PN JUNCTION DIODE
It is possible to manufacture a single piece of a semiconductor material one half of which is doped by p-type impurity and another half by n-type impurity. The plane dividing the two halves is called pn junction. When a pn junction is packed as a semiconductor device, it is called a PN junction diode.
Now, suppose the two pieces are joined to form pn junction, at the junction, there is a tendency for the free electrons to diffuse over the p - side and holes the n - side. This process is called diffusion. As the free electrons move across the junction from n-type to p-type positive donor ions are uncovered. Hence, a positive charge is built on the n - side of the junction. At the same time, the free electrons move across the junction and uncover the negative acceptor ions by filling in the holes. Therefore, a negative charge is established on p - the side of the junction. When a sufficient number of donor and acceptor ions is uncovered. further diffusion is prevented. Thus a barrier is set up again This is called a potential barrier. The potential barrier is of the further movement of charge carriers i.e., holes and electrons. order of 0.3 V for germanium and 0.7 V for silicon. The potential distribution diagram is shown in Fig. The electric field set up by a potential barrier prevents the respective majority of carriers from crossing the barrier region.
The outside of the barrier on each side of the junction is st neutral. Only inside the barrier, there is a positive charge n - side and a negative charge on the p - side. This region is called the depletion region. The circuit symbol of p - n diode is shown in Fig The arrow's head indicates the direction of the conventional current.
WORKING OF PN JUNCTION DIODE
The working of a PN junction diode can be explained under two different heads.
( a ) Forward Bias ( b ) Reverse Bias
FORWARD BIASED PN JUNCTION DIODE
When the external voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, it is called forward biasing.
Suppose the positive battery terminal is connected to a region of a semiconductor and the negative battery terminal to the n - region as shown in Fig . 1.11 is called forward bias. Forward bias permits the easy flow of current across the junction. The current flow may be explained in the following ways.
1. As soon as battery connections are made, holes are repelled by the positive battery terminal and electrons are repelled by the negative battery terminal with the result that both the elections and the holes are driven towards the junction. This movement of electrons and holes constitutes a large current flow through the semiconductor. The diode offers low resistance in the forwarding direction ..
2. The applied forward voltage reduces the height of the potential barrier at the junction. It allows more carriers across the junction, and more current to flow across the junction. Forward bias reduces the thickness of the depletion layer.
WORKING OF PN JUNCTION DIODE
When the external voltage applied to the junction is in such a direction that the potential barrier is increased, it is called reverse biasing. Suppose a negative terminal of the battery is connected to the p-region of the diode and the positive battery terminal the n - region as shown in Fig is called reveres bias. In this case, holes are attracted by the negative battery terminal and electrons by the positive terminal so that both holes and electrons move away from the junction. Since there is no current flow and the junction offers high resistance. The applied reverse voltage V increases the potential barrier thereby blocking the flow of the majority of carriers. The reverse bias increases the thickness of the depletion layer. Although under reverse bias condition, there is practically no current due to majority carriers, yet there is a small amount of current due to the flow of minority carriers. This current is called reverse saturation current Io. Since minority carriers are thermally generated I, are extremely temperature-dependent. Io is found to double to every 10 ° C rise for germanium and for every 6 ° C rise in silicon. I is in order of μA for germanium and nA for silicon.
If the reverse voltage is increased continuously the kinetic energy of minority electrons may become high enough to knockout electrons from the semiconductor atom. At this stage breakdown of the junction occurs, characterized by a sudden rise of reverse current and a sudden fall of the resistance of the barrier region. This may destroy the junction permanently
VOLT - AMPERE CHARACTERISTICS OF P - N DIODE
VI characteristics of a p - n diode are the curve between the voltage across the junction and the circuit current. Usually, voltage is taken along the x-axis and current along the y-axis. The characteristics are shown in Fig and can be studied under three heads
1. No Bias:
When the external voltage is zero i.e., the circuit is open, the potential barrier at the junction does not permit current flow. Therefore the circuit current is zero and is indicated by point O.
2. Forward Bias:
With a forward bias to the p - n junction, the potential barrier is reduced. At ordinary room temperature, a potential drop of about 0.3 u or 0.7 v is required to start conduction. This voltage is known as threshold voltage or cutin voltage. It is practically the same as barrier voltage VB . From now onwards that current increases exponentially with the increase in forwarding voltage. From the forward characteristics, it is see that in the first region OA, the current increases slowly and the curve is nonlinear. It is because the external voltage exceeds the potential barrier voltage the p- diode behaves like an ordinary conductor Therefore the current rises very sharply with an increase in the external voltage
3. Reverse Bias:
With reverse bias, the potential barrier is increased. Therefore the junction resistance becomes very high and practically no majority current flows through the circuit However in practice a very small current flows in the circuit with reverse bias shown in Fig .. This is called reverse saturation current and is due to minority carriers. Keeping the temperature constant, as the reverse voltage is increased, I is found to increase only slightly Ordinary the range of forwarding current over which a diode is operated is many orders of magnitude larger than the reverse saturation current. To display forward and reverse characteristics conveniently it is necessary to use two different scales.
The relation between voltage and current is given by
1 = 1₂ ( e ³ - 1 ) T ( k )
Where , V + = Volt equivalent to temperature = 11,600
1₂ = Reverse saturation current
V = Applied voltage
n = 1 for germanium 2 for silicon .
SPECIFICATIONS OF DIODE
particular In order to select a proper semiconductor diode for a application , an appropriate ratings / specifications must be followed . Usually a typical values are supplied by the manufacturer and while observing those , a proper pn - diode bas to select . Here some of the specifications are listed below .
1. Maximum Forward Current ( IF ) : It is the maximum current in forward bias that a pn - junction can conduct without damage to the junction .
2. Peak Inverse Voltage ( PIV ) : It is the maximum voltage that can be applied in reverse bias to the pn - junction without damage to the junction .
3. Reverse Break Down Voltage ( VBR ) : The minimum reverse voltage at which the diode may break down .
4. Power Rating ( P ) : The maximum power that the device can safely dissipate on a continuous basis in free air at 25 ° C .
