What is Diode

A diode is a semiconductor device with two terminals called the anode and the cathode. Typically, allowing the flow of current in one direction, called the forward direction, current trying to flow in the reverse direction is blocked. They’re like the one-way valve of electronics. If the voltage across a diode is negative, no current can flow, and the diode looks like an open circuit. In such a situation, the diode is said to be OFF or reverse biased.

What is Diode

As long as the voltage across the diode isn’t negative, it’ll "turn on" and conduct current. Ideally, a diode would act like a short circuit (0V across it) if it was conducting current. When a diode is conducting current, it’s forward biased.

Diode characteristics Diode Characteristics Operation Mode On (Forward biased) Off (Reverse biased) Current Through I>0 I=0 Voltage Across V=0 V<0 Diode looks like Short circuit Open circuit

Most diodes are made with semiconductor materials such as silicon, germanium, or selenium. Diodes can be used as rectifiers, signal limiters, voltage regulators, switches, signal modulators, signal mixers, signal demodulate, and oscillators.

A Diode's Operation Principle

1. Zero Bias – No external voltage potential is applied to the PN junction diode.

2. Forward Bias – The voltage potential is connected positively, (+ve) to the P-type material and negatively (-ve) to the N-type material across the diode, which has the effect of decreasing the PN junction diode width.

3. 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.

Zero Bias

The n side will have a large number of electrons and very few holes (due to thermal excitation), whereas the p side will have a high concentration of holes and very few electrons. Due to this, a process called diffusion takes place. In this process, free electrons from the n side will diffuse (spread) into the p side and combine with holes present there, leaving a positive immobile (not movable) ion on the n side. Hence, a few atoms on the p side are converted into negative ions. Similarly, a few atoms on the n-side will get converted to positive ions. Due to this, a large number of positive ions and negative ions will accumulate on the n-side and p-side respectively. This region, so formed, is called the depletion region. Due to the presence of these positive and negative ions, a static electric field called "barrier potential" is created across the p-n junction of the diode. It is called "barrier potential" because it acts as a barrier and opposes the further migration of holes and electrons across the junction.

Forward voltage

In a PN junction diode, when the forward voltage is applied, i.e., when the positive terminal of a source is connected to the p-type side, and the negative terminal of the source is connected to the n-type side, the diode is said to be in a forward biassed condition. We know that there is a barrier potential across the junction. This barrier potential is directed in the opposite direction of the forward applied voltage. So a diode can only allow current to flow in the forward direction when the forward applied voltage is more than the barrier potential of the junction.  This voltage is called forward biassed voltage. For a silicon diode, it is 0.7 volts. For a germanium diode, it is 0.3 volts. 

When forward applied voltage is more than this forward biassed voltage, there will be forward current in the diode, and the diode will become short-circuited. Hence, there will be no more voltage drop across the diode beyond this forward biassed voltage, and forward current is only limited by the external resistance">resistance connected in series with the diode. Thus, if forward applied voltage increases from zero, the diode will start conducting only after this voltage reaches just above the barrier potential or forward biassed voltage of the junction. The time taken by this input voltage to reach that value, or in other words, the time taken by this input voltage to overcome the forward-biased voltage, is called recovery time.

Reverse biased

If the diode is reverse biased, that is, the positive terminal of the source is connected to the n-type end and the negative terminal of the source is connected to the p-type end, no current will flow through the diode except reverse saturation current. This is because, at the reverse bias condition, the depilation layer of the junction becomes wider with increasing reverse-biased voltage. Although there is a tiny current flowing from the n-type end to the p-type end of the diode due to minority carriers, this tiny current is called reverse saturation current. 

Minority carriers are mainly thermally generated electrons and holes in p-type semiconductors and n-type semiconductors, respectively. If the reverse applied voltage across the diode is continuously increased, the depletion layer will be destroyed after a certain applied voltage, causing a massive reverse current to flow through the diode. If this current is not externally limited and it reaches beyond the safe value, the diode may be permanently destroyed. This is because, as the magnitude of the reverse voltage increases, the kinetic energy of the minority charge carriers also increases. 

These fast-moving electrons collide with the other atoms in the device to knock off some more electrons from them. The electrons so released further release much more electrons from the atoms by breaking the covenant bonds. This process is termed carrier multiplication and leads to a considerable increase in the flow of current through the p-n junction. The associated phenomenon is called Avalanche Breakdown.

Characteristics I-V

But before we can use the PN junction as a practical device or as a rectifying device, we need to first bias the junction, i.e. connect a voltage potential across it. On the voltage axis above, "Reverse Bias" refers to an external voltage potential that increases the potential barrier. An external voltage that 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:

A Junction Diode has a Junction Diode

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

The Forward Biased PN Junction Diode is

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 barrier 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.

The Forward Characteristics Curve for a Junction Diode

The application of a forward biassing 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.

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 are 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 reversed biased, the thickness of the depletion region increases, and the diode acts as an open circuit, blocking any current flow, (only a very small leakage current).

We have also seen above that the diode is a two-terminal non-linear device whose I-V characteristics are polarity dependent, as depending upon the polarity of the applied voltage, VD, the diode is either Forward Biased, VD > 0 or Reverse Biased, VD < 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

The typical applications of Diodes

• Rectifying a voltage, such as turning AC into DC voltages
• Isolating signals from the supply
• Referencing Voltage
• Controlling the size of a signal
• Mixing signals
• Detection signals
• Lighting
• Laser diodes

Power Conversion

One significant application of diodes is to convert AC power to DC power. A single diode or four diodes can be used to transform 110V household power to DC by forming a half-way (single diodes) or a full-wave (four diodes) rectifier. A diode does this by allowing only half of the AC waveform to travel through it. When this voltage pulse is used to charge a capacitor, the output voltage appears to be a steady DC voltage with a small voltage ripple.

Using a full-wave rectifier makes this process even more efficient by routing the AC pulses so both the positive and negative halves of the input sine wave are seen as only positive pulses, effectively doubling the frequency of the input pulses to the capacitor, which helps keep it charged and delivers a more stable voltage.

Diodes and capacitors can also be used to create a number of types of voltage multipliers to take a small AC voltage and multiply it to create very high voltage outputs. Both AC and DC outputs are possible using the right configuration of capacitors and diodes.

Demodulation of Signals

The most common use for diodes is to remove the negative component of an AC signal so it can be worked with easier with electronics. Since the negative portion of an AC waveform is usually identical to the positive half, very little information is effectively lost in this process. Signal demodulation is commonly used in radios as part of the filtering system to help extract the radio signal from the carrier wave.

Over-Voltage Protections

Diodes also function well as protection devices for sensitive electronic components. When used as voltage protection devices, the diodes are non-conducting under normal operating conditions but immediately short any high voltage spike to the ground where it cannot harm an integrated circuit. Specialized diodes called transient voltage suppressors are designed specifically for over-voltage protection and can handle very large power spikes for short time periods, typical characteristics of a voltage spike or electric shock, which would normally damage components and shorten the life of an electronic product.

Current Steering

The basic application of diodes is to steer current and make sure it only flows in the proper direction. One area where the current steering capability of diodes is used to good effect is in switching from power from a power supply to running from a battery. When a device is plugged in and charging, for example, a cell phone or uninterrupted power supply, the device should be drawing power only from the external power supply and not the battery and while the device is plugged in, the battery should be drawing power and recharging. As soon as the power source is removed, the battery should power the device so no interruption is noticed by the user.