The article covers various types of diode, including Schottky, PIN, Gunn, and IMPATT diodes, highlighting their construction, characteristics, and applications in electronics.
There are a number of two-terminal devices that have a single P–N junction or its equivalent that respond in some unconventional way. These devices have unique characteristics that distinguish them from conventional silicon diodes. As a rule, these devices may have a different method of operation, terminals, or construction. High-frequency oscillators and amplifiers, power control, and switching are typical applications for this group of diodes. Some of the devices found in this classification are Schottky, IMPATT, Gunn-effect, PIN, and switching diodes.
Schottky Diode
The Schottky diode is a two- terminal device that has a number of important applications in electronics. Its areas of application were first limited to the very high frequency (VHF) range of operation. Now, this device is found in low-voltage/high-current switching power supplies. It is also used in computer integrated circuits, radar systems, communication circuits, and instrumentation.
Figure 1. Device Structure of Schottky Diode
Schottky Diode Construction and Symbol
The construction of a Schottky diode is very different from that of the conventional silicon diode. The junction formed by this device is considered to be a metal-semiconductor structure. Figure 2 shows the structure of a Schottky diode and its schematic symbol. The semiconductor material is a piece of N-type silicon. The metal can be a variety of different materials, such as gold, silver, platinum, chrome, or tungsten. The construction techniques and materials of this device each result in a different frequency response and biasing voltage. In general, these characteristics are very similar in many respects. The device can be manufactured so that it will fit into a variety of different applications.
Figure 2. Schottky diode construction and symbol.
In the two materials of a Schottky diode, electrons are considered to be the majority current carriers. Metal naturally contains an abundance of electrons. The N-type of semiconductor material is purposely doped so that it has a large number of electrons that do not take part in the covalent bonding process. When the two materials are joined during the forming process, electrons from the N-type material immediately flow into the adjoining metal. These electrons possess a rather high level of energy compared with those of the metal piece. The injected electrons are commonly called hot carriers. In a conventional silicon diode, the current carriers are represented as electrons and holes. The energy level of the two current carriers is primarily the same. Schottky diodes are unique in that conduction is entirely by majority carriers. The heavy flow of electrons from N-type material to metal causes a depletion of current carriers near the metal junction. This depletion area is similar to that of a conventional silicon diode.
The absence of minority current carriers in a Schottky diode makes it an attractive high-speed switching device. The minority current carrier content of a diode normally slows the reverse recovery time of a solid-state device. With the minority carrier content at a minimum, the Schottky diode can be used to change states very quickly. Switching frequencies approaching 20 GHz are very common for the Schottky diode.
Schottky Diode I-V Characteristics
The Schottky diode exhibits unique I-V characteristics due to its metal-semiconductor junction. In the forward bias region, it has a significantly lower turn-on voltage, typically between 0.2V and 0.4V, depending on the material used. This low forward voltage drop minimizes power dissipation, making the diode efficient in high-current applications. The forward current increases exponentially with applied voltage, governed by the thermionic emission mechanism rather than charge carrier recombination as in PN junction diodes.
In the reverse bias region, Schottky diodes exhibit higher reverse leakage current compared to standard diodes. This leakage is due to the relatively small barrier height at the metal-semiconductor interface and is sensitive to temperature variations. The reverse breakdown voltage of Schottky diodes is lower, often in the range of 20V to 200V, which limits their use in high-voltage applications. These characteristics make Schottky diodes ideal for low-voltage, high-speed operations.
Figure 3. Schottky Diode I-V Characteristics Curve
Schottky Diode Applications
Schottky diodes find extensive use in modern electronics due to their fast-switching speed and low forward voltage drop. In power rectification, they are employed in switching power supplies and low-voltage DC-DC converters to minimize energy losses. Their ability to operate efficiently at high frequencies makes them suitable for RF circuits, where signal integrity and speed are crucial.
In clamping and protection circuits, Schottky diodes prevent overvoltage and reverse current damage in sensitive components, such as in battery protection systems and logic circuits. They are also used in solar photovoltaic systems to prevent reverse leakage currents at night. Furthermore, Schottky diodes are integral in high-speed digital systems, where they act as low-voltage drop components in voltage-level shifters and logic gates. Their versatility and efficiency make them essential in low-power and high-frequency electronic designs.
PIN Diode
Another type of diode is the PIN diode. The abbreviation PIN refers to the structure of the semiconductor material, which is quite different from that of a conventional silicon diode. The name “PIN” refers to the structure of the diode, which consists of three layers:
- P-type Layer: This layer is doped with acceptor impurities, creating an abundance of holes (positive charge carriers).
- Intrinsic Layer: This is a pure, undoped semiconductor layer that is sandwiched between the P-type and N-type layers. It has a high resistivity and acts as a barrier to charge carriers.
- N-type Layer: This layer is doped with donor impurities, creating an abundance of electrons (negative charge carriers).
Figure 4 shows the structure of a PIN diode and some of the schematic symbols that represent this device.
Figure 4. PIN diode crystal structure and symbols.
Probably the most important feature of a PIN diode is its ability to respond as almost a pure resistor at high radio frequencies. Resistance can be varied from 10,000 Q to less than 1 Q by the control of current passing through the diode. Most diodes have this characteristic to some degree. The PIN diode, however, is designed to achieve a relatively wide range of resistance with good linearity, low distortion, and low current drive. The PIN diode is widely used in high-speed switching applications, high-frequency control, and microwave circuits.
At radio frequencies, a forward-biased PIN diode behaves as a pure resistance. The resistance of a PIN diode is determined by the following:
- Bias voltage
- Thickness of the intrinsic layer
- Properties of the current carriers
The resistance is inversely proportional to the forward-bias current. Typically, only the on and off resistances are of major concern in the operation of this device as a switch.
Figure 5. Device Structure of PIN Diode
PIN Diode Characteristics
The PIN diode is a semiconductor device with a unique structure consisting of a P-region, an intrinsic layer, and an N-region. The intrinsic layer, positioned between the P and N regions, is devoid of free carriers and plays a critical role in the diode’s performance. In the forward bias condition, the diode exhibits low resistance due to carrier injection into the intrinsic layer, allowing high current flow. Conversely, under reverse bias, the intrinsic layer acts as an insulator, resulting in low reverse leakage current and a high breakdown voltage.
A distinctive feature of the PIN diode is its high-frequency response. The intrinsic layer increases the diode’s carrier transit time, enabling effective operation in RF and microwave frequencies. This property makes the PIN diode useful in applications requiring high power handling and minimal signal distortion. Additionally, the diode’s switching speed is slower compared to junction diodes, but its linearity in resistance is highly advantageous in controlled environments.
Applications of PIN Diode
PIN diodes are widely used in RF and microwave systems due to their excellent performance in high-frequency applications. In switching circuits, they function as RF switches to route signals efficiently with minimal loss. They are also employed in attenuators, where the diode’s resistance can be controlled to adjust signal amplitudes.
In photodetectors, the PIN structure enhances the efficiency of light absorption and carrier generation, making the diode suitable for optical communication systems and LIDAR applications. Furthermore, PIN diodes are utilized in power control circuits, such as in variable resistors or for isolating and protecting circuits from overvoltage conditions. Their ability to handle high RF power also makes them a key component in radar systems and amplitude modulators. The combination of high-speed performance and power handling capabilities ensures the PIN diode’s relevance in advanced electronic designs.
PIN Diode Housing
The housing, or packaging, of a PIN diode is similar in many respects to that of other high-frequency diodes. Figure 6 shows some representative high-frequency diode packages. The application of the device, its operating frequency range, power dissipation, and chip structure are some of the factors that dictate the packaging for a diode.
Figure 6. PIN diode package types.
Gunn Diode
Another semiconductor device that is used to control high-frequency AC is the Gunn diode. This particular device is primarily designed to operate in the microwave region. Typical operating frequencies are in the range of 5−100 GHz. This device is capable of controlling high-frequency AC with a minimum of parts and a low-voltage de-energizing source.
Figure 7. Gunn Diode Symbol
The Gunn diode differs from other semiconductor devices in its construction. It, for example, does not have a distinct P–N junction. Like the Schottky diode, the Gunn device has a piece of semiconductor material connected between two metal connections. This material is unevenly doped; so, the crystal will break into different conduction regions with fields of different intensities across them. With the application of a specific DC voltage value, the device will go into conduction and have a negative resistance effect. This effect can be used to generate or amplify RF signals. Operation depends on the amount or bulk of material involved in the structure. Gallium arsenide is used to form the semiconductor material of the device.
The Gunn diode must have a specific voltage polarity applied to its material to produce the negative resistance effect − that is, it must be properly biased to produce this characteristic. Most devices will be damaged if the polarity of the source voltage is reversed. Typical Gunn devices respond to voltages of 8−12 V dc. The operational current for a 10-mW device is 500−850 mA. In most microwave applications, the generated signal is radiated directly from the semiconductor material. An application of this device is in portable radar systems.
Characteristics of Gunn Diode
The Gunn diode, also known as a transferred electron device (TED), is a unique semiconductor device that exploits the Gunn effect to generate high-frequency oscillations. Unlike conventional diodes, it does not rely on a P-N junction but instead uses a single N-type semiconductor material, typically gallium arsenide (GaAs) or indium phosphide (InP). The key feature of the Gunn diode is the presence of negative differential resistance in its I-V characteristics, which occurs due to intervalley electron transfer in the material when a critical electric field is applied (approximately 3.3 kV/cm for GaAs).
In the negative resistance region, an increase in voltage results in a decrease in current, enabling the diode to sustain oscillations. The Gunn diode operates in microwave and millimeter-wave frequency ranges, typically between 1 GHz and 100 GHz, depending on the device geometry and material properties. It exhibits high efficiency in RF power generation and can deliver power levels ranging from milliwatts to several watts in specific configurations. The diode’s performance is influenced by factors like doping concentration, device dimensions, and heat dissipation mechanisms.
Figure 8. I-V characteristics curve for a Gunn diode
- Ohmic Region (0–2 V): The current increases linearly with voltage.
- Negative Resistance Region (2–6 V): The current decreases with increasing voltage, showing a unique negative resistance behavior.
- Saturation Region (>6 V): The current stabilizes or increases slightly with further increases in voltage.
Applications of Gunn Diode
The Gunn diode is widely used as a microwave oscillator in communication systems, radar systems, and laboratory test equipment. Its ability to generate stable and tunable high-frequency signals makes it an ideal choice for use in local oscillators for frequency synthesis and mixers in RF systems. The Gunn diode is also employed in Doppler radar applications, particularly for motion detection and speed measurement due to its high sensitivity and compact design.
In addition to communication, the Gunn diode finds applications in industrial sensing and proximity detection. It is a common component in devices like automatic door openers and intrusion alarms, where reliable microwave signal generation is essential. Moreover, the diode is utilized in millimeter-wave imaging systems, where high-frequency signals are critical for resolution and penetration capabilities. Its robustness, compactness, and efficiency make the Gunn diode a vital component in both commercial and defense-grade electronic systems.
IMPATT Diode
The IMPATT diode is similar in many respects to a conventional silicon diode in crystal construction and operation. The term IMPATT stands for IMPact Avalanche and Transit Time. The term avalanche indicates the area of operation, which is the reverse-bias area near the avalanche region of conduction. A small change in reverse voltage causes the diode to produce a negative resistance characteristic. This effect can be used to amplify or generate high-frequency AC signals. RF signal control is in the range of 2−10 GHz.
Characteristics of IMPATT Diode
The IMPATT (Impact Avalanche Transit Time) diode is a high-frequency device designed for microwave and millimeter-wave applications. Its I-V characteristics are distinct, operating in the reverse breakdown region due to its reliance on avalanche breakdown for functionality. The device exhibits negative resistance, a critical property for generating high-frequency oscillations. When reverse voltage is applied beyond the breakdown threshold, an avalanche multiplication of carriers occurs, followed by a transit time delay, resulting in high-power RF generation.
The IMPATT diode typically operates at very high breakdown voltages, often in the range of 50–100 V. The efficiency of the diode is generally between 5% and 20%, depending on the operating frequency. However, due to the avalanche mechanism, IMPATT diodes experience significant thermal heating, requiring proper heat dissipation techniques. Additionally, the noise levels are relatively high, which can affect applications where signal purity is critical. These diodes are generally fabricated from materials such as silicon, gallium arsenide, or indium phosphide, depending on the required frequency range and application.
Applications of IMPATT Diode
The IMPATT diode is widely utilized in high-frequency applications, especially in the microwave and millimeter-wave frequency ranges. It is commonly employed in radar systems, satellite communication, and high-power RF transmitters due to its capability to operate at frequencies from several gigahertz to hundreds of gigahertz. The diode is a preferred choice for continuous-wave (CW) and pulsed microwave generation in industrial heating and medical applications.
IMPATT diodes are also used in local oscillators for microwave communication systems and signal sources for frequency synthesis. Their compact size, ability to generate high-power signals, and suitability for integration with other circuits make them attractive for use in solid-state transmitters. However, their high noise and thermal sensitivity often limit their use in applications requiring precise signal clarity, such as sensitive communication links or low-noise microwave systems.
IMPATT Diode Housing
The housing or packaging of an IMPATT diode is very similar to that of other high-frequency diodes. Most of the packages used for the PIN diode could be used for the IMPATT diode. One very popular package not shown in Figure 6 has the diode mounted inside a small threaded bolt structure. This type of package permits the diode to be placed in a metal cavity that responds to microwave signals and also permits heat to be conducted away from the small P–N junction. Packaging is primarily dictated by the application of the device and the frequency being controlled. The IMPATT diode is very similar in operation to the Gunn-effect diode, but the IMPATT diode has a P–N junction, while the Gunn-effect diode has a metal N-type material structure.
Key Takeaways
- Schottky diodes are two-terminal devices that are constructed of metal and a piece of semiconductor material.
- The electrons of the N-type material in a Schottky diode possess a rather high level of energy compared with the electrons of the metal.
- When the metal and N-type semiconductor material of a Schottky diode are joined during the forming process, the injected electrons from the
- N-type material become hot carriers.
- The construction of a Schottky diode causes it to respond entirely to majority current carriers.
- Switching frequencies approaching 20 GHz are common with a Schottky diode.
- A PIN diode is constructed of an intrinsic layer of semiconductor material placed between the P-type and N-type materials of the junction.
- The resistance of a PIN diode can be varied from 10,000 Q to less than 1 Q by control of the current passing through the device.
- PIN diodes have the ability to respond as resistors to high radio frequencies.
- PIN diodes are widely used as high-speed switching devices in microwave control circuits.
- The Gunn-effect diode does not have a distinct P–N junction in its construction; it has a piece of semiconductor material connected between two metal terminals.
- When a particular DC voltage is applied to a Gunn-effect diode, the diode responds by producing a negative resistance.
- The negative resistance produced by a Gunn-effect diode can be used to amplify or generate microwave signals.
- The operating frequencies of a Gunn-effect diode are in the range of 5−100 GHz.
- IMPATT diodes are high-frequency devices.
- The term IMPATT refers to impact avalanche and transit time.
- IMPATT diodes operate in the reverse-bias region near the avalanche point of conduction.
- A small change in reverse voltage causes an IMPATT diode to have a negative resistance characteristic.
- The negative resistance produced by an IMPATT diode can be used to control RF signals in the 2−10 GHz range.
- Although these IMPATT diodes have a rather low-efficiency rating, under normal circumstances, this efficiency rating is much better than that of other high-frequency devices.
Review Questions
1. A(n) _____ diode has a metal-semiconductor type of construction.
2. In a Schottky diode, _____ are the majority current carriers.
3. The absence of minority current carriers in a Schottky diode improves its ability to perform high-speed _____ operations.
4. The high-energy electrons of a Schottky diode are generally called _____.
5. A device that has a piece of intrinsic material separating the N-type and P-type semiconductors is called a(n) _____ diode.
6. An important feature of the PIN diode is that it responds as a(n) _____ at high radio frequencies.
7. A Gunn-effect diode has a piece of _____ material connected between two metal terminals.
8. An IMPATT diode operates in reverse bias near the _____ region.
Answers
- Schottky
- electrons
- switching
- hot carriers
- PIN
- resistor
- semiconductor
- avalanche