Tunnel diodes are heavily doped P-N diodes in which electron tunneling from the conduction band in the N-type material to the valence band in the P-type region produces a region of negative resistance. This negative-resistance region is the most important area of operation. As the voltage is increased, the current decreases. This feature makes tunneling diodes especially useful in oscillators and radio frequency (RF) applications. There are two basic types of tunnel diodes: standard tunnel diodes and back tunnel diodes. With a back tunnel diode, the peak current is very small or totally suppressed. This causes the device to resemble a conventional PN junction diode. Back diodes have a very small reverse breakdown voltage that produces a very high current. Consequently, conduction is heavier in reverse bias than in forward bias. The terms “back” and “backward” imply this type of conduction.
Performance specifications for tunnel diodes include peak current, valley current, maximum allowable reverse voltage, forward voltage, maximum permissible power dissipation, terminal capacitance, and junction operating temperature. Peak current is the maximum current and valley current is the minimum current in the tunneling (negative resistance) region. Forward voltage is measured across the diode terminals and results from the flow of current in the forward direction. Power dissipation is determined when tunneling diodes are in the ON state. Terminal capacitance or diode capacitance is measured when the specified reverse voltage and frequency are applied. Junction operating temperature is the range of temperatures over which tunnel diodes are designed to operate.
IC Package Types
Tunnel diodes use several different IC package types. Choices include: diode outline (DO), transistor outline (TO), small outline diode (SOD), small outline transistor (SOT) discrete package (DPAK), and metal electrode leadless face (MELF). DO-4, DO-5, DO-8, DO-9, DO-15, DO-27, DO-34, DO-35, DO-41 and DO-201 are diode outline (DO) packages. TO-3, TO-66, TO-92, TO-202, TO-220, TO-237 and TO-247 are transistor outline (TO) packages. SOD-80, SOD-106, SOD-123, SOD-323, and SOD-523 are small outline diode (SOD) packages. SOT23, SOT26, SOT89, SOT143, SOT223, SOT323, SOT343, SOT346, SOT353, SOT363, SOT416, SOT457, and SOT523 are small outline transistor (SOT) packages. QuadroMELF, MicroMELF, and MiniMELF are types of MELF packages for tunnel diodes. D2PAK is a large surface-mounted package for tunnel diodes that includes a heat sink. SC-59, SC-74, and SC-76 are plastic, surface-mounted packages with three leads.
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