# Homework Help: Transient nature of diode.

1. Mar 29, 2013

### hhi.rabbit

A pn junction diode is driven by a voltage +E to -E volts preodically
when voltage is +E its forward biases and vice versa.
when in forward biased a current I flows through it, now if voltage is reversed to -E, current is still I (in reverse direction). This then stay constant for a time (storage delay time) and then falls of exponentially.
now my question is
is this transient nature of diode due to diffusion capacitance or due to junction capacitance of diode

2. Mar 29, 2013

### rude man

Neither.

It's due to minority carrier lifetime. Although there is charge stored when the diode is forward-biased, the reverse-biased current I_r does not behave like a discharging capacitor. Rather, it is almost constant and = Q_s/T_s where Q_s is the charge stored during forward bias and T_s ("storage time") is the time I_r flows before more or less abruptly stopping. So we have

forward bias: Q_s = I_forward * T_mcl where T_mcl is the minority carrier lifetime.
reverse bias: Q_s = I_r * T_s.

3. Mar 30, 2013

### hhi.rabbit

thnks a lot man... :D

also plz can u tell me any role of capacitance in diode..
i mean when they come into play or when thier effect is seen...

4. Mar 30, 2013

### rude man

Big question. Obviously, in any circuit affected by capacitance, the (reverse-biased) diode capacitance has to be considered.

One interesting example is the varactor diode, where the capacitance, being a function of reverse bias voltage, is used to tune an L-C or active circuit by varying the bias voltage.

5. Mar 30, 2013

### hhi.rabbit

ok thank u

6. Mar 30, 2013

### AugustCrawl

The whole minority/majority arguments with their holes & electrons has always seemed rather mysterious to me.

The first thing I can add to this is to refer you to shockley's equation:
http://en.wikipedia.org/wiki/P–n_diode

The other things I can point you to is to have a look at the arguments that I didn't understand which explain when conduction occurs in terms of excitation of holes (which need energy) to cross something called a band gap.

I think there is some distribution in terms of carriers which could be gaussian & the average energy of these carriers is called the Fermi-Level.

http://en.wikipedia.org/wiki/Fermi_level

When you bias the diode in different ways I think the holes & electrons move around to widen & decrease the bandgap, which is significant for reasons I do not fully get.

Somehow I passed an introductory module in Semiconductor Materials without fully grasping the intricacies of crystal structures, pn- junctions, bjts & mosfets - so I cant give you a clear answer. But what I can tell you with certainty is that understanding the arguments in terms of pn junctions is the first step in terms of an understanding arguments that explain the operation of all of these devices.

Our handouts were based on a simplification of some of this book:
http://www.amazon.co.uk/Physics-Semiconductor-Devices-Simon-Sze/dp/0471143235

I found the book impenetrable myself, but I think it might be a place to look for more answers.

7. Mar 31, 2013

### hhi.rabbit

I have just started studying about semiconductors.
Im really confused about all these things.
and i am trying very hard to get basics of junction
will try to get SM Sze from library..
thanks