1. 1 SEMICONDUCTORS Tunnel an Varactor Diodes

2. 2 SEMICONDUCTORS PN diodes and zener diodes have lightly doped PN junctions and similar V-I characteristics. Tunnel diodes are heavily doped and very different from PN and zener diodes PN diodes and zener diodes have lightly doped PN junctions and similar V-I characteristics. Tunnel diodes are heavily doped and very different from PN and zener diodes

3. 3 SEMICONDUCTORS Tunnel diodes have a high internal barrier voltage and a narrow depletion region. It also has an extremely low reverse breakdown voltage (almost zero) which means it conducts large currents when its reverse biased. At low forward bias voltages, electrons are forced through the depletion region at extremely high velocities. Tunnel diodes have a high internal barrier voltage and a narrow depletion region. It also has an extremely low reverse breakdown voltage (almost zero) which means it conducts large currents when its reverse biased. At low forward bias voltages, electrons are forced through the depletion region at extremely high velocities.

4. 4 SEMICONDUCTORS Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow P–N junction barrier because filled electron states in the conduction band on the N-side become aligned with empty valence band hole states on the P- side of the P-N junction. As voltage increases further these states become more misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage. Under normal forward bias operation, as voltage begins to increase, electrons at first tunnel through the very narrow P–N junction barrier because filled electron states in the conduction band on the N-side become aligned with empty valence band hole states on the P- side of the P-N junction. As voltage increases further these states become more misaligned and the current drops – this is called negative resistance because current decreases with increasing voltage.

5. 5 SEMICONDUCTORS As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the P–N junction, and no longer by tunneling through the P–N junction barrier. Thus the most important operating region for a tunnel diode is the negative resistance region. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the P–N junction, and no longer by tunneling through the P–N junction barrier. Thus the most important operating region for a tunnel diode is the negative resistance region.

6. 6 SEMICONDUCTORS Here is the V-I characteristics of a tunnel diode

7. 7 SEMICONDUCTORS Tunnel diodes operate as oscillators capable of operating in the microwave frequencie range and are often used as UHF oscillators in TV tuners Tunnel diode applications also include trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits.oscilloscopes Tunnel diodes operate as oscillators capable of operating in the microwave frequencie range and are often used as UHF oscillators in TV tuners Tunnel diode applications also include trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits.oscilloscopes

8. 8 SEMICONDUCTORS Varactor diodes are sometimes referred to as variable capacitor diodes (varicap) because they have a usable amount of capacitance created at the PN junction.

9. 9 SEMICONDUCTORS Varactors are operated with a reverse-bias voltage that is less than its reverse breakdown voltage rating. As the reverse voltage increases, the depletion region widens and acts as a wider dielectric between the N and P sections A decrease in reverse bias voltage will cause an increase in the diode’s internal junction capacitance. Varactors are operated with a reverse-bias voltage that is less than its reverse breakdown voltage rating. As the reverse voltage increases, the depletion region widens and acts as a wider dielectric between the N and P sections A decrease in reverse bias voltage will cause an increase in the diode’s internal junction capacitance.

10. 10 SEMICONDUCTORS Here is an example of the operation of a varicap. Here is an example of the operation of a varicap.

11. 11 SEMICONDUCTORS Varicotor diodes are used to vary the frequeny of a resonant circuit. They also find uses in high frequency amplifiers and frequency multipliers. They are also used as automatic frequency control (AFC) circuits found in FM radios. Varicotor diodes are used to vary the frequeny of a resonant circuit. They also find uses in high frequency amplifiers and frequency multipliers. They are also used as automatic frequency control (AFC) circuits found in FM radios.

12. 12 SEMICONDUCTORS The Schottky diode also known as (hot carrier diode) is a semiconductor diode with a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless can be considered primitive Schottky diodes. The Schottky diode also known as (hot carrier diode) is a semiconductor diode with a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless can be considered primitive Schottky diodes.

13. 13 SEMICONDUCTORS When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6– 1.7 volts, while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency. When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6– 1.7 volts, while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency.

14. 14 SEMICONDUCTORS The most important difference between the P - N and Schottky diode is reverse recovery time, when the diode switches from conducting to non-conducting state. Where in a P-N diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e. no charge carrier depletion region at the junction). The most important difference between the P - N and Schottky diode is reverse recovery time, when the diode switches from conducting to non-conducting state. Where in a P-N diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e. no charge carrier depletion region at the junction).

15. 15 SEMICONDUCTORS Commonly encountered Schottky diodes include the 1N5817 series (1 ampere) rectifiers. Schottky metal–semiconductor junctions are featured in the successors to the 7400 TTL family of logic devices, the 74S, 74LS and 74ALS series, where they are employed as clamps in parallel with the collector- base junctions of the bipolar transistors to prevent their saturation, thereby greatly reducing their turn-off delays. Commonly encountered Schottky diodes include the 1N5817 series (1 ampere) rectifiers. Schottky metal–semiconductor junctions are featured in the successors to the 7400 TTL family of logic devices, the 74S, 74LS and 74ALS series, where they are employed as clamps in parallel with the collector- base junctions of the bipolar transistors to prevent their saturation, thereby greatly reducing their turn-off delays.

16. 16 SEMICONDUCTORS A Gunn diode, also known as a transferred electron device (TED), is a form of diode used in high- frequency electronics.

17. 17 SEMICONDUCTORS Its internal construction is unlike other diodes in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between. Its internal construction is unlike other diodes in that it consists only of N-doped semiconductor material, whereas most diodes consist of both P and N-doped regions. In the Gunn diode, three regions exist: two of them are heavily N-doped on each terminal, with a thin layer of lightly doped material in between.

18. 18 SEMICONDUCTORS When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity, preventing further conduction and current starts to fall. When a voltage is applied to the device, the electrical gradient will be largest across the thin middle layer. Conduction will take place as in any conductive material with current being proportional to the applied voltage. Eventually, at higher field values, the conductive properties of the middle layer will be altered, increasing its resistivity, preventing further conduction and current starts to fall.

19. 19 SEMICONDUCTORS Because of their high frequency capability, Gunn diodes are mainly used at microwave frequencies and above. Their most common use is in oscillators, but they are also used in microwave amplifiers to amplify signals. Because of their high frequency capability, Gunn diodes are mainly used at microwave frequencies and above. Their most common use is in oscillators, but they are also used in microwave amplifiers to amplify signals.

20. 20 SEMICONDUCTORS Gunn diode oscillators are used to generate microwave power for: Airborne collision avoidance radar, anti-lock brakes, car radar detectors, pedestrian safety systems, motion detectors, traffic signal controllers, automatic door openers, burglar alarms Gunn diode oscillators are used to generate microwave power for: Airborne collision avoidance radar, anti-lock brakes, car radar detectors, pedestrian safety systems, motion detectors, traffic signal controllers, automatic door openers, burglar alarms

21. 21 SEMICONDUCTORS An IMPATT diode (IMPact ionization Avalanche Transit- Time) is a form of high-power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields. An IMPATT diode (IMPact ionization Avalanche Transit- Time) is a form of high-power diode used in high-frequency electronics and microwave devices. They are typically made with silicon carbide owing to their high breakdown fields.

22. 22 SEMICONDUCTORS They operate at frequencies between about 3 and 100 GHz or more. A main advantage is their high-power capability. These diodes are used in a variety of applications from low- power radar systems to alarms. A major drawback of using IMPATT diodes is the high level of phase noise they generate. They operate at frequencies between about 3 and 100 GHz or more. A main advantage is their high-power capability. These diodes are used in a variety of applications from low- power radar systems to alarms. A major drawback of using IMPATT diodes is the high level of phase noise they generate.

23. 23 SEMICONDUCTORS This results from the statistical nature of the avalanche process. Nevertheless these diodes make excellent microwave generators for many applications. This results from the statistical nature of the avalanche process. Nevertheless these diodes make excellent microwave generators for many applications.

24. 24 SEMICONDUCTORS Can you identify the different diode types below?

25. 25 SEMICONDUCTORS The most commonly used diode in electronics