Semiconductor Diodes

A diode is a device that allows current to flow only in one direction. It is made from p-type or n-type semiconductors joined together. It has a depletion layer/p-n junction/potential barrier.

How the depletion layer is formed

The depletion layer is formed when a p-type and n-type are joined. The electrons in the n-type semiconductor adjacent to the p-type semiconductor move to fit in the holes which are also at the junction. This results in the creation of positive and negative ions which don’t have any charge carriers. This region is called the depletion layer. It can also be called a potential barrier since the positive ions on the n-side are at a higher potential than the negative ions in the p-side. The movement of ions is called diffusion. The diffusion process stops after sometime because the positive ions will repel the holes and the negative ions will repel the electrons.

Forward and Reverse biasing of Diodes

Consider the circuits illustrated below:

In a forward biased diode, the anode is connected to positive terminal of the power supply while the cathode is connected to the negative terminal. This leads to a continuous supply of electrons and continuous supply of holes at the junction which makes current to flow hence the bulb lights up.

If the battery polarity is reversed, so that the diode anode is connected to the negative terminal while the cathode is connected to the positive terminal of power supply, the width of depletion layer is enlarged due to majority of carriers (electrons) moving away from the junction. In this case we have a reverse-biased diode. There is only minimal current which flows in the diode due to minority charge carriers. This current is referred to as a leakage current and is expressed in terms of μA which cannot make the bulb to light up.

The forward-bias voltage drop exhibited by the diode is due to the action of the depletion region formed by the pn-junction under the influence of the applied voltage. If no voltage is applied across the semiconductor diode, a thin depletion region exists around the region of the p-n junction preventing current flow. The depletion region is devoid of available charge carriers and acts as an insulator.

The schematic symbol of the diode is shown above in figure 1(d) such that the anode (pointing end/arrowhead) corresponds to the p-type semiconductor while the cathode bar (non-pointing end) corresponds to the n-type material.

The cathode stripe on the diode physical part corresponds to the cathode on the symbol.

Voltage-Current (VI) characteristics of a diode

Consider the circuit below:

In the forward bias connection germanium starts conduction at 0.3 V while silicon starts conducting at 0.7 V. From this point onwards, the current increases with increase in forward voltage hence a rising curve as illustrated in figure (g). The voltage at which the diodes start conducting current is called knee voltage.

In reverse-bias, when the voltage is at zero, there is a very small amount of current which flows called the leakage current. The increase in voltage does not affect the current provided it is within the limit the diode can withstand. If the voltage is increased beyond a point where the diode cannot withstand, it breaks down i.e. the minority electrons moving at high speed detach the electrons from the semiconductor atoms. This voltage at which the pn junction breaks down is called breakdown voltage or peak inverse voltage (PIV) i.e. characterized by a sudden rise of reverse current and a sudden fall of resistance of the barrier region. The electrons at this point are called avalanche free electrons. This current may damage the diode permanently.

An equation describes the exact current through the diode; given the voltage drop across the junction, the temperature of the junction, and several physical constants. It is known as the diode equation which is given as:

Where,

I_D = diode current in amperes

I_s = saturation current in amperes (typically 1 x 10^-12 A)

e = Euler’s constant ~ 2.718281828

q = charge of electron (1.6 x 10^-19 coulombs)

V_D = Voltage applied across the diode in volts

N= nonideality or ‘’emission’’ coefficient (typically 1 and 2)

k = Boltzmann’s constant (1.38 x 10^-23)

T = junction temperature in kelvins

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 V_t of the junction. At room temperature this is about 26 millivolts.

Related: The Basics of Semiconductor Physics as the Foundation of Electronics

Types of Diodes

We have several types of diodes, some of them include:

Light Emitting Diodes (LED)

This is a diode that emits light when forward voltage is applied across it.

LEDs are used in digital displays, indicators and networks like fiber optics where it converts electrical energy to light energy.

Photodiode

This is a diode made from photo conducting material such as germanium that changes light energy to electric energy i.e. it is a transducer.

Photodiodes are used alarm circuits, fiber optics networks to change light energy to electric energy, automatic switching systems, etc.

Tunnel diode

A tunnel diode exhibits negative resistance between two points of forward voltage.

Tunnel diodes are employed in oscillators e.g. tuning circuits and fast switches.

Varactor diode

This is a diode that behaves like a variable capacitor.

The varactor diode is commonly used in tuning circuits.

You can also read: Optoelectronics