P-N Junction Diode

The symbol of diode is shown in fig.
The terminal connected to p-layer is called anode (A) and the terminal connected to n-layer is called cathode (K).

Reverse Bias:

If positive terminal of dc source is connected to cathode and negative terminal is connected to anode, the diode is called reverse biased as shown in fig.

When the diode is reverse biased then the depletion region width increases, majority carriers move away from the junction and there is no flow of current due to majority carriers but there are thermally produced electron hole pair also.
If these electrons and holes are generated in the vicinity of junction then there is a flow of current.
The negative voltage applied to the diode will tend to attract the holes thus generated and repel the electrons.
At the same time, the positive voltage will attract the electrons towards the battery and repel the holes.
This will cause current to flow in the circuit.
This current is usually very small (interms of micro amp to nano amp).
Since this current is due to minority carriers and these number of minority carriers are fixed at a given temperature therefore, the current is almost constant known as reverse saturation current ICO.
In actual diode, the current is not almost constant but increases slightly with voltage.
This is due to surface leakage current.
The surface of diode follows ohmic law (V=IR).
The resistance under reverse bias condition is very high 100k to mega ohms.
When the reverse voltage is increased, then at certain voltage, then breakdown to diode takes place and it conducts heavily.
This is due to avalanche or zener breakdown.
The characteristic of the diode is shown in fig.

Forward bias:

When the diode is forward bias, then majority carriers are pushed towards junction, when they collide and recombination takes place.
Number of majority carriers are fixed in semiconductor.
Therefore as each electron is eliminated at the junction, a new electron must be introduced, this comes from battery.
At the same time, one hole must be created in p-layer.
This is formed by extracting one electron from p-layer.
Therefore, there is a flow of carriers and thus flow of current.

Relationship between Diode Current and Diode Voltage

An exponential relationship exists between the carrier density and applied potential of diode junction as given in equation.
This exponential relationship of the current iD and the voltage vD holds over a range of at least seven orders of magnitudes of current – that is a factor of 10^7.

Since the problem contains only a dc source, we use the diode equivalent circuit, as shown in fig.(b).
Once we determine the state of the ideal diode in this model (i.e., either open circuit or short circuit), the problem becomes one of simple dc circuit analysis.
It is reasonable to assume that the diode is forward biased.
This is true since the only external source is 10 V, which clearly exceeds the turn-on voltage of the diode, even taking the voltage division into account.
The equivalent circuit then becomes that of fig.(b) with the diode replaced by a short circuit.
The Thevenin’s equivalent of the circuit between A and B is given by fig. 1(c).
The output voltage is given by

Diode Approximation: (Large signal operations)

Ideal Diode:

When diode is forward biased, resistance offered is zero, When it is reverse biased resistance offered is infinity.
It acts as a perfect switch. The characteristic and the equivalent circuit of the diode is shown in fig.

Second Approximation:

When forward voltage is more than 0.7 V, for Si diode then it conducts and offers zero resistance.
The drop across the diode is 0.7V.
When reverse biased it offers infinite resistance. The characteristic and the equivalent circuit is shown in fig.

3rd Approximation:

When forward voltage is more than 0.7 V, then the diode conducts and the voltage drop across the diode becomes 0.7 V and it offers resistance Rf (slope of the current)
VD= 0.7 + ID Rf.
The output characteristic and the equivalent circuit is shown in fig.

Diode Applications

To understand the concept of working of diode & its applications, few examples are given here:

Example – 1:

Calculate the voltage output of the circuit shown in fig. for following inputs

(a) V₁ = V₂ = 0.

(b) V₁ = V, V₂ = 0.

Forward resistance of each diode is Rf.

(a) When both V1 and V2 are zero , then the diodes are unbiased. Therefore, Vo = 0 V
(b) When V₁ = V and V₂ = 0, then one upper diode is forward biased and lower diode is unbiased.
The resultant circuit using third approximation of diode will be as shown in fig.

(c) When both V₁ and V₂ are same as V, then both the diodes are forward biased and conduct. The resultant circuit using third approximation of diode will be as shown in Fig.

Half wave Rectifier:

The single phase half wave rectifier is shown in fig.

In positive half cycle, D is forward biased and conducts.
Thus the output voltage is same as the input voltage.
In the negative half cycle, D is reverse biased, and therefore output voltage is zero. The output voltage waveform is shown in fig.
The average output voltage of the rectifier is given by:

When the diode is reverse biased, entire transformer voltage appears across the diode.
The maximum voltage across the diode is Vm.
The diode must be capable to withstand this voltage. Therefore PIV half wave rating of diode should be equal to Vm in case of single-phase rectifiers.
The average current rating must be greater than Iavg.

Full Wave Rectifier:

A single phase full wave rectifier using center tap transformer is shown in fig.
It supplies current in both half cycles of the input voltage.

In the first half cycle D1 is forward biased and conducts.
But D2 is reverse biased and does not conduct.
In the second half cycle D2 is forward biased, and conducts and D1 is reverse biased.
It is also called 2 pulse midpoint converter because it supplies current in both the half cycles. The output voltage waveform is shown in fig.

The average output voltage is given by

Bridge Rectifier:

The single phase full wave bridge rectifier is shown in fig.
It is the most widely used rectifier.
It also provides currents in both the half cycle of input supply.

In the positive half cycle, D1 & D4 are forward biased and D2 & D3 are reverse biased.
In the negative half cycle, D2 & D3 are forward biased, and D1 & D4 are reverse biased.
The output voltage waveform is shown in fig. and it is same as full wave rectifier but the advantage is that PIV rating of diodes are Vm and only single secondary transformer is required.
The main disadvantage is that it requires four diodes.
When low dc voltage is required then secondary voltage is low and diodes drop (1.4V) becomes significant.
For low dc output, 2 pulse center tap rectifier is used because only one diode drop is there.
The ripple factor is the measure of the purity of dc output of a rectifier and is defined as:

Clippers:

Clipping circuits are used to select that portion of the input wave which lies above or below some reference level.

Clipper Circuit 1:

The circuit shown in fig., clips the input signal above a reference voltage (V_R).

In this clipper circuit,

      If v_i < V_R, diode is reversed biased and does not conduct. Therefore, v_o = v_i

      if v_i > V_R, diode is forward biased and thus, v_o= V_R.

The transfer characteristic of the clippers is shown in fig.

Clipper Circuit 2:

The clipper circuit shown in fig. clips the input signal below reference voltage V_R.

In this clipper circuit,

          If v_i > V_R, diode is reverse biased. v_o = v_i

and, If v_i < V_R, diode is forward biased. v_o = V_R

The transfer characteristic of the circuit is shown in fig.

Clipper Circuit 3:

To clip the input signal between two independent levels (V_R1< V_R2 ), the clipper circuit is shown in fig.

The diodes D1 & D2 are assumed ideal diodes.

For this clipper circuit, when vi Less than equal to VR1, vo=VR1

and, vi greater than equal to V_R2, v_o= V_R2
and, V_R1 < v_i < V_R2 vo = vi

The transfer characteristic of the clipper is shown in fig.

Example 1:

Draw the transfer characteristic of the circuit shown in fig.

Clamper Circuits:

Clamping is a process of introducing a dc level into a signal.
For example, if the input voltage swings from -10 V and +10 V, a positive dc clamper, which introduces +10 V in the input will produce the output that swings ideally from 0 V to +20 V.
The complete waveform is lifted up by +10 V.

Negative Diode clamper:

A negative diode clamper is shown in fig., which introduces a negative dc voltage equal to peak value of input in the input signal.

Let the input signal swings form +10 V to -10 V.
During first positive half cycle as Vi rises from 0 to 10 V, the diode conducts.
Assuming an ideal diode, its voltage, which is also the output must be zero during the time from 0 to t1.
The capacitor charges during this period to 10 V, with the polarity shown.
At that Vi starts to drop which means the anode of D is negative relative to cathode, ( VD = vi – vc ) thus reverse biasing the diode and preventing the capacitor from discharging.
Since the capacitor is holding its charge it behaves as a DC voltage source while the diode appears as an open circuit, therefore the equivalent circuit becomes an input supply in series with -10 V dc voltage as shown in fig. , and the resultant output voltage is the sum of instantaneous input voltage and dc voltage (-10 V).

Positive Clamper:

The positive clamper circuit is shown in fig., which introduces positive dc voltage equal to the peak of input signal.
The operation of the circuit is same as of negative clamper.

Let the input signal swings form +10 V to -10 V.
During first negative half cycle as Vi rises from 0 to -10 V, the diode conducts.
Assuming an ideal diode, its voltage, which is also the output must be zero during the time from 0 to t1.
The capacitor charges during this period to 10 V, with the polarity shown.
After that Vi starts to drop which means the anode of D is negative relative to cathode, (V_D= v_i – v_C) thus reverse biasing the diode and preventing the capacitor from discharging.
Since the capacitor is holding its charge it behaves as a DC voltage source while the diode appears as an open circuit, therefore the equivalent circuit becomes an input supply in series with +10 V dc voltage and the resultant output voltage is the sum of instantaneous input voltage and dc voltage (+10 V).

To clamp the input signal by a voltage other than peak value, a dc source is required. As shown in fig., the dc source is reverse biasing the diode.
The input voltage swings from +10 V to -10 V.
In the negative half cycle when the voltage exceed 5V then D conduct.
During input voltage variation from -5 V to -10 V, the capacitor charges to 5 V with the polarity shown in fig.
After that D becomes reverse biased and open circuited.
Then complete ac signal is shifted upward by 5 V. The output waveform is shown in fig.

Zener Diode:

The diodes designed to work in breakdown region are called zener diode.
If the reverse voltage exceeds the breakdown voltage, the zener diode will normally not be destroyed as long as the current does not exceed maximum value and the device closes not over load.
When a thermally generated carrier (part of the reverse saturation current) falls down the junction and acquires energy of the applied potential, the carrier collides with crystal ions and imparts sufficient energy to disrupt a covalent bond.
In addition to the original carrier, a new electron-hole pair is generated.
This pair may pick up sufficient energy from the applied field to collide with another crystal ion and create still another electron-hole pair.
This action continues and thereby disrupts the covalent bonds.
The process is referred to as impact ionization, avalanche multiplication or avalanche breakdown.
There is a second mechanism that disrupts the covalent bonds.
The use of a sufficiently strong electric field at the junction can cause a direct rupture of the bond.
If the electric field exerts a strong force on a bound electron, the electron can be torn from the covalent bond thus causing the number of electron-hole pair combinations to multiply.
This mechanism is called high field emission or Zener breakdown.
The value of reverse voltage at which this occurs is controlled by the amount of doping of the diode.
A heavily doped diode has a low Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage.
At voltages above approximately 8V, the predominant mechanism is the avalanche breakdown.
Since the Zener effect (avalanche) occurs at a predictable point, the diode can be used as a voltage reference.
The reverse voltage at which the avalanche occurs is called the breakdown or Zener voltage.
A typical Zener diode characteristic is shown in fig.
The circuit symbol for the Zener diode is different from that of a regular diode, and is illustrated in the figure.
The maximum reverse current, IZ(max), which the Zener diode can withstand is dependent on the design and construction of the diode.
A design guideline that the minimum Zener current, where the characteristic curve remains at VZ (near the knee of the curve), is 0.1/ IZ(max).

The power handling capacity of these diodes is better. The power dissipation of a zener diode equals the product of its voltage and current.

P_Z= V_Z I_Z

The amount of power which the zener diode can withstand ( V_Z I_Z(max) ) is a limiting factor in power supply design.

Zener Regulator:

When zener diode is forward biased it works as a diode and drop across it is 0.7 V.
When it works in breakdown region the voltage across it is constant (VZ) and the current through diode is decided by the external resistance.
Thus, zener diode can be used as a voltage regulator in the configuration shown in fig. for regulating the dc voltage.
It maintains the output voltage constant even through the current through it changes.

The load line of the circuit is given by Vs= Is Rs + Vz.
The load line is plotted along with zener characteristic in fig.
The intersection point of the load line and the zener characteristic gives the output voltage and zener current.
To operate the zener in breakdown region Vs should always be greater then Vz. Rs is used to limit the current.
If the Vs voltage changes, operating point also changes simultaneously but voltage across zener is almost constant.
The first approximation of zener diode is a voltage source of Vz magnitude and second approximation includes the resistance also.
The two approximate equivalent circuits are shown in fig.

If second approximation of zener diode is considered, the output voltage varies slightly as shown in fig.
The zener ON state resistance produces more I * R drop as the current increases.
As the voltage varies form V1 to V2 the operating point shifts from Q1 to Q2.