Historically, some crude but useable semiconductor rectifiers were made even before high purity materials were available. Ferdinand Braun invented a lead sulfide (PbS, Galena) based point contact rectifier in 1874. Cuprous Oxide (Cu2O) rectifiers were used as power rectifiers in 1924. Forward voltage drop for Cu2O rectifiers is 0.2 V. Perhaps the linear characteristic curve of Cu2O rectifiers is why Cu2O was used as a rectifier for the AC scale on D'Arsonval based multimeters. Cu2O diodes are also photosensitive.
Selenium Oxide rectifiers were used before modern power diode rectifiers became available. Selenium Oxide rectifiers and Cu2O rectifiers are polycrystalline devices. Photoelectric cells were once made from Selenium.
Before the modern semiconductor era, diodes were used as radio frequency detectors, recovering audio signals from radio signals. The “semiconductor” was a polycrystalline piece of the mineral Galena (lead sulfide, PbS). A pointed wire known as a cat whisker was brought in contact with a spot on a crystal within the polycrystalline mineral. (Figure below) Operators labored to find a “sensitive” spot on the Galena by moving the cat whisker about. There are P- and N-type spots randomly distributed throughout Galena crystals due to variability of uncontrolled impurities. Less often, the mineral Iron Pyrite (Fool’s Gold) was used, as were the minerals Carborundum (Silicon Carbide, SiC). Another type of detector, part of a foxhole radio, consisted of a sharpened pencil lead bound to a bent safety pin, touching a rusty blue-blade disposable razor blade. These all required searching for a sensitive spot, easily lost because of vibration.
Replacing one of these minerals with an N-doped semiconductor makes the whole surface sensitive, so that searching for a specific sensitive spot is not required. This device was perfected by G. W. Pickard in 1906. A pointed metal contact produces a localized P-type region within the semiconductor. The metal point was fixed in place, and the whole point contact diode was encapsulated in a cylindrical body for mechanical and electrical stability. (Figure d) Note that a bar marking at the cathode end of the schematic symbol corresponds to a bar on a physical diode.
Silicon point contact diodes made an important contribution to radar in World War II by detecting giga-hertz radio frequency echo signals in radar receivers. To be clear, the point contact diode preceded the junction diode and modern semiconductors by several decades. Even to this day, the point contact diode is a practical means of microwave frequency detection because of its low capacitance. Germanium point contact diodes were once more readily available than today, being preferred in some applications (e.g., self-powered crystal radios) for their lower 0.2 V forward voltage. Point contact diodes, though sensitive to a wide bandwidth, have low current capacity compared with junction diodes.
Most diodes today are silicon junction diodes. The cross-section in Figure (b) looks a bit more complex than a simple PN junction, though, it is still a PN junction. Starting at the cathode terminal, the N+ indicates this region is heavily N-doped (the + means “more” rather than “electrically positive”). This reduces the series resistance of the diode. The N- region is lightly doped as indicated by the (-). Light doping produces a diode with a higher reverse breakdown voltage, important for high voltage power rectifier diodes. Lower voltage diodes, even low voltage power rectifiers, would have lower forward losses with heavier doping. The heaviest levels of doping produce Zener diodes which are designed for low reverse breakdown voltages. However, heavy doping increases the reverse leakage current. The P+ region at the anode contact is heavily doped P-type semiconductor, a good contact strategy. Glass-encased small signal junction diodes are capable of 10's to 100's of mA of current. Plastic or ceramic encapsulated power rectifier diodes can handle up to 1000's of amperes of current.
REVIEW:
Point contact diodes have superb high frequency characteristics, making them useable well into microwave frequencies.
Junction diodes range in size from small signal diodes to power rectifiers capable of 1000's of amperes.
The level of doping near a diode’s junction determines that diode’s reverse breakdown voltage. Light doping produces a diode with a high reverse breakdown voltage. Heavy doping produces a diode with a low reverse breakdown voltage, and increases the diode’s reverse leakage current. Zener diodes have lower breakdown voltages because of heavy doping.
Selenium Oxide rectifiers were used before modern power diode rectifiers became available. Selenium Oxide rectifiers and Cu2O rectifiers are polycrystalline devices. Photoelectric cells were once made from Selenium.
Before the modern semiconductor era, diodes were used as radio frequency detectors, recovering audio signals from radio signals. The “semiconductor” was a polycrystalline piece of the mineral Galena (lead sulfide, PbS). A pointed wire known as a cat whisker was brought in contact with a spot on a crystal within the polycrystalline mineral. (Figure below) Operators labored to find a “sensitive” spot on the Galena by moving the cat whisker about. There are P- and N-type spots randomly distributed throughout Galena crystals due to variability of uncontrolled impurities. Less often, the mineral Iron Pyrite (Fool’s Gold) was used, as were the minerals Carborundum (Silicon Carbide, SiC). Another type of detector, part of a foxhole radio, consisted of a sharpened pencil lead bound to a bent safety pin, touching a rusty blue-blade disposable razor blade. These all required searching for a sensitive spot, easily lost because of vibration.
Replacing one of these minerals with an N-doped semiconductor makes the whole surface sensitive, so that searching for a specific sensitive spot is not required. This device was perfected by G. W. Pickard in 1906. A pointed metal contact produces a localized P-type region within the semiconductor. The metal point was fixed in place, and the whole point contact diode was encapsulated in a cylindrical body for mechanical and electrical stability. (Figure d) Note that a bar marking at the cathode end of the schematic symbol corresponds to a bar on a physical diode.
Silicon point contact diodes made an important contribution to radar in World War II by detecting giga-hertz radio frequency echo signals in radar receivers. To be clear, the point contact diode preceded the junction diode and modern semiconductors by several decades. Even to this day, the point contact diode is a practical means of microwave frequency detection because of its low capacitance. Germanium point contact diodes were once more readily available than today, being preferred in some applications (e.g., self-powered crystal radios) for their lower 0.2 V forward voltage. Point contact diodes, though sensitive to a wide bandwidth, have low current capacity compared with junction diodes.
Most diodes today are silicon junction diodes. The cross-section in Figure (b) looks a bit more complex than a simple PN junction, though, it is still a PN junction. Starting at the cathode terminal, the N+ indicates this region is heavily N-doped (the + means “more” rather than “electrically positive”). This reduces the series resistance of the diode. The N- region is lightly doped as indicated by the (-). Light doping produces a diode with a higher reverse breakdown voltage, important for high voltage power rectifier diodes. Lower voltage diodes, even low voltage power rectifiers, would have lower forward losses with heavier doping. The heaviest levels of doping produce Zener diodes which are designed for low reverse breakdown voltages. However, heavy doping increases the reverse leakage current. The P+ region at the anode contact is heavily doped P-type semiconductor, a good contact strategy. Glass-encased small signal junction diodes are capable of 10's to 100's of mA of current. Plastic or ceramic encapsulated power rectifier diodes can handle up to 1000's of amperes of current.
REVIEW:
Point contact diodes have superb high frequency characteristics, making them useable well into microwave frequencies.
Junction diodes range in size from small signal diodes to power rectifiers capable of 1000's of amperes.
The level of doping near a diode’s junction determines that diode’s reverse breakdown voltage. Light doping produces a diode with a high reverse breakdown voltage. Heavy doping produces a diode with a low reverse breakdown voltage, and increases the diode’s reverse leakage current. Zener diodes have lower breakdown voltages because of heavy doping.