voltage to go into conduction than silicon. This characteristic is a distinct advantage in low-voltage circuits. Note also that a germanium diode requires less voltage drop across it for different values of current. This means that germanium has a lower resistance to forward current flow. Germanium, therefore, appears to be a better conductor than silicon. Silicon is more widely used, however, because of its low leakage current and lower production cost. The IR of a silicon diode is very small compared with that of a germanium diode.
Figure 5. Combined Forward and Reverse Characteristics of PN Junction Diode
Reverse current is determined primarily by the minority current con tent of the material, a condition influenced primarily by temperature. For germanium diodes, $I_{R}$ doubles for each $10^{o}C$ rise in temperature. In a silicon diode, the change in $I_{R}$ is practically negligible for the same rise in temperature. As a result, silicon diodes are preferred over germanium diodes in applications where large changes in temperature occur. Comparisons of this type are quite obvious through the study of characteristic curves.
PN Junction Diode Review Questions
- In a diode, forward current ($I_{F}$) is related to the value of forward voltage ($V_{F}$).
- Forward voltage ($V_{F}$) is measured a diode.
- Forward current ($I_{F}$) is a measure of the current passing a diode.
- When $V_{F}$ overcomes the barrier potential of a P–N junction, there is a large increase in .
- The knee voltage ($V_{k}$) is 0.3 V for and 0.7 V for .
- When the electrons of a diode are pulled from their covalent bonding in a strong reverse-biased electric field, breakdown occurs.
- The breakdown of a diode is due to an energy-related reverse- biased condition.
Answers
- directly
- across
- through
- forward current or IF
- germanium, silicon
- Zener
- avalanche
PN Junction Diode Key Takeaways
The understanding of PN junction diode characteristics, including forward and reverse bias behavior, is crucial for designing and optimizing electronic circuits. These characteristics influence how diodes are used in various applications such as rectifiers, voltage regulation, and signal processing. The forward current-voltage relationship ensures efficient current flow in applications like power supplies, while the reverse bias behavior, especially breakdown mechanisms like Zener and avalanche, is essential for designing protection circuits and voltage regulators.