Diodes

What is a Diode?

A diode is a nonlinear semiconductor device which has two electrodes.

A current will result, whenever an electric charge should flow through the diode's electrodes.

The essential feature of a diode is that the magnitude of the current greatly depends on the direction of the applied voltage's polarity.

An ideal diode can only conduct electricity in one direction. It acts like a switch that opens when the voltage polarity is applied in one certain order and closes again once this polarity is reversed.

Where are Diodes Used?

Diode Structure

The diode's structure is responsible for it's specific electrical properties.
It is formed by a single semiconductor crystal. One part of this semiconductor is a p-type, the other is an n-type (as shown in the figure).

The border zone between the p-type and the n-type regions is called a pn junction. This pn junction made various kinds of inventions possible including diodes, transistors and integrated circuits.

Diode Schematic Symbol

Junction diode is just another name for a pn crystal. Its p-side is called an anode and its n-side a cathode.

The figure illustrates the schematic symbol of a junction diode.

It is a line with an arrow that points from the anode to the cathode. This is to remind the observer that the current will only flow easily in this direction.

Diode's Mode of Operation

A diode's mode of operation is dependent on the variation of the conditions that are possible at the pn junction. They are caused by the polarity of the applied voltage.

The pn junction diode makes up an integral part in transistors and integrated circuits. So understanding a pn junction is the key to understanding all semiconductor devices. We shall therefore discuss the physics of pn junction operations in further detail.

Unbiased Diode

Before we discuss the influence of voltage polarity on the electric behavior of a diode, we shall first cover the conditions that exist at an isolated pn junction when no voltage is applied. Such an isolated pn junction is called an unbiased diode.

This pn junction actually exists within one single crystal. But to simplify things, we will consider the crystal to be formed by adjoining two separate n-type and p-type semiconductor crystals. This will make the matter easier to understand when we proceed to explain how the diode operates.

Separate Semiconductors

Let us recall that in a silicon crystal each donor atom gives one free electron and turns itself into a positive ion. In a similar manner, each acceptor atom produces one hole, and it becomes a negative ion as soon as it accepts one electron. Both materials also contain a few thermally produced minority carriers.

The circled plus and minus symbols in the figure represent donor and acceptor ions, and the little blue and red balls represent free electrons and holes. Note that each semiconductor piece is electrically neutral as a whole because of its equal numbers of plus and minus charges.

pn Junction Formation

A separate n-type or p-type semiconductor piece will function alone as a resistor.
But if we imagine a p-type and an n-type piece uniting to become one diode, new conditions will arise for the entire new crystal.

In this united diode, free electrons and holes will tend to diffuse (spread) in all directions as free carriers. Some of them will also diffuse across the junction. However, the acceptor and donor ions will remain fixed, which is due to their covalent bonds in the crystal structure. Unlike the free carriers, the ions will never move.

Majority Carriers Movement

The concentration of the free electrons (majority carriers) in the n region is much greater than the concentration of the free electrons in the p region (minority carriers) close to the junction. This difference in concentration levels provides the driving force for the diffusion of free electrons from the n-type to the p-type region.

Whenever a free electron enters the p-type region it will become a minority carrier, which will usually be for a short time. For soon after it crosses the junction, this free electron will fall into a hole and cause both the charge of itself and that of the hole to neutralize. Merging free electrons with holes is called recombination. The same processes are observed when it comes to holes crossing the pn-junction in the opposite direction.

Depletion Layer

Whenever an electron leaves the n side, it will leave behind an uncompensated positive ion. A positive space charge will therefore be created to the right of the junction in the n-region.

Similarly, whenever a hole leaves the p-side, it will leave behind an uncompensated negative ion which in turn will create a negative space charge to the left of the junction in the p-region.

Eventually we will have regions of ionic space charges that are almost completely depleted of their mobile charge carriers. These regions are always located near the boundary and are collectively referred to as the depletion layer do. Most of the semiconductor material in the depletion layer has a net charge density of zero.

Barrier Potential & Electric Field

The uncompensated positively- and negatively-charged ions that are within the depletion layer generate an electric field Eo. This field prevents any further diffusion of free electrons and holes across the junction. If additional free electrons or holes enter the depletion layer, the electric field will tend to drive them back.

This built-in electric field (voltage divided by distance) is equivalent to the difference of potential which is also referred to as the barrier potential Uo. At room temperature (25 oC) the barrier potential has for silicon diodes a voltage of approximately 0.7V and for germanium diodes a voltage of about 0.3V.

Diffusion and Drift Currents

While the electric field is generated in the depletion layer, only the more energetic majority carriers will diffuse against the action of the field and cross the junction. Those charge carriers that will still cross the junction despite the barrier will constitute the diffusion current ID. It consists of both, electron and hole components. Recall that the flow of positive charges which conventionally represent the direction of the current.

However, the electric field in the center does help minority carriers cross the junction, if they are moving in the opposite direction to the diffusion current. The flow of minority carriers in the opposite direction to the diffusion current constitute the drift current IE.

An exact balance between the diffusion current and the drift current is reached at equilibrium. For unbiased diodes the diode net will result in a zero current across the pn junction.

Biased Diode

This figure shows a diode that is connected to an external voltage source Us. for reasons of simplicity the depletion layer is shown as a bi-colored strip situated in the center. Depending on the direction of its polarity, the applied voltage will either widen or diminish the built-in barrier potential, and will accordingly cause the electric field E in the depletion layer to change.

The current will almost completely stop when the minus/plus polarity of the power source is applied to its opposite plus/minus counterparts in the diode. In this case the applied voltage will almost totally diminish across the diode's depletion layer, because this layer will have an even higher resistance than each of the p-type and n-type regions taken separately. Hence the magnitude of the current will change by altering the voltage polarity.

Forward Bias

Whenever the negative battery terminal is connected to the n-side and the positive terminal is connected to the p-side, the connection will have a forward bias.

In this case the battery will force free electrons and holes to flow toward the junction. These excess carriers will reduce the width of the depletion layer d. This is the same as decreasing the barrier potential or decreasing the electric field E in the depletion layer. The barrier potential will be reduced to Uo - Us and the electric field to E < Eo.

Forward Current

Currents will easily flow in forward-biased diodes. A forward current is always composed of majority carriers whose energies are sufficient to overcome the barrier potential.

With the barrier potential being reduced through the forward flow, the majority carriers will meet even less resistance while diffusing across the junction. This will cause the forward current IF to increase even further.

Reverse Bias

If the negative battery terminal is connected to the p side and the positive battery terminal to the n side, we will have a reverse bias.

The negative battery terminal will attract the holes from the p region and the positive battery terminal the free electrons from the n region. As the free electrons and holes will move away from the junction, they will leave behind uncompensated positive and negative ions. These new ions will increase the difference of potential and widen the depletion layer. The new barrier potential will therefore be Uo + Us and E > Eo.

Reverse Current

Here the reverse voltage has increased, and so have the depletion layer and the barrier potential too. The diffusion of majority carriers across the junction has therefore been greatly diminished.

However, a very small reverse current IR, which is still crossing the junction, has remained. It consists of thermally produced minority carriers that have been created by the build-in electric field. This makes the reverse current IR independent from the direction of the voltage polarity and of the barrier potential. It will always remain no matter what we change.

Diode's VA Characteristics

This figure illustrates the volt ampere (VA) characteristics of a pn junction diode.

A diode is a nonlinear device. It will conduct in only one direction.

If the diode is forward-biased, the current will increase very rapidly with U. But if, in contrast, the diode is reverse-biased, the reverse current will be very small.

An ideal diode should function like a switch that closes when it is reverse-biased but opens again when it is forward-biased.

The Forward Region

This figure shows a forward biased diode's VA characteristics. Note that the value of the forward current IF can be determined by an exponential function of the forward voltage UF. This means that small increases in the diode's voltage will lead to disproportionate high rises in the diode's current. The higher the value of the applied voltage is, the lower the barrier potential and the higher the magnitude of the forward current will be.

The Turn-on Voltage

Note that the current is small for the first few tenths of a volt. The voltage level where the current begins to increase rapidly is called the turn-on (or knee) voltage UTo. This voltage is approximately equal to the barrier potential of the diode.

At room temperature, the turn-on voltage for a silicon diode will be approximately 0.7 V whereas for a germanium diode it will be about 0.3V.

The equivalent circuit of a silicon diode, which has commonly found application as a switch, will therefore have a barrier potential of 0.7 V too. This means it will conduct electricity as soon as the applied voltage is higher than 0.7 V.

Play & Learn - Barrier Potential and Temperature

Recall that at room temperature the barrier potential is approximately 0.7V for silicon diodes and 0.3V for germanium diodes. The rule of thumb for estimating the change in the barrier potential is the following: For both, germanium and silicon diodes, the barrier potential will decrease by 2 mV for each Celsius degree in temperature rise.

What to do:

- Change the temperature

- Observe the change in the barrier potential value.

Explain:

- Why do values for barrier potential changes depend on temperature.

Forward Voltage and Temperature

Forward voltage decreases with rising temperatures. Because barrier potentials decrease at higher temperatures, less voltage needs to be applied in order to achieve the same diode current that would normally exist if temperatures did not change. The effect of temperature is represented by the temperature coefficient. This coefficient can be inferred from the voltage change per degree change. A forward-biased diode will have a negative temperature coefficient.

This is an important characteristic of forward-biased diodes. It is used for temperature compensation and also in many other applications such as temperature sensors.

The Reverse Region

This figure shows the reverse region of a diode's VA characteristics. Note that the reverse current is independent from changes in the reverse voltage.

The reverse current, which is caused by thermally created minority carriers, is also called the saturation current. It is symbolized by Is. The term saturation is derived from the fact that enhancing the reverse voltage UR will not increase the number of thermally created minority carriers any further. Such an increase would only be caused by temperature alone.

Reverse Saturation Current

Because the reverse current is caused by thermally created minority carriers, it will also be highly sensitive to temperature changes. A useful relation to remember is the following:Is doubles with every 10oC increase. The higher the temperature becomes, the greater the saturation current will be.

There are fewer minority carriers in silicon (Si) diodes than in germanium (Ge) diodes. This means that a silicon diode has a much smaller Is than a germanium diode. In most applications, the reverse current in a silicon diode is too small to be noticed. It is important to always keep in mind that the current is close to zero in reverse-biased diodes.

The Total Reverse Current

The total reverse current of a diode consists of a saturation current (which is very small and dependent on the temperature) and a surface-leakage current (which is also very small and dependent on the applied voltage). The surface-leakage current is generated by imperfections in the crystal structure.

As we know, the diode's current will be very small for all reverse voltages that are lower than the breakdown voltage UBR. At breakdown level, however, the current will rapidly increase with only small rises in the voltage.

In general, diodes are never operated in the breakdown region. The only exception is the Zener diode, which is a special-purpose diode that is covered in a separate e-Tr@iner.

Load Line

These figures discuss the most elementary diode circuit. By applying Kirchoff's law to the circuit, we will end up with a linear relation between the current and the voltage across the diode with U = Us - I.R. Us as the source voltage, and U and I being respectively the voltage and the current. The equation represents a straight line that is called the load line.

This load line can be easily plotted with its intercepts on the horizontal and on the vertical axes being respectively (I=0,U=Us) and (U=0, I=Us/R). The slope of the load line is 1/R.

Operating Point

The load line is used to find the exact values of a diode's current and voltage in the circuit.

The intersection point between the load line and the diode's VA characteristics is referred to as the quiescent operating point Q. At this point there will be no variation, neither in voltages nor in currents.

With the source voltage being Us and the load resistor being R, the coordinates of point Q (IQ ,UQ) will represent the exact voltage and diode current of a given circuit.

Diode Parameters - DC Resistance

The DC resistance of a diode is determined by the ratio of the total diode voltage to the total diode current at a given operating point.

In case of a forward direction, the dc resistance will be represented by RF. For the operating point Q (as shown in the figure) the diode's forward dc resistance will be represented as RF = UQ / IQ. In case of a reverse direction, the dc resistance will be denoted by RR. Here the diode will have a low value of RF (less than 100 ohms) and a high value of RR (from about a few hundreds of Kohms to one Mohm).

Since the diode is a nonlinear device, its forward dc resistance will vary with the current. The dc resistance value will decrease whenever the current increases. At any rate, the forward dc resistance will be low.

Diode Testing

An example for go/no-go testing is using an ohmmeter to check the diode resistance. A good diode has a low resistance in one direction and a high resistance in the other.

The exact value of the resistance is not so important. The main thing to look for while testing is a high ratio of reverse to forward resistance. For typical silicon diodes the ratio should be higher than 1000:1.

Diode problems are indicated as follows: low resistance in both directions will cause the diode to be shorted, high resistance in both directions will cause the diode to open.

Diode Ratings - Maximum Power

The maximum power rating strongly depends on the settings of two temperatures which are the ambient temperature and the maximum temperature. Ta is the ambient temperature of the surrounding air, and Tj the junction temperature of the pn junction inside the diode.

As soon as power is dissipated in a diode, the temperature of the diode will tend to rise. When the junction temperature becomes too high (i.e. more than the maximum junction temperature Tjmax), the diode will be destroyed. Tjmax is approximately 150oC for silicon diodes and around 75oC for germanium diodes.

The maximum power rating Pmax can be easily inferred from a function in which the maximum level of dissipated power is defined as the point where the junction temperature will reach Tjmax.

Heat Emission

The actual increase in junction temperature depends on the rate at which the thermal energy can be conducted into the ambient surroundings. Thermal energy can be easily reduced through conduction and radiation from the device's case.

These illustrations show some common packages for diodes (not in real-size) which operate at different power levels. Devices that do operate at higher power levels are usually larger in size and accordingly more conductive. The sizes shown in the above illustrations are not standard sizes and therefore are only applicable for small power dissipations.

Heat Sink

For usage at higher power levels, the surface area will need to be increased by mounting the device onto a heat sink (a mass of metal). The diode can be fastened to a large heat sink with fins that will conduct heat out of the device more efficiently.

These figures show several types of heat sinks. No matter what kind of heat sink is used, its purpose will always be to reduce the case temperature. The idea behind this is that heat will emit faster into the surrounding air and increase the power rating of the diode. For operations at very high power levels, cooling can be additionally improved by having a fan blow at the heat sink.

Maximum Forward Current

This figure illustrates the importance of maximum current ratings. Maximum forward current and maximum voltage ratings must be specified. If the current becomes too large, excessive heat will destroy the diode. But approaching the burnout value without completely reaching it can also shorten a diode's life and impair its functionability.

Therefore the maximum forward current IFmax must always be lower than the burnout level in order to specify the maximum current level that a diode can safely handle without shortening diode's life or diminishing its characteristics. Power that has been dissipated in a diode can either be expressed with the current or with the voltage.

Current-Limiting Resistor

In this figure we can see a current-limiting resistor. This resistor has the function to keep the diode current smaller than the maximum rating.

A current which runs through the series resistor can be expressed as I = (Us - U) /R, where Us is the source voltage and U the voltage that runs across the diode. We obtain this equation by applying Ohm's law to the current-limiting resistor.

Since a current is the same in all parts of a series circuit, the diode current will have the same value as the current running through the current-limiting resistor.

Reverse Breakdown Voltage

Should the reverse voltage become too large, a reverse breakdown will occur.

At a reverse breakdown voltage UBR, the current will rapidly increase with only small changes in the voltage. However, the breakdown current will also destroy the diode. Therefore the same rule for the forward voltage also applies to the reverse voltage. The reverse voltage will always have a limit that a diode can handle.

In many applications a diode is used with the purpose to conduct in only one direction. Its reverse breakdown voltage will therefore not be exceeded very often. Breakdown voltages usually ranges between 30 and 120 V for germanium rectifier diodes, and for silicon diodes even between 100 V and 2000 V.

Diode Capacitance

Diode capacitances are very important either when high frequency signals are applied to a diode or when a diode will need to switch very quickly.

Diode capacitances relate the changes in the charge to changes in the applied voltage.

The junction capacitance Cj (also referred to as the barrier capacitance) is dependent on the charge that is generated by the ionized donors and acceptors in the depletion layer. The animation shows changes that will occur in the charge as a response to changes in the ac signal um.

Under forward-bias conditions, the storage capacitance Cs (also referred to as the diffusion capacitance) is dependent on the excess minority carrier charge near the junction. The forward bias transfers this charge into the neutral region.

High Frequency Diode Structure

Diode capacitances reduce the effectivity of diodes as rectifiers at high frequencies. They also limit the speed at which diodes can switch.

Charging and discharging the capacitances determine the switching speed and the frequency response of the diode. The value of the diode capacitance should be as low as possible for most high-frequency or frequent-switching applications.

The above figure shows a model of the so-called point-contact diode which is used for high frequency application. This diode has a very small junction area and a very small capacitance (less than 1 pF). The ring indicates the cathode, and the ring's color indicates the diode's breakdown voltage.