As we have already known that reverse recovery time is the time it takes to invert the minority charge distribution of diode from forward biase to minority charge distribution in reverse biase. Hence when we increase the doping of material, the concentration fo minority charge carriers decrease. Hence as the peaks of charge distribution have fallen, it takes lesser time to invert the charge distribution. Hence we can say that with increase in doping, the reverse recovery time decrease and with decrease in doping level the reverse recovery time increases.
Now let’s analyze that what would happen when we change diode state from forward biase from reverse biase. This state change takes time which is known as reverse recovery time. Consider the following circuit of diode to analyze the switching time of diode.
When the diode is in forward biase, the minority charge distribution of diode is as:
And when diode is in reverse biase position, the minority charge distribution is as:
So to change state from forward to reverse biase, the whole minority charge distribution needs to be inverted as we can see from the figures above.
Now let’s see what happens during the period in which state changes.
Firstly we are in forward biase state when voltage applied is +V. So there are many minority carriers near the junction and then there is an exponential decrease in the concentration of minority carriers and there is a continuous flow of majority carriers across the junction. We assume the current as I in the forward biase. We depict this in the following graph of current across the junction with time:
Now we change the applied voltage to –V at time t=t1 i.e. diode is now reverse biased. As minority carrier concentration in both sides was large near junction in the forward biase, when we have instantly changed the state to reverse biased, those minority carriers start moving in the opposite direction. And due to large concentration of such minority carriers, the amount of current flowing remains the same, only direction changes as shown below:
*Red coloured lines are of reverse biased region.
But the high reverse current continues for small time because the concentration of the stored minority carriers start decreasing and the current also starts decreasing exponentially as shown below:
The time gap t2 – t1 in which the reverse current is high (i.e. equal to I) is known as storage time and the time gap from t2 to t3 i.e. the time reverse current becomes equal to reverse saturation current is known as transient time. The total time from t1 to t3 is known as reverse recovery time.
Normal (un-biased) state:
Apply the relation given below
n * p = ni2 at constant temperature (Mass action law)
Now we apply the above relation to p-type:
p i.e. the concentration of majority carriers (holes) is larger as doping of p-side is high and we have the value of ni2as constant at fixed temperature. Hence from the above relation we find that number of minority carriers ( electrons) is less in p-type material while as doping of n-side is normal, hence number of majority carriers (i.e. electrons) in n-side is not large with the value of ni2as constant and hence number of minority carriers is larger as compared to that in p-side. We depict the above as below:
Npo is defined as the concentration of minority carriers in N-type material i.e. holes and Pno is defined as the concentration of minority carriers in P-type material i.e. electrons when diode is in un-biased state (i.e. diode is neither forward biased or reverse biased).
Forward Biased state:
When diode is forward biased, the majority carriers of both sides cross the junction and after reaching the other side, the charge carriers start combining. So holes from p-side start moving towards n-side and electrons from n-side start moving to p-side. When holes enter the n-side they become the minority carriers and just at the junction there would be high concentration of holes in n-side as the recombining has just started. Also all the holes can not recombine at the junction. Hence when we move away from the junction in the n-side, the concentration of holes is decreasing as more and more holes are recombining. This is also shown in the figure below. Similarly in the p-side, concentration of the electrons is high near the junction and it starts decreasing as we move away from the junction in the p-side.
The red curve shows the level of concentration of minority carriers at different distances on the both sides of junction and the shaded blue part shows the increase in the concentration of minority carriers after forward biasing the diode. There is a difference in the peak level of minority carriers as we have the difference in the doping level of both sides.
Reverse Biased state:
When we reverse biase any diode, the minority carriers from both sides cross the junction and then recombine after reaching the other side. Hence the holes from n-side move towards p-side and after reaching p-type material become majority carriers. These holes combine with minority carriers of p-side i.e. electrons. So the minority carriers at junction i.e. holes in the n-side which are near junction would immediately cross the junction on reverse biase and other holes move slowly. Similar to the above, electrons of p-side move to n-side. Hence the concentration of minority carriers falls on both sides as shown below:
The switching time of a diode is defined as the time which a diode takes to change its state from forward biased state to reverse biased state or in other words the forward current through diode doesn’t reduce to reverse saturation current immediately as the reverse voltage is applied. In fact it takes time for the current to reduce from forward current to reverse saturation current. This time is also called reverse recovery time.
To discuss more about the switching time, we first need to discuss charge distribution of diode in normal state, forward biased state and reverse biased state assuming doping of p-type is more than n-type.
The following graph shows the effect of temperature on the characteristics of diode
A-B curve: This curve shows the characteristics of diode for different temperatures in the forward biase. As we can see from the figure given above, that curve moves towards left as we increase the temperature. We know with increase in temperature, conductivity of semiconductors increase. The intrinsic concentration (ni) of the semiconductors is dependent on temperature as given by:
Eg is the energy gap
K is a voltage man constant
A is a constant independent of temperature
When temperature is high, the electrons of the outermost shell take the thermal energy and become free. So conductivity increases with temperature. Hence with increase in temperature, the A-B curve would shift towards left i.e. curve would rise sharply and the breakdown voltage would also decrease with increase in temperature.
A-C curve: This curve shows the characteristics of diode in the reverse biased region till the breakdown voltage for different temperatures. We know ni concentration would increase with increase in temperature and hence minority charges would increase with increase in temperature. The minority charge carriers are also known as thermally generated carriers and the reverse current depends on minority carriers only. Hence as the number of minority charge carriers increase, the reverse current would also increase with temperature as shown in the figure given on the previous page.
The reverse saturation current gets double with every 10 C increase in temperature.
C-D curve: This curve shows the characteristics of a diode in reverse biased region from the breakdown voltage point onwards. As with increase in temperature, loosely bonded electrons are already free and to free the other electrons, it would take more voltage than earlier. Hence breakdown voltage increases with increase in temperature as depicted in the figure shown in the figure given on the previous page..
When the reverse voltage applied across diode becomes greater than the breakdown voltage, then the diode breaks down and very high current starts flowing in the circuit. There are generally two types of breakdowns in a diode:
- Zener breakdown
- Avalanche breakdown
And based on the above classifications of breakdown of diode, we have the two special types of diode as
- Zener Diode
- Avalanche Diode
The difference between the Zener Diode and avalanche Diode is the doping level. The doping level of Zener diode is more than avalanche diode or we can say diodes which have higher doping level undergo Zener breakdown when reverse bias voltage is increased while diodes with lesser doping level undergo Avalanche breakdown.
As we have already mentioned doping level of Zener diode is very high and hence width of depletion region is less. As we know
E = VB / d
VB is the barrier voltage
E is the electric field
d is the depletion width
As doping is high, hence width (d) is less and as barrier voltage varies with doping as stated by the formula:
From the formula we can get that the voltage varies proportional to log of doping and hence the barrier voltage is almost constant.
So from the above discuss we find that Electric field in the depletion region would be large as VB is almost constant and d has decreased. Due to this large electric field, electrons from the outer shell of the atom in the depletion region are expelled out and hence carriers are generated within the depletion region. The high electric field in the depletion region pulls out large number of electrons from the large number of atoms. This leads to large current flow and this type of breakdown is called Zener breakdown.
The diode which have lesser doping undergo avalanche breakdown when high reverse voltage is applied. The lesser doping means the depletion width is large and so electric field within depletion region is not so high. Hence the electric field would not be able to pull out electrons from the outer shell of atoms and breakdown doesn’t occur in depletion region. But as the depletion region is large and hence when the minority charge carriers move through the depletion region, they get accelerated by the electric field and that even for larger time (as distance through which acceleration is provided is large). Hence minority charge carriers acquire high velocity and so high kinetic energy. When these charge carriers strike with atoms in the n-type and p-type regions, the high kinetic energy gets converted to thermal energy and hence due to this energy electrons from the outermost shell are pulled out and large current starts flowing. This type of breakdown is called avalanche breakdown.
But due to the high thermal energy, the temperature rises and diode gets burned. Due to this reason the simple diodes (where avalanche breakdown occurs) is not used in the applications and instead Zener diode is used in the application circuits of breakdown diodes such as regulating power supply.
Differences between Zener breakdown and Avalanche breakdown:
Zener breakdown Avalanche breakdown
- The Zener breakdown occurs in HIGH 1. The avalanche breakdown occurs in LOW doping diodes. doping diodes.
- The breakdown occurs within the 2. The breakdown occurs outside the depletion depletion region. region.
- The breakdown voltage is lesser than 3. The breakdown voltage is more than zener that of avalanche breakdown. breakdown voltage.
As Zener breakdown voltage is less than that of avalanche breakdown voltage, hence Zener breakdown is said to occur before the avalanche breakdown.
Hence we can say if we increase the doping of a diode, the chances of zener breakdown increases and hence breakdown voltage decreases
CHARACTERISTICS OF AN IDEAL DIODE
When we talk about the ideal diode, the diode is a device which acts as a short circuit when forward biased and acts as open circuit when reverse biased. Hence the behavior of ideal diode can be shown in the following graph:
In forward biased, current is zero till the point forward voltage is less than breakdown voltage and after that diode offers no resistance while in the reverse biased, there is no current flow at all.
CHARACTERISTICS OF A PRACTICAL DIODE
When the polarity of the external voltage source is opposite to the above case i.e. when positive terminal is connected to n-side and negative terminal to p-side, then diode is said to be reverse biased as shown below:
When we reverse bias the diode, the majority carriers have again 3 types of forces acting on them as shown below but in this case both forces due to barrier electric field and the external voltage act in the same direction and hence pull the majority carriers away from the junction.
Hence the space charge region or say depletion region is enlarged and minority carriers are attracted to move to the other region crossing the barrier as shown:
But as number of minority carriers is very less and current is proportional to the charge carriers flowing across the barrier, hence current through the diode is less and measured in micro-amperes
When we apply voltage across the diode, as shown below:
When the voltage applied across the diode is greater than the barrier voltage, the electrons and the holes present in the n-type and the p-type regions of the diode acquire enough energy to cross the barrier at the junction.
Now if we talk in terms of forces acting on the charge carriers, then there are 3 kinds of forces acting on both types of carrier as shown:
FE1 is force acting on electrons (in p-type) due to electric field.
FE2 is force acting on holes (in n-type) due to electric field.
FA1 is force acting on electrons (in p-type) due to hole-electron attraction.
FA2 is force acting on holes (in n-type) due to hole-electron attraction.
FV1 is force acting on electrons (in p-type) due to external voltage source.
FV2 is force acting on holes (in n-type) due to external voltage source.
The forces satisfy FE1 + FV1 > FA1 & FE2 + FV2 > FA2 and hence holes & electrons are able to cross the barrier and hence flow of charge carriers continue.
The flow of charges doesn’t stop as long external battery is connected because battery itself generates electrons and holes.
Q- Would there be any current flow when we connect both ends of a diode and can we measure it using multimeter?
Ans: Well when we connect both sides of a diode, it is actually the same case when we connect the n-type and p-type material at the junction. There would be diffusion of charges i.e. holes flowing from p-side to n-side and electrons flowing from n-side to p-side. Hence there would be recombination of electrons and holes and so there be formation of depletion region at the ends too as shown below:
Hence there would be flow of charge carriers for very small time till the barrier voltage is developed. So we’ll have very small amount of current flowing that even for very small time.
But this current cannot be measured or detected using multimeter.