What are signal diodes?

A diode is the simplest electronic device. There are many different types of diodes. Most are semiconductor diodes, while many diodes have a different construction than typical PN junctions. All diodes are, by definition, electronic valves controlled by two-terminal unidirectional voltage.

Signal diodes are often used in electronic circuits. Often the term signal diode is used to refer to “small signal diodes”. They are designed to operate small currents and high-frequency signals, which is why they are commonly used in radio, television, communications, and switching circuits.

These general-purpose semiconductor diodes can be easily identified by their relatively small size and are either “glass-passivated” diodes or “point-contact” diodes. In a glass-passivated diode, the PN junction is encapsulated in glass to protect it from contamination. There is a red or black stripe at one end to identify the cathode terminal. The power and current ratings of the signal diodes are minimal – typically 500mW and 150mA, respectively. Signal diodes are best suited for high frequency applications or switching applications involving short pulse waveforms.

Point contact diodes are slightly different in construction. A point contact diode is constructed by creating a PN junction between a gold or tungsten wire and an n-type germanium material. The gold wire allows high current to pass through the junction. Point contact diodes are used to detect high frequency signals as these diodes have the characteristic of blocking DC signals during the passage of high frequency AC.

Almost all signal diodes are silicon or germanium, but both types have slightly different specifications. A silicon diode has high reverse resistance, while a germanium diode has lower. The turn-on voltage of a silicon diode is 0.6 to 0.7 V, and a germanium diode is 0.2 to 0.3 V. Silicon diodes have lower forward resistance, which allows high forward current and reverse voltage spikes. The symbol of the signal diodes is the same as that of the generic diode.

VI signal diode characteristics

The VI characteristics of a signal diode are the same as those of any semiconductor diode.

The diode conducts current when positive supply is applied to the anode and negative supply is applied to the cathode. The diode blocks any current if the voltage is applied in the reverse direction. Ideally, a signal diode should completely block a negative signal. If the negative signal peak is beyond the Zener voltage of the signal diode, it must break down and may be permanently damaged. Therefore, a signal diode is expected to work as a perfect one-way valve for high-frequency electronic signals. The positive signal must be greater than the diode's knee voltage to be visible through the diode's cathode. Typically, communication and switching circuits operate with signals of 4V and higher. Therefore, a voltage drop of 0.3 V for a germanium diode or 0.7 V for a silicon diode has a negligible impact on the output signal levels.

Signal diode construction

Small signal diodes are constructed as PN diodes with a small junction area. A smaller junction area gives the diode a smaller junction capacitance. This also reduces the diode's reverse recovery time to a few nanoseconds or less. The small junction capacitance and fast response make the diode suitable for high-frequency applications. The simplest construction of a signal diode is the infusion of p-type silicon onto n-type silicon. However, simple construction has less predictable specifications.

A better construction of a signal diode is the Mesa diode. In a Mesa diode, a portion of the PN block is written. This gives the diode a high-topped plateau structure. Furthermore, the N-type layer is divided into two regions – one near the p-type material that is lightly doped and another heavily doped layer near the cathode contact. The entire structure is isolated by an encapsulating layer of silicon oxide or passivated glass to protect the diode from oxidation. The Mesa'diode construction makes the signal diode more reliable and results in better predictability of its parameters.

Signal diode specifications

Signal diodes are manufactured with a wide range of diode specifications. These static characteristics play an important role in selecting a specific model for a given application. Some of the important specifications of the signal diode are as follows –

1. Maximum forward current – ​​This is the maximum forward current allowed to flow through the diode. When the diode is forward biased, the current passing through it increases exponentially. The flow of electrical current through the diode dissipates energy in the form of heat. As more direct current flows through the diode, more heat is produced at its junction. This leads to thermal overload, causing failure or damage to the diode. This is why all signal diodes have a maximum forward current rating. Typically, it is in mA and supplied for room temperature (25˚C). A resistor in series with the signal diode can be connected to protect the diode from high currents. The maximum direct current is indicated as 'continuous direct current' for DC signals and 'repetitive direct current' for AC signals. The rating of repetitive direct current is always higher (almost double or greater) than the rating of continuous direct current. The maximum current rating is also indicated as a non-repetitive peak forward current. This is normally given in relation to the square waves of the time periods tested. This classification is useful in determining diode usefulness in high-frequency semiconductor applications whose peak signal amplitudes and frequencies are well defined.
2. Maximum reverse voltage – This is the maximum reverse operating voltage that can be applied to the signal diode without breaking and damaging it. It is always lower than the avalanche breakdown voltage and can vary from a few volts to thousands of volts. PIV rating is a crucial factor to be considered whenever the signal diode has to be applied in AC applications. It must be considered whenever the diode is replaced in a circuit. The maximum reverse voltage is indicated as 'continuous reverse voltage' for DC signals and 'repetitive reverse voltage' for AC signals. Typically, both classifications are the same. Therefore, signal diodes have the same PIV rating for DC and AC applications.
3. Total power dissipation – It is the maximum power that the signal diode can dissipate in the forward bias condition. As already mentioned, the signal diode dissipates electrical energy in the form of heat across the junction. The current increases exponentially through the diode for a small change in forward voltage. Power dissipation is derived from the multiplication of the forward voltage and current through the diode (P = V*I). The total power dissipation of the signal diodes is in the mW range provided for room temperature.
4. Maximum operating temperature – This is indicated as the junction temperature on a diode data sheet. It is related to the power dissipation of the diode. It is supplied in degrees centigrade. The junction temperature depends on both the direct current and the ambient temperature. Both factors increase the junction temperature or operating temperature of the diode.
5. Forward Voltage – Forward voltage is supplied for different current levels. Typically it is 0.6V to 1V for silicon diodes and 0.2 to 0.5V for germanium diodes.
6. Reverse current – ​​Total reverse current includes reverse saturation current and current due to metallic contacts in reverse bias condition. It is supplied for a given reverse voltage and is typically in the uA or nA range.
7. Diode Capacitance – It is the junction capacitance of the signal diode. It is provided for a specific frequency and is typically in the pF range.
8. Reverse recovery time – It is an important factor for diode selection in switching applications. It is supplied for a given level of current within a given range of forward and reverse current for a known load resistance. It's usually in nanoseconds.
9. Forward Recovery Voltage – The voltage required to reverse the forward current level of the signal diode. It is a useful parameter for high-speed switching applications. It is generally greater than the forward voltage of the diode.

In addition to the specifications mentioned above, the datasheet of a signal diode has many graphs. It is important to note that continuous forward current is plotted against ambient temperature, reverse current is plotted against junction temperature, forward current is plotted against forward voltage, repetitive peak forward voltage is plotted against pulse duration, and the diode capacitance is plotted as a function of the reverse voltage. These plots show the interdependence of various parameters and their applicability in forward and reverse characteristic regions.

The following table lists some of the popular signal diodes with their notable parameters.

Please note that this list is by no means exhaustive. It can not be. However, this list can be a starting point for exploring signal diodes. Look for the technical sheets of the signal diodes mentioned above. Note the common parameters and specifications provided for them. You will find some datasheets quite detailed and elaborate, while some datasheets are extremely accurate. For example, the following 1N4148 signal diode datasheet contains many details. However, the following datasheet for 1N4151 is very accurate.

Compare the two data sheets and find out the common parameters and specifications that are essentially stated in both data sheets. There are thousands of signal diodes available from hundreds of manufacturers. Search and discover the most popular signal diodes. Try exploring why some signal diodes are more popular while others have specific applications.

Signal Diode Applications

Some of the common applications of signal diodes are as follows:

Wave Shaping or Clipping – Signal diodes are often used to clip AC signals. One or more signal diodes can be used to clip an input signal. A signal diode can cut only one positive cycle or only one negative cycle. Using two signal diodes can cut both cycles of an incoming AC signal. The signal diode ignores the clipped signal in the forward bias condition if it is connected in parallel to the input port. The unclipped output signal is obtained for the reverse bias period of the diode. If both cycles are shaped using signal diodes, two signal diodes will be connected to the circuit in the opposite configuration. Therefore, one diode operates in one cycle and the other operates in the other cycle of the AC wave. The signal diode ignores the unclipped signal if connected in series with the input signal. If the peak voltage of the AC signal is not very high, the forward voltage of the diode can have a significant impact on wave formation.

DC Clamping and Restoring – Signal diodes are used to change the DC level of AC signals. This is called diode clamping or DC restoration. A clamper circuit essentially involves a signal diode and a capacitor. Diode clamping is often used in digital circuits to reset the upper and lower limits of square wave signals to communicate signals between two systems of different signal levels or for other specific requirements such as adjusting the pixel brightness of a display screen. .

Protection Diodes – Signal diodes are also used to protect other semiconductor devices and interfaces against sudden voltage spikes and excessive signal voltages. Such protection mechanisms are often required in control systems where semiconductor circuits or interfaces control high-power actuators. Freewheeling diodes are widely used to protect semiconductor devices. A signal diode is connected in parallel to an inductive load which is useful for suppressing voltage spikes and transients. Freewheeling diodes are commonly used to protect MOSFET and power transistors from inductive loads such as motors and damage caused by battery reverse protection.

Voltage regulation – In some cases, signal diodes can also be used for simple voltage regulation. In the forward bias region, a signal diode has a constant voltage drop. By connecting many signal diodes in series, a fixed voltage can be eliminated from the input voltage. For voltage regulation, Zener diodes are a better alternative. Still, simple voltage regulation can be achieved using signal diodes.

Signal Diode Packages

There are thousands of signal diode models available from hundreds of manufacturers. Signal diodes are most commonly available as a discrete component in through-hole and SMD packages. There are also signal diode arrays available to protect data lines in computers and digital circuits and protect data ports against voltage spikes and ESD. Diode arrays are available as isolated diodes, common cathode, and common anode arrays.