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Diode small signal model

I have some questions about pn junction:

1)Typical small signal model for a pn junction consists of a conductance in parallel with a capacitor conductance=(conductance p-neutral region)+(conductance n-neutral region)+(conductance space charge region) capacitor=(diffusion capacitor n-neutral region)+(diffusion capacitor p-neutral region)+(junction capacitor) I don't understand why these capacitors (and resistrors) that come from three series layers are not in series but in parallel

1)What is the difference between zener diode and tunnel diode? These two diodes are based on the same phenomena but first is used in reverse bias and the second in foward bias,is it right?

2)I've found that is not possible obtain diode breakdown with tunnel phenomena, but a lot of links said the contrary...This is true or not?

Thanks

Reply to
lionelgreenstreet
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That model simulates the conduction through the forward-biased junction in parallel with the capacitor formed by that same junction.

In much the same way as a fork lift and a shopping cart both roll on wheels, yes, they are both based on the same basic phenomena.

Do you mean in the reverse direction? All diodes will break down when reverse biased -- are you working with real diodes, or playing simulator games?

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
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Reply to
Tim Wescott

As i remember, the Esaki diode AKA tunnel diode is totally different, due to the very high level of doping needed to make a "barrier" for ordinary electrons at low electric fields (voltages). Electrons pass thru by way of tunneling, a probabilistic, quantum effect that Einstein did not like. "Diode breakdown"? Oh, you mean reverse breakdown as in zenering. Inthe reverse direction, the tunneling is extremely pronounced, and the V/I curve cannot possibly intercept such a point.

Reply to
Robert Baer

A lot of engineers wonder about this at length when studying p-n junctions.

The easiest answer is that it is congruent to observed behavior, just as the dependent current source in small-signal hybrid-Pi model is congruent with observed behavior in a transistor circuit. Ask yourself what would happen if you modeled the elements as being in series. A forward-bias would result in steady-state current of zero, which would be inconsistent with observed behavior.

[Note that have to pay attention to the depletion capacitance as well as the diffusion capacitance.]

IIRC, one difference is that a Zener diode has monotonic I-V characteristic (dI/dV > 0), whereas tunnel diode has regions of negative incremental resistance (dI/dV < 0), and this can be exploited to achieve what is impossible using "normal" passive components in a "normal" passive circuit: placing small-signal poles on jw axis.

Do you mean normal reverse flow, or thermal runaway? As Tim pointed out, you can destroy pretty much anything with sufficient reverse bias. I once saw a firefighter demonstrate how tree limbs will conduct with sufficient bias in any direction. :)

-Le Chaud Lapin-

Reply to
Le Chaud Lapin

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