Extra Class question of the day: Semiconductor diodes

September 04, 2012 Dan KB6NU

Diodes are the simplest semiconductor devices. In their simplest form, they have two terminals and conduct current in only one direction, from the cathode to the anode. By manipulating the characteristics of the semiconductor material, manufacturers can make diodes useful in a wide variety of applications.

Take, for example, the Zener diode. The most useful characteristic of a Zener diode is a constant voltage drop under conditions of varying current. (E6B01) This makes it useful in voltage regulator circuits.

Another example is the varactor diode. The varactor diode is a semiconductor device designed for use as a voltage-controlled capacitor. (E6B04) Varactor diodes are often used in tuning circuits.

A PIN diode is a semiconductor device that operates as a variable resistor at RF and microwave frequencies. One common use for PIN diodes is as an RF switch. (E6B12)The characteristic of a PIN diode that makes it useful as an RF switch or attenuator is a large region of intrinsic material. (E6B05) The forward DC bias current is used to control the attenuation of RF signals by a PIN diode. (E6B11)

Two types of diodes used in RF circuits are the tunnel diode and hot-carrier diode. The tunnel diode is a special type of diode is capable of both amplification and oscillation. (E6B03) Tunnel diodes are capable of operating well into the microwave region. A hot-carrier diode is commonly used as a VHF / UHF mixer or detector. (E6B06)

Metal-semiconductor junction is a term that describes a type of semiconductor diode. (E6B08) A Schottky diode is an example of a metal-semiconductor diode. An important characteristic of a Schottky diode as compared to an ordinary silicon diode when used as a power supply rectifier is that it has less forward voltage drop.

In Figure E6-3 (below), 5 is the schematic symbol for a light-emitting diode. (E6B10) Forward bias is required for an LED to emit light. (E6B13)

Figure E6-3

No matter what kind of diode you are using, it’s very important to not exceed the forward current specification. Doing so, will cause it to fail. Excessive junction temperature is the failure mechanism when a junction diode fails due to excessive current. (E6B07)

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