Basic Electronics on the Go - The Signal Diode

From http://www.electronics-tutorials.ws/diode/diode_4.html

 The semiconductor Signal Diode is a small non-linear semiconductor devices generally used in electronic circuits, where small currents or high frequencies are involved such as in radio, television and digital logic circuits.

The signal diode which is also sometimes known by its older name of the Point Contact Diode or the Glass Passivated Diode, are physically very small in size compared to their larger Power Diode cousins.

Generally, the PN junction of a small signal diode is encapsulated in glass to protect the PN junction, and usually have a red or black band at one end of their body to help identify which end is the cathode terminal. The most widely used of all the glass encapsulated signal diodes is the very common 1N4148 and its equivalent 1N914 signal diode.

Small signal and switching diodes have much lower power and current ratings, around 150mA, 500mW maximum compared to rectifier diodes, but they can function better in high frequency applications or in clipping and switching applications that deal with short-duration pulse waveforms.


The characteristics of a signal point contact diode are different for both germanium and silicon types and are given as:

• 1. Germanium Signal Diodes – These have a low reverse resistance value giving a lower forward volt drop across the junction, typically only about 0.2 to 0.3v, but have a higher forward resistance value because of their small junction area.
• 2. Silicon Signal Diodes – These have a very high value of reverse resistance and give a forward volt drop of about 0.6 to 0.7v across the junction. They have fairly low values of forward resistance giving them high peak values of forward current and reverse voltage.

The electronic symbol given for any type of diode is that of an arrow with a bar or line at its end and this is illustrated below along with the Steady State V-I Characteristics Curve.

Silicon Diode V-I Characteristic Curve

 

 

The arrow always points in the direction of conventional current flow through the diode meaning that the diode will only conduct if a positive supply is connected to the Anode, ( a ) terminal and a negative supply is connected to the Cathode ( k ) terminal thus only allowing current to flow through it in one direction only, acting more like a one way electrical valve, ( Forward Biased Condition ).
However, we know from the previous tutorial that if we connect the external energy source in the other direction the diode will block any current flowing through it and instead will act like an open switch, ( Reversed Biased Condition ) as shown below.

Forward and Reversed Biased Diode

 

 

Signal Diode Parameters

Signal Diodes are manufactured in a range of voltage and current ratings and care must be taken when choosing a diode for a certain application. There are a bewildering array of static characteristics associated with the humble signal diode but the more important ones are.

1. Maximum Forward Current

The Maximum Forward Current ( I_F(max) ) is as its name implies the maximum forward current allowed to flow through the device. When the diode is conducting in the forward bias condition, it has a very small “ON” resistance across the PN junction and therefore, power is dissipated across this junction ( Ohm´s Law ) in the form of heat.

 hen, exceeding its ( I_F(max) ) value will cause more heat to be generated across the junction and the diode will fail due to thermal overload, usually with destructive consequences. When operating diodes around their maximum current ratings it is always best to provide additional cooling to dissipate the heat produced by the diode.

For example, our small 1N4148 signal diode has a maximum current rating of about 150mA with a power dissipation of 500mW at 25^oC. Then a resistor must be used in series with the diode to limit the forward current, ( I_F(max) ) through it to below this value.

2. Peak Inverse Voltage

The Peak Inverse Voltage (PIV) or Maximum Reverse Voltage ( V_R(max) ), is the maximum allowable Reverse operating voltage that can be applied across the diode without reverse breakdown and damage occurring to the device. This rating therefore, is usually less than the “avalanche breakdown” level on the reverse bias characteristic curve. Typical values of V_R(max) range from a few volts to thousands of volts and must be considered when replacing a diode.

The peak inverse voltage is an important parameter and is mainly used for rectifying diodes in AC rectifier circuits with reference to the amplitude of the voltage where the sinusoidal waveform changes from a positive to a negative value on each and every cycle.

3. Total Power Dissipation

Signal diodes have a Total Power Dissipation, ( P_D(max) ) rating. This rating is the maximum possible power dissipation of the diode when it is forward biased (conducting). When current flows through the signal diode the biasing of the PN junction is not perfect and offers some resistance to the flow of current resulting in power being dissipated (lost) in the diode in the form of heat.

As small signal diodes are non-linear devices, the resistance of the PN junction is not constant - it is a dynamic property so we cannot use Ohms Law to define the power in terms of current and resistance or voltage and resistance as we can for resistors. Then to find the power that will be dissipated by the diode we must multiply the voltage drop across it times the current flowing through it: P_D = VxI

4. Maximum Operating Temperature

The Maximum Operating Temperature actually relates to the Junction Temperature ( T_J ) of the diode and is related to maximum power dissipation. It is the maximum temperature allowable before the structure of the diode deteriorates and is expressed in units of degrees centigrade per Watt, ( ^oC/W ).
This value is linked closely to the maximum forward current of the device so that at this value the temperature of the junction is not exceeded. However, the maximum forward current will also depend upon the ambient temperature in which the device is operating so the maximum forward current is usually quoted for two or more ambient temperature values such as 25^oC or 70^oC.
Then there are three main parameters that must be considered when either selecting or replacing a signal diode and these are:

• The Reverse Voltage Rating
• The Forward Current Rating
• The Forward Power Dissipation Rating
•  

Signal Diode Arrays

When space is limited, or matching pairs of switching signal diodes are required, diode arrays can be very useful. They generally consist of low capacitance high speed silicon diodes such as the 1N4148 connected together in multiple diode packages called an array for use in switching and clamping in digital circuits. They are encased in single inline packages (SIP) containing 4 or more diodes connected internally to give either an individual isolated array, common cathode, (CC), or a common anode, (CA) configuration as shown.

Signal diode arrays can also be used in digital and computer circuits to protect high speed data lines or other input/output parallel ports against electrostatic discharge, (ESD) and voltage transients.
By connecting two diodes in series across the supply rails with the data line connected to their junction as shown, any unwanted transients are quickly dissipated and as the signal diodes are available in 8-fold arrays they can protect eight data lines in a single package.

Signal diode arrays can also be used to connect together diodes in either series or parallel combinations to form voltage regulator or voltage reducing type circuits or even to produce a known fixed reference voltage.

We know that the forward volt drop across a silicon diode is about 0.7v and by connecting together a number of diodes in series the total voltage drop will be the sum of the individual voltage drops of each diode.

However, when signal diodes are connected together in series, the current will be the same for each diode so the maximum forward current must not be exceeded.

Connecting Signal Diodes in Series

Another application for the small signal diode is to create a regulated voltage supply. Diodes are connected together in series to provide a constant DC voltage across the diode combination. The output voltage across the diodes remains constant in spite of changes in the load current drawn from the series combination or changes in the DC power supply voltage that feeds them. Consider the circuit below.

As the forward voltage drop across a silicon diode is almost constant at about 0.7v, while the current through it varies by relatively large amounts, a forward-biased signal diode can make a simple voltage regulating circuit. The individual voltage drops across each diode are subtracted from the supply voltage to leave a certain voltage potential across the load resistor, and in our simple example above this is given as 10v - ( 3*0.7V ) = 7.9V.

This is because each diode has a junction resistance relating to the small signal current flowing through it and the three signal diodes in series will have three times the value of this resistance, along with the load resistance R, forms a voltage divider across the supply.

By adding more diodes in series a greater voltage reduction will occur. Also series connected diodes can be placed in parallel with the load resistor to act as a voltage regulating circuit. Here the voltage applied to the load resistor will be 3*0.7v = 2.1V. We can of course produce the same constant voltage source using a single Zener Diode. Resistor, R_D is used to prevent excessive current flowing through the diodes if the load is removed.

Freewheel Diodes

Signal diodes can also be used in a variety of clamping, protection and wave shaping circuits with the most common form of clamping diode circuit being one which uses a diode connected in parallel with a coil or inductive load to prevent damage to the delicate switching circuit by suppressing the voltage spikes and/or transients that are generated when the load is suddenly turned “OFF”. This type of diode is generally known as a “Free Wheeling Diode”, “Flywheel Diode” or simply Freewheel diode as it is more commonly called.

The Freewheel diode is used to protect solid state switches such as power transistors and MOSFET’s from damage by reverse battery protection as well as protection from highly inductive loads such as relay coils or motors, and an example of its connection is shown below.

Use of the Freewheel Diode

 

 

Modern fast switching, power semiconductor devices require fast switching diodes such as free wheeling diodes to protect them form inductive loads such as motor coils or relay windings. Every time the switching device above is turned “ON”, the freewheel diode changes from a conducting state to a blocking state as it becomes reversed biased.

However, when the device rapidly turns “OFF”, the diode becomes forward biased and the collapse of the energy stored in the coil causes a current to flow through the freewheel diode. Without the protection of the freewheel diode high di/dt currents would occur causing a high voltage spike or transient to flow around the circuit possibly damaging the switching device.


Previously, the operating speed of the semiconductor switching device, either transistor, MOSFET, IGBT or digital has been impaired by the addition of a freewheel diode across the inductive load with Schottky and Zener diodes being used instead in some applications. But during the past few years however, freewheel diodes had regained importance due mainly to their improved reverse-recovery characteristics and the use of super fast semiconductor materials capable at operating at high switching frequencies.

Other types of specialized diodes not included here are Photo-Diodes, PIN Diodes, Tunnel Diodes and Schottky Barrier Diodes. By adding more PN junctions to the basic two layer diode structure, other types of semiconductor devices can be made.

For example a three layer semiconductor device becomes a Transistor, a four layer semiconductor device becomes a Thyristor or Silicon Controlled Rectifier and five layer devices known as Triac’s are also available.

In the next tutorial about diodes, we will look at the large signal diode sometimes called the Power Diode. Power diodes are silicon diodes designed for use in high-voltage, high-current mains rectification circuits.

 

Basic Electronics on the Go - PN Junction Diode

From http://www.electronics-tutorials.ws/diode/diode_3.html

 The effect described in the previous tutorial is achieved without any external voltage being applied to the actual PN junction resulting in the junction being in a state of equilibrium.
 
However, if we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the potential barrier.

The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other. The behaviour of the PN junction with regards to the potential barrier’s width produces an asymmetrical conducting two terminal device, better known as the PN Junction Diode.

 A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential current-voltage ( I-V ) relationship and therefore we can not described its operation by simply using an equation such as Ohm’s law.

If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.

Applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode.

The depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resulting in physical changes taking place. One of the results produces rectification as seen in the PN junction diodes static I-V (current-voltage) characteristics. Rectification is shown by an asymmetrical current flow when the polarity of bias voltage is altered as shown below.

Junction Diode Symbol and Static I-V Characteristics.

 

 

But before we can use the PN junction as a practical device or as a rectifying device we need to firstly bias the junction, ie connect a voltage potential across it. On the voltage axis above, “Reverse Bias” refers to an external voltage potential which increases the potential barrier. An external voltage which decreases the potential barrier is said to act in the “Forward Bias” direction.
There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:

• 1. Zero Bias – No external voltage potential is applied to the PN junction diode.
• 2. Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width.
• 3. Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN junction diodes width.
•  

Reverse Biased PN Junction Diode

When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material.
The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode.
The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.

Increase in the Depletion Layer due to Reverse Bias

 

 

This condition represents a high resistance value to the PN junction and practically zero current flows through the junction diode with an increase in bias voltage. However, a very small leakage current does flow through the junction which can be measured in micro-amperes, ( μA ).
One final point, if the reverse bias voltage Vr applied to the diode is increased to a sufficiently high enough value, it will cause the diode’s PN junction to overheat and fail due to the avalanche effect around the junction. This may cause the diode to become shorted and will result in the flow of maximum circuit current, and this shown as a step downward slope in the reverse static characteristics curve below.
 

Reverse Characteristics Curve for a Junction Diode

Sometimes this avalanche effect has practical applications in voltage stabilising circuits where a series limiting resistor is used with the diode to limit this reverse breakdown current to a preset maximum value thereby producing a fixed voltage output across the diode. These types of diodes are commonly known as Zener Diodes and are discussed in a later tutorial.

Forward Biased PN Junction Diode

When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow.
This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the “knee” on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below.

Forward Characteristics Curve for a Junction Diode

  The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the “knee” point.

Reduction in the Depletion Layer due to Forward Bias

 

This condition represents the low resistance path through the PN junction allowing very large currents to flow through the diode with only a small increase in bias voltage. The actual potential difference across the junction or diode is kept constant by the action of the depletion layer at approximately 0.3v for germanium and approximately 0.7v for silicon junction diodes.

Since the diode can conduct “infinite” current above this knee point as it effectively becomes a short circuit, therefore resistors are used in series with the diode to limit its current flow. Exceeding its maximum forward current specification causes the device to dissipate more power in the form of heat than it was designed for resulting in a very quick failure of the device.

 

Junction Diode Summary

The PN junction region of a Junction Diode has the following important characteristics:

• Semiconductors contain two types of mobile charge carriers, Holes and Electrons.
• The holes are positively charged while the electrons negatively charged.
• A semiconductor may be doped with donor impurities such as Antimony (N-type doping), so that it contains mobile charges which are primarily electrons.
• A semiconductor may be doped with acceptor impurities such as Boron (P-type doping), so that it contains mobile charges which are mainly holes.
• The junction region itself has no charge carriers and is known as the depletion region.
• The junction (depletion) region has a physical thickness that varies with the applied voltage.
• When a diode is Zero Biased no external energy source is applied and a natural Potential Barrier is developed across a depletion layer which is approximately 0.5 to 0.7v for silicon diodes and approximately 0.3 of a volt for germanium diodes.
• When a junction diode is Forward Biased the thickness of the depletion region reduces and the diode acts like a short circuit allowing full current to flow.
• When a junction diode is Reverse Biased the thickness of the depletion region increases and the diode acts like an open circuit blocking any current flow, (only a very small leakage current).

We have also seen above that the diode is two terminal non-linear device whose I-V characteristic are polarity dependent as depending upon the polarity of the applied voltage, V_D the diode is either Forward Biased, V_D > 0 or Reverse Biased, V_D < 0. Either way we can model these current-voltage characteristics for both an ideal diode and for a real diode.

Junction Diode Ideal and Real Characteristics

 

In the next tutorial about diodes, we will look at the small signal diode sometimes called a switching diode which is used in general electronic circuits. As its name implies, the signal diode is designed for low-voltage or high frequency signal applications such as in radio or digital switching circuits.
Signal diodes, such as the 1N4148 only pass very small electrical currents as opposed to the high-current mains rectification diodes in which silicon diodes are usually used. Also in the next tutorial we will examine the Signal Diode static current-voltage characteristics curve and parameters.

(to be updated)