3 Different Types of Diodes

Right from the electronic design to production and repair, diodes are extensively used for several applications. These are of different types and transfers the electric current based on the properties and specifications of that particular diode. These are mainly P-N junction diodes, Photosensitive diodes, Zener diodes, Schottky diodes, Varactor diodes. Photosensitive diodes include LED’s, Photodiodes and Photovoltaic cells. Some of these are explained briefly in this article.

1. P-N Junction Diode

A P-N junction is a semiconductor device, which is formed by P-type and N-type semiconductor material. P-type has a high concentration of holes and N-type has a high concentration of electrons. Holes diffusion is from p-type to n-type and electron diffusion is from n-type to p-type.

The donor ions in the n-type region become positively charged as the free electrons move from the n-type to p-type. Hence, a positive charge is built on the N-side of the junction. The free electrons across the junction are the negative acceptor ions by filling in the holes, then the negative charge established on the p-side of the junction is shown in the figure.

 An electric field formed by the positive ions in the n-type region and negative ions in p-type regions. This region is called the diffusion region. Since the electric field quickly sweeps free carriers out, hence the region is depleted of free carriers. A built-in potential Vbi due to Ê is formed at the junction is shown in the figure.

Functional Diagram of P-N Junction Diode:

Functional Diagram of P-N Junction Diode
Functional Diagram of P-N Junction Diode

Forward Characteristics of P-N Junction:

When the positive terminal of the battery is connected to P-type and the negative terminal is connected to N-type is called forward bias of P-N junction is shown figure below.

Forward Characteristics of P-N Junction
Forward Characteristics of P-N Junction

If this external voltage becomes greater than the value of the potential barrier, approximately 0.7 volts for silicon and 0.3V for Ge, the potential barrier is crossed and the current starts flowing due to movement of electrons across the junction and same for the holes.

P-N Junction Forward Bias Characteristics
P-N Junction Forward Bias Characteristics

Reverse Characteristics of P-N Junction:

When a positive voltage is given to the n-part and negative voltage to the p-part of the diode, it is said to be in reverse bias condition.

P-N Junction Reverse Characteristics Circuit
P-N Junction Reverse Characteristics Circuit

When a positive voltage is given to N-part of the diode, the electrons move towards the positive electrode and the application of negative voltage to the p-part makes the holes move towards the negative electrode. As a result, the electrons cross the junction to combine with the holes in the opposite side of the junction and vice versa. As a result, a depletion layer is formed, having a high impedance path with a high potential barrier.

P-N Junction Reverse Bias Characteristics
P-N Junction Reverse Bias Characteristics

Applications of P-N Junction Diode:

P-N junction diode is a two-terminal polarity sensitive device, the diode conducts when in forwarding bias and diode not conducts when reverse bias. Due to these characteristics, P-N junction diode is used in many applications like

  1. Rectifiers in DC power supply
  2. Demodulation circuits
  3. Clipping and clamping networks

2. Photodiode

The photodiode is a kind of diode which generates current proportional to the incident light energy. It is a light to voltage/current converter that finds applications in security systems, conveyors, automatic switching systems, etc. The photodiode is similar to an LED in construction but its p-n junction is highly sensitive to light. The p-n junction may be exposed or packaged with a window to enter light into the P-N junction. Under the forward biased state, current passes from the anode to cathode, while in the reverse-biased state, photocurrent flows in the reverse direction. In most cases, the packaging of Photodiode is similar to LED with anode and cathode leads projecting out from the case.

Photo Diode
Photo Diode

There are two kinds of Photodiodes – PN and PIN photodiodes. The difference is in their performance. The PIN photodiode has an intrinsic layer, so it must be reverse biased. As a result of reverse biasing, the width of the depletion region increases, and the capacitance of the p-n junction decreases. This allows the generation of more electrons and holes in the depletion region. But one disadvantage of reverse biasing is that it generates noise current that may reduce the S/N ratio. So reverse biasing is suitable only in applications that require higher bandwidth. The PN photodiode is ideal for lower light applications because the operation is unbiased.

PhotodiodeThe photodiode works in two modes namely Photovoltaic mode and Photoconductive mode. In the photovoltaic mode (also called Zero bias mode), the photocurrent from the device is restricted and a voltage builds up. The photodiode is now in the Forward biased state and a “Dark current” starts flowing across the p-n junction. This flow of dark current occurs opposite to the direction of the photocurrent. The dark current generates in the absence of light. The dark current is the photocurrent induced by the background radiation plus the saturation current in the device.

The Photoconductive mode occurs when the photodiode is reverse biased. As a result of this, the width of the depletion layer increases and leads to a reduction in the capacitance of the p-n junction. This increases the response time of the diode. Responsivity is the ratio of the photocurrent generated to the incident light energy. In the Photoconductive mode, the diode generates only a small current called Saturation current or back current along its direction. The photocurrent remains the same in this condition. The photocurrent is always proportional to the luminescence. Even though the Photoconductive mode is faster than the Photovoltaic mode, the electronic noise in higher in photoconductive mode. Silicon-based photodiodes generate less noise than germanium based photodiodes since silicon photodiodes have a greater bandgap.

3. Zener Diode

zenerZener diode is a type of Diode that allows the flow of current in the forward direction similar to a rectifier diode but at the same time, it can permit the reverse flow of current also when the voltage is above the breakdown value of the Zener. This is typically one to two volts higher than the rated voltage of the Zener and is known as the Zener voltage or Avalanche point. The Zener was named so after Clarence Zener who discovered the electrical properties of the diode. Zener diodes find applications in voltage regulation and to protect semiconductor devices from voltage fluctuations. Zener diodes are widely used as voltage references and as shunt regulators to regulate the voltage across circuits.

The Zener diode uses its p-n junction in the reverse bias mode to give the Zener Effect. During the Zener effect or Zener breakdown, the Zener holds the voltage close to a constant value known as the Zener voltage. The conventional diode also has the property of reverse bias, but if the reverse bias voltage is exceeded, the diode will be subjected to high current and it will be damaged. The Zener diode, on the other hand, is specially designed to have a reduced breakdown voltage called Zener voltage. The Zener diode also exhibits the property of a controlled breakdown and allows the current to keep the voltage across the Zener diode close to the breakdown voltage. For example, a 10 volt Zener will drop 10 volts across a wide range of reverse currents.

ZENER SYMBOLWhen the Zener diode is reverse biased, its p-n junction will experience an Avalanche breakdown and the Zener conducts in the reverse direction. Under the influence of the applied electric field, the valance electrons will be accelerated to knock and release other electrons. This ends in the Avalanche effect. When this occurs, a small change in the voltage will result in a large current flow. The Zener break down depends on the applied electric field as well as the thickness of the layer on which the voltage is applied.

ZENER BREAKDOWNThe Zener diode requires a current limiting resistor in series to it to restrict the current flow through the Zener. Typically the Zener current is fixed as 5 mA. For example, if a 10 V Zener is used with a 12 volt supply, a 400 Ohms (Near value is 470 Ohms) is ideal to keep the Zener current as 5 mA. If the supply is 12 volts, there are 10 volts across the Zener diode and 2 volts across the resistor. With 2 volts across the 400 ohms resistor, then the current through the resistor and Zener will be 5 mA. So as a rule 220 Ohms to 1K resistors are used in series with the Zener depending upon the supply voltage. If the current through the Zener is insufficient, the output will be unregulated and less than the nominal breakdown voltage.

1The following formula is useful to determine the current through the Zener:

Zener = (VIn – V Out) / R Ohms

The value of the Resistor R must satisfy two conditions.

  1. It must be a low value to permit sufficient current through the Zener
  2. The power rating of the resistor must be high enough to protect the Zener.

Photo Credit:

  • Zener by wikimedia
  • Functional Diagram of P-N Junction Diode by Cuer


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