The article provides an overview of the key specifications of diode, including maximum ratings, operating conditions, and their temperature-dependent behavior. It also covers topics such as junction capacitance, switching time, and various diode packaging styles based on current and voltage ratings.
Selection of a diode for a specific application requires knowledge of its specifications. This type of information is usually made available through the manufacturer. Diode specifications generally include absolute maximum ratings, typical operating conditions, mechanical data, lead identification, mounting procedures, and characteristic curves. This article provides you with the knowledge needed to interpret a diode data sheet.
General Diode Specifications
Some of the important rating and operating condition specifications are explained in the following:
- Maximum reverse voltage, $V_{RM}$: The absolute maximum or peak reverse-bias voltage that can be applied to a diode.
- Reverse breakdown voltage, $V_{BV}$: The minimum steady-state reverse voltage at which breakdown occurs.
- Maximum forward current, $I_{F}$: The absolute maximum repetitive forward current that can pass through a diode at $25^{o}C$ ($77^{o}F$). This is reduced for operation at higher temperatures.
- Maximum forward surge current, $I_{FM}$ (surge): The maximum current that can be tolerated for a short interval of time. This current value is much greater than IFM. This represents the increase in current that occurs when a circuit is first turned on.
- Maximum reverse current, $I_{RM}$: The absolute maximum reverse current that can be tolerated at device operating temperature.
- Forward voltage, $V_{FM}$: Maximum forward voltage drop for a given forward current at device operating temperature.
Some other specifications that may be found in a diode data sheet include the following:
- Mechanical data – this refers to the type of material used to construct the diode (usually molded plastic), maximum temperature values, and dimensions of the device.
- Lead identification – shows the diode symbol or a band on the cathode side to specify polarity.
- Mounting procedures – any special instructions in terms of temperature and dimensions/size restrictions are included.
- Characteristic curves – datasheets can include a forward current derating curve that plots maximum average forward current ($I_{F}$) at various ambient temperatures.
Other values that might be included on a data sheet include the following:
- Power dissipation, $P_{D}$: The maximum power that the device can safely absorb on a continuous basis in free air at $25^{o}C$ ($77^{o}F$). This may not appear on all data sheets. It can be calculated as $P_{D}= I_{FM}V_{FM}$.
- Reverse recovery time, T: The maximum time it takes the device to switch from its on state to its off state. This is not identified on the data sheet.
- Diode Temperature
The operation of a diode is directly related to temperature. All semiconductor materials are similar in this respect. The primary effect of temperature is the generation of additional electron hole pairs because more valence electrons are thermally excited and gain enough energy to move into the conduction band. These additional current carriers cause a decided change in the I–V characteristics of the diode.
The relationship between temperature and the I–V characteristics of a silicon diode are shown in Figure 1. These operating temperatures are indicated in degrees Celsius. Three main differences should be noted. The first of these is in the forward-bias region. This shows that less voltage is needed to produce conduction at higher temperatures. The other differences are in the reverse- bias area. One of these shows that there is a great deal more leakage current at higher temperatures. The third difference is in the breakdown voltage. This indicates that lower temperatures produce higher breakdown voltages. These considerations must all be taken into account when selecting a diode for a circuit application.
Figure 1. Relationship between Diode I-V Characteristics and Temperature
Electronic circuits that employ diodes are called on to operate in a rather wide range of temperatures. Consumer-grade diodes are usually rated for a range of $-50^{o}C$ to $+100^{o}C$. The extremes of this range are more meaningful if related to the boiling point of water ($100^{o}C$ or $212^{o}F$). Military-grade diodes are rated at $65^{o}C$ to $125^{o}C$. This type of diode is much more expensive than consumer-grade devices.
Junction Capacitance
A capacitor is defined as two or more conductive plates separated by an insulating material, called a dielectric. Remember that a capacitor develops an electrostatic charge between two conductive plates. The strength of the charge depends on the applied voltage, the size of the conductive plates, and the dielectric constant of the insulating material. The closer the plates are, the more charge may be set up between them.
A reverse-biased diode has a structure that responds as a capacitor. The two independent crystal materials serve as conductor plates, with the depletion zone acting as a dielectric material. The term junction capacitance is used to describe this effect. The value of a capacitor is determined by the thickness of the dielectric material and the area of the two conducting plates. The value of diode junction capacitance depends on the thickness of its depletion zone. A decrease in bias voltage causes a decrease in the depletion zone and an increase in junction capacitance. An increase in bias voltage causes an increase in the depletion zone and a decrease in junction capacitance.
Junction capacitance is a variable value that is dependent on bias voltage. Small amounts of reverse bias, and, in some cases, forward bias, produce the largest values of junction capacitance because the width of the depletion zone is reduced. A decrease in the depletion zone or dielectric thickness causes a corresponding increase in capacitance. When the reverse-bias voltage of a diode is increased, there is a decrease in junction capacitance because the width of the depletion zone is increased. With a thicker dielectric material, there is a smaller junction capacitance.
Figure 2. Junction Capacitance in a Reverse Biased Diode
The dielectric constant of a silicon depletion zone is approximately 12. This means that the depletion zone is 12 times better than air as a dielectric. As a result, some rather significant capacitance values appear across a junction diode. This value is extremely important in high-frequency circuit applications. A reverse-biased diode can, therefore, respond as a voltage- controlled capacitor. Special devices known as vari-cap diodes are designed to perform this function.
Switching Time
One rather important characteristic for some diodes is switching time, which refers to the time it takes for a diode to switch from one state to the other. This condition is important in computer applications that involve rapid turn-on and turn-off times.
When a junction diode is forward biased, its depletion zone begins to fill with current carriers. This action produces a large number of electron hole recombinations near the junction. If the bias voltage is suddenly removed, the recombination process does not stop instantly. Current carriers have a type of inertia that causes them to continue moving once they are placed in motion. The nonconduction state cannot be reached until all the current carriers have cleared the depletion zone. This means that the depletion zone has a current-carrier storage function.
Changing state from reverse bias to forward bias is also time dependent in a diode. The delay involved in this characteristic is due to junction capacitance. This capacitance tends to absorb the initial forward-bias current when it is being charged. After the charging operation has been completed, current carriers become available for normal conduction. Special switching diodes are now available for most high-speed control operations.
Diode Packaging
Diodes are manufactured in a wide range of case styles and packages. A person working with these devices must be familiar with some of the common methods of packaging. Element identification and lead marking techniques are essential for proper installation and testing. A diode improperly connected in a circuit may be damaged or may cause damage to other circuit parts.
The low-current diodes are the smallest of all pack ages. The body length of these packages rarely exceeds 0.3 cm. The cathode is usually denoted by a painted color band. Glass-packaged diodes often have two or three color bands to indicate specific number types. This group of diodes is generally capable of passing forward current values of approximately 100 mA. The peak reverse voltage rating rarely exceeds 100 V. Low reverse current values for these devices are typically 5 mA at $25^{o}C$.
The medium-current diodes are slightly larger in size than the low-current devices. Body size is approximately 0.5 cm with larger connecting leads. Diodes in this group can pass forward current values up to 5 A. Peak reverse voltage ratings generally do not exceed 1000 V. The anode (+) and cathode (-) terminals may be identified by a diode symbol on the body of the device. A color band near one end of the case is also used to identify the cathode. Low- and medium-current diodes are usually mounted by soldering. Any heat generated during operation is carried away by air or lead conduction.
High-current, or power, diodes are the largest of all diode types. These devices normally generate a great deal of heat. Air convection of heat is generally not adequate for most installations. These devices are designed to be mounted on metal heat sinks, which conduct heat away from the diode. Diodes of this classification can pass hundreds of amperes of forward current. Peak reverse voltage ratings are in the 1000-V range. Numerous packaging types and styles are used to house power diodes. As a rule, the rating of the device and method of installation usually dictate its package type.
Review Questions
- The absolute maximum reverse-bias voltage that can be applied to a diode is called the .
- $I_{FM}$ refers to the absolute maximum that can pass through a diode.
- The I–V characteristics of a silicon diode change with .
- The crystal structure of a reverse-biased diode responds as a(n) .
- The depletion zone of a reverse-biased diode serves as the material of a capacitor.
- A diode responds as a voltage-controlled capacitor.
- The ability of a diode to change from one state to another is called .
- Diodes are generally packaged according to their rating.
Answers
- peak inverse voltage
- forward current
- temperature
- capacitor
- dielectric
- vari-cap
- switching time
- power
Diode Specifications Key Takeaways
Understanding diode specifications, such as maximum ratings, temperature effects, junction capacitance, and switching time, is crucial for selecting the right diode for specific applications. These parameters ensure that the diode operates efficiently and safely under varying conditions, thereby enhancing the performance and reliability of electronic circuits.