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Everyone knows that temperature affects every semiconductor device you can think of, and diodes are no exception.

From the below graph, we can see that the temperature has marked effects on pn junction diode and zener diode.

PN Junction diode enter image description here

Zener Diode enter image description here Source : Concept electronics

Now we can see that in an PN junction diode,in the forward bias region increase in temperature causes decrease in the cut-off voltage,whereas in the reverse bias the opposite thing happens.

I know that these can modeled using the diode-current equation. But conceptually I can't understand what happens to the charges with change in temperature which causes these change in the characteristics.

I also can't understand why the zener diode behaves differently from that of the junction diode in the reverse bias region.Please explain me this too conceptually.

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  • \$\begingroup\$ Temperature affect the intrinsic property of material. When temperature rise, the internal resistance of material rise because the atom have more kinetic energy therefore, electrons ( current) got more difficulty to flow. That explain the change of function in the most part because there is a resistance internally in a diode. \$\endgroup\$ – MathieuL Jul 27 '15 at 15:44
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Semiconductors work in general because thermal energy lifts some number of electrons from their "ground states", where they are bound to a particular nucleus, into the conduction band, where they are free to move about. The number of electrons in the conduction band is a strong function of temperature, but it is also a function of the relative doping levels in the various parts of a semiconductor device.

The relative levels of conduction-band population is what determines the electrical characteristics of the device. Whether one population rises faster or slower than another with respect to temperature can make the difference between having a positive or negative temperature coefficient in the electrical characteristics.

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  • \$\begingroup\$ en.wikipedia.org/wiki/Band_gap \$\endgroup\$ – JonRB Jul 27 '15 at 16:39
  • \$\begingroup\$ Seemed simpler to add a comment to compliment rather than a new reply. \$\endgroup\$ – JonRB Jul 27 '15 at 17:07
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Ok, there are many things at play here. Let me just quickly define a few things for you. I'm assuming some background knowledge since you mentioned the Shockley equation.

A diode is formed by joining a piece of n-type and p-type semiconductor. This leads to diffusion of electrons and holes which creates a current. As a result, the space charge region (SCR) is formed. The space charge region creates an electric field that creates a drift current that cancels off the diffusion current. Hence at thermal equilibrium, there is no current.

A zener diode depends on quantum tunneling. This means that the breakdown voltage is achieved once the p-region valence band edge is raised above the n-region conduction band edge. This allows the electrons in the p-type valence band to tunnel to the conduction band of the n-type region. This creates a current.

An avalanche diode (that's not a regular diode, its an avalanche diode), depends on the avalanche effect. When the SCR field exceeds a certain amount (known as critical field), electrons get accelerated to very high speeds and start knocking other electrons into the conduction band. This creates a huge current. Notice the difference in operating principle between the zener and the avalanche diodes.

Ok, now to tackle the questions.

This analysis is simplified but should be good enough. In a regular diode, when you raise the temperature, the carrier concentrations rise greatly. This affects diffusion current only minimally as the rise is around the same on both sides so we can approximate diffusion current to be constant for small increases in temperature. Drift current increases proportional to the carrier concentrations however and so drift current increases greatly. This means that a smaller electric field is required in the SCR to offset the diffusion current. Due to this smaller electric field, the turn-on voltage of the diode decreases.

In a zener diode, when you raise the temperature, the energy of electrons increases. Consequently, the tunnelling probability increases and the reverse breakdown voltage drops. (not entirely sure of this but seems plausible)

In an avalanche diode, when the temperature is higher, the built in field drops as per the previous explanation. Hence, a larger applied voltage is needed to reach the critical field and so breakdown voltage increases.

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because they are made of semiconductor materials. In low temperature a semiconductor does not conduct current but as the temperature rises its conductivity also rises. That's where the names come from, not conductor not insulator but a middle thing called semiconductor.

In other words, when you increase the temperature of a material, the movement of the electrons inside the material also increases, which results more possibility for conductivity.

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Forward Bias of PN Junction Diode:

As you increase the temperature, the intrinsic carrier concentration increases. This pushes the fermi level closer to the intrinsic fermi level (the middle of the band gap). Since the built-in potential of a diode is determined by the difference in fermi-levels in the p-type and n-type regions, the fermi level in each region moves closer to the middle of the gap, and the built-on potential is decreased.

Reverse Bias:

Intrinsic concentration would increase with increase in temperature and hence minority charges would increase with increase in temperature.The reverse current depends on minority carriers. Hence as the number of minority charge carriers increase, the reverse current would also increase with temperature

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  • \$\begingroup\$ What's the source of your citations? Citations should be attributed. \$\endgroup\$ – Lorenzo Donati supports Monica Jul 27 '15 at 16:39

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