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Frequently Asked Questions


Are a diode and a rectifier the same?

In general, yes, however the diode is usually thought of as a signal device, with currents of 1 Amp or less; and a rectifier, a power device with currents up to 1,000 amps or more. Also, the term rectifier is usually associated with rectification of some sort whereas a diode can be a rectifier, a zener, a four-layer device, etc.

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What is a depletion zone and why is it important?

When a PN junction is reverse biased, a region of high resistance (or a region depleted of carriers) is formed. See the figures below .The extent of this depletion is proportional to the reverse voltage applied. In figure A, this region, devoid of carriers, forms a junction capacitance and that capacitance is often a major limitation insofar as switching is concerned. On the other hand, it is the basis of the varactor diode; wherein the junction capacitance can be varied by varying the reverse voltage.


PN Junction
Figure A

In another example, a depiction of the junction FET is shown in figure B. Again, as the reverse gate junction voltage is increased, the depletion region (high resistance region) extends into the conducting n-channel eventually pinching off the N-channel current.


Junction FET
Figure B
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What is the meaning of Vf as it relates to a diode or rectifier?

Vf is the forward voltage drop of a semiconductor. It consists of two components; an intrinsic offset voltage (0.3 volt for germanium ; 0 .7 for silicon) plus If x R, where R is the resistance of the semiconductor and leads multiplied by the current, If. Vf is undesirable and is minimized whenever possible as it adversely affects the efficiency of the circuit and results in a power loss, that loss is Vf xIf.

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What is a schottky diode?

A schottky diode is a fast switching silicon diode with a low forward voltage drop.

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How do I specify a zener diode?

The minimum specifications should include:

  1. The zener voltage
  2. The tolerance
  3. The test current
  4. Power dissipation

Standard Zener diode Power classifications are w, 1/2w ,1w , 5w,10w,and 50 watt.

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Why is the semiconductor device called a tunnel diode?

With a standard germanium diode a potential barrier of 0.3 volts must be exceeded before the diode can conduct. However in a tunnel diode, according to quantum theory, a particle, in a highly doped diode or PN junction, can bypass this barrier by tunneling beneath the barrier, appearing instantaneously on the other side of the junction.

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What is the chief application of the tunnel diode?

Since its characteristic curve incorporates a negative resistance region, it switches extremely quickly, greater than 5 Mhz, making it an excellent high frequency device.

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If I exceed the reverse breakdown voltage of a diode, will I destroy it?

Possibly, since current multiplies rapidly in the reverse breakdown region, the power dissipated can rapidly exceed the power dissipation of the diode and the destroy the part.

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Is a schottky diode the same as a shockley diode?

No. A schottky diode is a fast switching diode with a low forward voltage drop. A shockley diode on the other hand, also known as a four layer diode, is a thyristor that switches once its breakover voltage is reached.

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What is a back diode?

A back diode is a tunnel diode with a suppressed peak voltage. Since a tunnel diode is highly doped, it has a low reverse breakdown voltage with an accompanying large current. Conduction is less in the forward direction. The term "backward" is meant to imply heavier conduction with a negative rather than positive applied voltage; the reverse of most diodes or rectifiers.

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What is a unijunction transistor and is it a transistor?

The unijunction transistor is more a thyristor than a transistor. It has one junction and was once referred to as a double-base diode. The UJT, unijunction transistor, doesn’t conduct until a peak voltage, Vp, is reached. At that time the emitter conducts, resulting in a positive pulse at B1 and a negative pulse at B2 either of which can be used to trigger another device such as an SCR.

Unijunction Transistor

The UJT is often used with an RC timer circuit (see below), and can operate up to 1 Mhz.

Basic Relaxation Oscillator

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What is the structural difference between a bipolar transistor, a diode and a unijunction transistor?

AC

A) is an npn transistor.

B) is a diode.

C) is a unijunction transistor.

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What is a PUT?

PUT stands for Programmable Unijunction Transistor. Its characteristics are similar to a unijunction transistor, however the peak or firing voltage can programmed by two external resistors. Frequencies of up to 10Khz are obtainable with the PUT.

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What are Thyristors?

Thyristors are switching devices that don’t require any control current once they are turned on. A small control pulse of current is all that is required to turn them on and when that pulse is removed the thyristor continues to conduct ,assuming a minimum holding current is maintained e.g. SCRs, Triacs. Then there are pulse generating thyristors in which a holding current is meaningless e.g. unijunction transistors, shockley diodes.

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What is the difference between an SCR and a Triac?

An SCR is a unidirectional thyristor and a triac is bidirectional thyristor. A positive SCR gate signal turns that device on. The triac is similar to two reverse parallel SCRs, and can conduct in each direction. See the figure below. The main SCR circuit is anode to cathode and is controlled by a positive gate voltage. The triac conducts in both directions. When MT2 is positive relative to MT1, a positive gate current will cause conduction; and when MT1 is positive relative to MT2, a negative gate signal will cause conduction. See figure below.

Triac

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Why is dV/dt important in SCRs and Triacs?

An SCR or a triac can be switched into conduction by a sharp increase in off-state voltage. This dV/dt is the rate of increase of voltage. The forward breakover needn’t be exceeded for switching to occur. A rapid voltage rise charges the thyristor’s junction capacitance and when the current exceeds the gate trigger current, the thyristor is triggered into conduction.

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What is an FET and how is it different from a bipolar transistor?

A bipolar transistor is a current amplifying device. An input current results in an amplified output current Hfe=Ic/Ib, where Ic is the output or collector current and Ib is the input or base current. The FET on the other hand is a transconductance device, that is an input voltage results in an output current G=Id/Vg, where Id is the output or drain current and Vg is the input or gate voltage. This assumes a common emitter connection for the bipolar transistor and a common source connection for the FET.

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What is the pinchoff voltage of an FET?

Pinchoff occurs when the depletion zone, caused by increased reverse gate voltage, extends into the FET channel to the extent that the FET channel resistance reaches some maximum value and the channel current is minimum.

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How does a photodiode and phototransistor work?

When light strikes a semiconductor it supplies energy to knock electrons free of their atomic orbits, creating holes and electrons. This light sensitive behavior occurs in the depletion zone of a reverse biased diode. The phototransistor behaves the same way when light impinges upon its reverse biased collector-base junction depletion layer. Of course the transistor amplifies this current so that a much larger current flows in the collector-emitter circuit than in diode. Lenses are used to enhance the light sensitivity. Also, keep in mind that all semiconductors are light sensitive and one function of their package is to shut out light!

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How does an LED work?

An LED , Light Emitting Diode, works just the opposite of a photosensitive diode; it converts electrical energy to light, rather than light to electrical current. When an electron falls into a hole (with application of external voltage), it creates a particle of light, a photon. Many photons make a visible light and an LED important.

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