A diode is an electrical device allowing current to move through it in one direction with far greater ease than in the other. The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. Semiconductor diodes are symbolized in schematic diagrams such as Figure) The term “diode” is customarily reserved for small signal devices, I ≤ 1 A. The term rectifier is used for power devices, I > 1 A.
Semiconductor diode schematic symbol: Arrows indicate the direction of electron current flow.
When placed in a simple battery-lamp circuit, the diode will either allow or prevent current through the lamp, depending on the polarity of the applied voltage. (Figure).
Diode operation: (a) Current flow is permitted; the diode is forward biased. (b) Current flow is prohibited; the diode is reversed biased.
When the polarity of the battery is such that electrons are allowed to flow through the diode, the diode is said to be forward-biased. Conversely, when the battery is “backward” and the diode blocks current, the diode is said to be reverse-biased. A diode may be thought of as like a switch: “closed” when forward-biased and “open” when reverse-biased.
Oddly enough, the direction of the diode symbol's “arrowhead” points against the direction of electron flow. This is because the diode symbol was invented by engineers, who predominantly use conventional flow notation in their schematics, showing current as a flow of charge from the positive (+) side of the voltage source to the negative (-). This convention holds true for all semiconductor symbols possessing “arrowheads:” the arrow points in the permitted direction of conventional flow, and against the permitted direction of electron flow.
Diode behavior is analogous to the behavior of a hydraulic device called a check valve. A check valve allows fluid flow through it in only one direction as in Figure.
Hydraulic check valve analogy: (a) Electron current flow permitted. (b) Current flow prohibited.
Check valves are essentially pressure-operated devices: they open and allow flow if the pressure across them is of the correct “polarity” to open the gate (in the analogy shown, greater fluid pressure on the right than on the left). If the pressure is of the opposite “polarity,” the pressure difference across the check valve will close and hold the gate so that no flow occurs.
Like check valves, diodes are essentially “pressure-” operated (voltage-operated) devices. The essential difference between forward-bias and reverse-bias is the polarity of the voltage dropped across the diode. Let's take a closer look at the simple battery-diode-lamp circuit shown earlier, this time investigating voltage drops across the various components in Figure.
Diode circuit voltage measurements: (a) Forward biased. (b) Reverse biased.
A forward-biased diode conducts current and drops a small voltage across it, leaving most of the battery voltage dropped across the lamp. If the battery's polarity is reversed, the diode becomes reverse-biased, and drops all of the battery's voltage leaving none for the lamp. If we consider the diode to be a self-actuating switch (closed in the forward-bias mode and open in the reverse-bias mode), this behavior makes sense. The most substantial difference is that the diode drops a lot more voltage when conducting than the average mechanical switch (0.7 volts versus tens of millivolts).
This forward-bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the P-N junction under the influence of an applied voltage. If no voltage applied is across a semiconductor diode, a thin depletion region exists around the region of the P-N junction, preventing current flow. (Figure)(a) The depletion region is almost devoid of available charge carriers, and acts as an insulator:
Diode representations: PN-junction model, schematic symbol, physical part.
The schematic symbol of the diode is shown in Figure  (b) such that the anode (pointing end) corresponds to the P-type semiconductor at (a). The cathode bar, non-pointing end, at (b) corresponds to the N-type material at (a). Also note that the cathode stripe on the physical part (c) corresponds to the cathode on the symbol.
If a reverse-biasing voltage is applied across the P-N junction, this depletion region expands, further resisting any current through it. (Figure )
Depletion region expands with reverse bias.
Conversely, if a forward-biasing voltage is applied across the P-N junction, the depletion region collapses becoming thinner. The diode becomes less resistive to current through it. In order for a sustained current to go through the diode; though, the depletion region must be fully collapsed by the applied voltage. This takes a certain minimum voltage to accomplish, called the forward voltage as illustrated in Figure below.
Inceasing forward bias from (a) to (b) decreases depletion region thickness.
For silicon diodes, the typical forward voltage is 0.7 volts, nominal. For germanium diodes, the forward voltage is only 0.3 volts. The chemical constituency of the P-N junction comprising the diode accounts for its nominal forward voltage figure, which is why silicon and germanium diodes have such different forward voltages. Forward voltage drop remains approximately constant for a wide range of diode currents, meaning that diode voltage drop is not like that of a resistor or even a normal (closed) switch. For most simplified circuit analysis, the voltage drop across a conducting diode may be considered constant at the nominal figure and not related to the amount of current.
Actually, forward voltage drop is more complex. An equation describes the exact current through a diode, given the voltage dropped across the junction, the temperature of the junction, and several physical constants. It is commonly known as the diode equation:
The term kT/q describes the voltage produced within the P-N junction due to the action of temperature, and is called the thermal voltage, or Vt of the junction. At room temperature, this is about 26 millivolts. Knowing this, and assuming a “nonideality” coefficient of 1, we may simplify the diode equation and re-write it as such:
You need not be familiar with the “diode equation” to analyze simple diode circuits. Just understand that the voltage dropped across a current-conducting diode does change with the amount of current going through it, but that this change is fairly small over a wide range of currents. This is why many textbooks simply say the voltage drop across a conducting, semiconductor diode remains constant at 0.7 volts for silicon and 0.3 volts for germanium. However, some circuits intentionally make use of the P-N junction's inherent exponential current/voltage relationship and thus can only be understood in the context of this equation. Also, since temperature is a factor in the diode equation, a forward-biased P-N junction may also be used as a temperature-sensing device, and thus can only be understood if one has a conceptual grasp on this mathematical relationship.
A reverse-biased diode prevents current from going through it, due to the expanded depletion region. In actuality, a very small amount of current can and does go through a reverse-biased diode, called the leakage current, but it can be ignored for most purposes. The ability of a diode to withstand reverse-bias voltages is limited, as it is for any insulator. If the applied reverse-bias voltage becomes too great, the diode will experience a condition known as breakdown (Figure), which is usually destructive. A diode's maximum reverse-bias voltage rating is known as the Peak Inverse Voltage, or PIV, and may be obtained from the manufacturer. Like forward voltage, the PIV rating of a diode varies with temperature, except that PIV increases with increased temperature and decreases as the diode becomes cooler -- exactly opposite that of forward voltage.
Diode curve: showing knee at 0.7 V forward bias for Si, and reverse breakdown.
Typically, the PIV rating of a generic “rectifier” diode is at least 50 volts at room temperature. Diodes with PIV ratings in the many thousands of volts are available for modest prices.
  • A diode is an electrical component acting as a one-way valve for current.
  • When voltage is applied across a diode in such a way that the diode allows current, the diode is said to be forward-biased.
  • When voltage is applied across a diode in such a way that the diode prohibits current, the diode is said to be reverse-biased.
  • The voltage dropped across a conducting, forward-biased diode is called the forward voltage. Forward voltage for a diode varies only slightly for changes in forward current and temperature, and is fixed by the chemical composition of the P-N junction.
  • Silicon diodes have a forward voltage of approximately 0.7 volts.
  • Germanium diodes have a forward voltage of approximately 0.3 volts.
  • The maximum reverse-bias voltage that a diode can withstand without “breaking down” is called the Peak Inverse Voltage, or PIV rating.


Tunnel Diode(Definition):
                                                                  "A heavily doped junction diode that has a negative resistance at very low voltage in the forward bias direction, due to quantum-mechanical tunneling, and a short circuit in the negative bias direction" Also known as Esaki tunnel diode.

A tunnel diode or Esaki diode is a type of semiconductor diode which is capable of very fast operation, well into the microwave frequency region, by using quantum mechanical effects.It was invented in August 1957 by Leo Esaki when he was with Tokyo Tsushin Kogyo (now known as Sony), who in 1973 received the Nobel Prize in Physics for discovering the electron tunneling effect used in these diodes.These diodes have a heavily doped p–n junction only some 10 nm (100 Å) wide. The heavy doping results in a broken bandgap, where conduction band electron states on the n-side are more or less aligned with valence band hole states on the p-side. Tunnel diodes were manufactured by SONY for the first time in 1957 followed by General Electric and other companies from about 1960, and are still made in low volume today. Tunnel diodes are usually made from germanium, but can also be made in gallium arsenide and silicon materials. They can be used as oscillators, amplifiers, frequency converters and detectors.

Reverse bias operation:

When used in the reverse direction they are called back diodes and can act as fast rectifiers with zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction).Under reverse bias filled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction – this is the Zener effect that also occurs in zener diodes.

Technical comparisons:

A rough approximation of the VI curve for a tunnel diode, showing the negative differential resistance regionIn a conventional semiconductor diode, conduction takes place while the p–n junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the “reverse breakdown voltage” when conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction. However, when forward-biased, an odd effect occurs called “quantum mechanical tunnelling” which gives rise to a region where an increase in forward voltage is accompanied by a decrease in forward current. This negative resistance region can be exploited in a solid state version of the dynatron oscillator which normally uses a tetrode thermionic valve (or tube).

The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it would operate at frequencies far greater than the tetrode would, well into the microwave bands. Applications for tunnel diodes included local oscillators for UHF television tuners, trigger circuits in oscilloscopes, high speed counter circuits, and very fast-rise time pulse generator circuits. The tunnel diode can also be used as low-noise microwave amplifier However, since its discovery, more conventional semiconductor devices have surpassed its performance using conventional oscillator techniques. For many purposes, a three-terminal device, such as a field-effect transistor, is more flexible than a device with only two terminals. Practical tunnel diodes operate at a few millamperes and a few tenths of a volt, making them low-power devices The Gunn diode has similar high frequency capability and can handle more power.Tunnel diodes are also relatively resistant to nuclear radiation, as compared to other diodes. This makes them well suited to higher radiation environments, such as those found in space applications.



Definition:"A semiconductor diode that uses a germanium crystal pellet as the rectifying element. Also known as germanium rectifie"

Diodes are a very important component of most alternative energy generating systems - for example in PV solar panels they are used to stop energy being radiated back out into the night sky from the battery bank, and in wind turbine generators they are used to rectify AC into DC electricity.

Most diodes are made of silicon because of its ease of processing and stability, however they have one disadvantage: a silicon diode has a forward voltage drop of around 0.7 volts. This means that if you had a 7.0 Volt rated solar panel charging a battery via a silicon diode, only 7 - 0.7 = 6.3 Volts would be seen by the battery - the remainder is lost as heat in the diode.

In a Bridge Rectifier the current passes through two diodes. Therefore the total forward voltage drop is a whopping 1.4 volts - a significant amount in low voltage systems.

A selection of diodes including some vintage germanium diodes

Germanium Diodes:

1N34A Germanium Diode
A Germanium Diode (such as the 1N34 pictured above) will typically have a forward voltage drop of just 0.3 volts which means they are much more efficient. Older germanium diodes had a larger leakage of current at a reverse voltage, but now American Microsemiconductor and others supply a range of improved low current leakage germanium diodes. Diodes such as the most common 1N34A can cost as little as 5 pence each.

Buy Germanium Diodes:

OA47 germanium diodes

It is not always easy to find germanium diodes as they are not a very popular item. One of the best ways to obtain them is to purchase vintage stock made available for sale on eBay. At the time of updating this article (August 2008) there were OA47, OA81, OA90, OA91, OAZ210, IS689, 1N34A, 1N42, 1N60, AAZ17, and AA119 germanium diodes all listed for sale - some new and some vintage / used
More Information about Germanium Diodes: While silicon diodes are resistant to the heat from soldering, germanium diodes can very easilty be damaged. Therefore a crocodile clip or other suitable heat sink should be clipped onto the lead between the diode body and the joint to be soldered.

An interesting article about the practical use of Germanium Diodes in a crystal radio set is here, and discusses the forward
voltage drop of a selection of germanium diodes in real world testing. The OA47 came out tops with a forward voltage drop of under 250mV.

                       :DIODE TESTING PROCEDURE:

Diode test with an analogue multimeter

The basic diode test is very simple to perform. Just two tests are needed with the multimeter to ensure that the diode works satisfactorily:

... the band on the diode package represents the cathode....

  Diode circuit representation=  
Step by step instructions:
  1. Set the meter to its ohms range - any range should do, but the middle ohms range if several are available is probably best.
  2. Connect the cathode terminal of the diode to the terminal marked positive on the multimeter, and the anode to the negative or common terminal.
  3. Set the meter to read ohms, and a "lowish" reading should be obtained.
  4. Reverse the connections.
  5. This time a high resistance reading should be obtained.
Diode test using a multimeter
  • In step 3 above the actual reading will depend upon a number of factors. The main thing is that the meter deflects, possibly to half way or more. The variation depends on many items including the battery in the meter, and the range used. The main point to note is that the meter deflects significantly.
  • When checked in the reverse direction, silicon diodes are unlikely to show any meter deflection whatsoever. Germanium ones that have a much higher level of reverse leakage current may easily show a small deflection if the meter is set to a high ohms range.
This simple analogue multimeter test of a diode is very useful because it gives a very quick indication of whether the diode is basically operational. It cannot, however, test more complicated parameters such as the reverse breakdown, etc.

Transistor test using an analogue multimeter

The diode test using an analogue multimeter can be extended to give a simple and straightforward confidence check for bipolar transistors. Again the test using a multimeter only provides a confidence check that the device has not blown, but it is still very useful.
The test relies on the fact that a transistor can be considered to comprise of two back to back diodes, and by performing the diode test between the base and collector and the base and emitter of the transistor using an analogue multimeter, the basic integrity of the transistor can be ascertained.
Transistor equivalent circuit
It should be noted that a transistor cannot be functionally replicated using two separate diodes because the operation of the transistor depends upon the base which is the junction of the two diodes, being one physical layer, and also very thin.
Step by step instructions:
The instructions are given primarily for an NPN transistor as these are the most common types in use. The variations are shown for PNP varieties - these are indicated in brackets (.. .. ..):
  1. Set the meter to its ohms range - any range should do, but the middle ohms range if several are available is probably best.
  2. Connect the base terminal of the transistor to the terminal marked positive (usually coloured red) on the multimeter
  3. Connect the terminal marked negative or common (usually coloured black) to the collector and measure the resistance. It should read open circuit (there should be a deflection for a PNP transistor).
  4. With the terminal marked positive still connected to the base, repeat the measurement with the positive terminal connected to the emitter. The reading should again read open circuit (the multimeter should deflect for a PNP transistor).
  5. Now reverse the connection to the base of the transistor, this time connecting the negative or common (black) terminal of the analogue test meter to the base of the transistor.
  6. Connect the terminal marked positive, first to the collector and measure the resistance. Then take it to the emitter. In both cases the meter should deflect (indicate open circuit for a PNP transistor).
  7. It is next necessary to connect the meter negative or common to the collector and meter positive to the emitter. Check that the meter reads open circuit. (The meter should read open circuit for both NPN and PNP types.
  8. Now reverse the connections so that the meter negative or common is connected to the emitter and meter positive to the collector. Check again that the meter reads open circuit.
  9. If the transistor passes all the tests then it is basically functional and all the junctions are intact.
  • The final checks from collector to emitter ensure that the base has not been "blown through". It is sometimes possible that there is still a diode present between collector and base and the emitter and the base, but the collector and emitter are shorted together.
  • As with the germanium diode, the reverse readings for germanium transistors will not be as good as for silicon transistors. A small level of current is allowable as this results from the presence of minority carriers in the germanium.