p-n junction question

Discussion in 'Electronic Basics' started by vic, Jun 3, 2007.

1. vicGuest

When various textbooks explain the operation of a p-n junction or a
diode, they usually do not talk about metal contacts. That is usually
a separate topic. But let's consider the whole thing together since
this the only way the diode can operate and let's assume that both
anode and cathode contacts are made from the same metal and therefore
have the same energy level of free electrons.

The energy of an electron flowing through the diode under a forward
bias condition would have to first be elevated from the metal energy
level to the energy level of the conduction band of the N-type
semiconductor, then it would drop when the electron crosses the
junction an falls into a hole in the P-type semiconductor (and
releases thermal or light energy) and then elevated again to the
original metal energy level.

The voltage drop on a diode will be equal to the band gap and the
energy spent to get an electron through the diode will be eaual to the
energy released during the recombination. This energy is needed to
elevate the energy of an electron before it plunges to a hole and,
again, to get it out of a hole back to the metal, but any textbook
would tell you that the voltage drop occurs on the junction itself to
overcome a barrier created there though carrier diffusion before an
external voltage was applied.

As far as I can see there is no barrier to overcome at the junction in
the forward direction - it's a fall, like in a waterfall. But you do
need to overcome a barrier between the metal terminal and the N-type
semiconductor of the cathode to get to that high energy level and
that's where an external energy is spent.

Am I missing something basic here? Does everyone else understand the
story about the depletion region and how a dynamic balance (diffusion
+ drift) existing before an external voltage is applied to a diode
somehow affects the operation of a diode ever after?

2. MarraGuest

In practice all you need know is its 0v7 forward volts drop and it
wont conduct reverse polarity.

I can walk but dont understand how my legs work !

3. Guest

Yep, this is a weird and fascinating topic. Search for info on
"nonrectifying junction", also "ohmic contact"

If manufacturers just stuck some metal contacts on the semiconductor,
they'd form Schottky diodes in series with the main diode (metal/
silicon junctions.) The diode as a whole would always turn off
regardless of polarity.

So instead they apply heavy doping to the semiconductor surface before
adding the metal contacts. This converts the semiconductor surface
into a "metal-like" conductor. In diode diagrams you'll often see a
layer of p+ next to the metal contact on the p-doped layer, and a
layer of n- next to the metal touching the n-doped layer. In that
case there still is an energy shift, but it doesn't present a
barrier. Don't forget that energy shifts are NOT BARRIERS, they are
more like ideal batteries. Only a depletion layer can act as a high-
resistance barrier. ALso don't forget the thermocouple effect: that
whenever you use copper and aluminum or iron wires in the same
circuit, there is an energy shift at the metal junctions. These
mismatched "work functions" don't create a high resistance because the
energy shift around the circuit as a whole is zero. Any energy gain
at one metal-metal junction will be cancelled by an energy loss at
other, opposite metal-metal junctions. It's similar to hooking two
batteries back to back in series: the voltages cancel out, but the
batteries still form a low-resistance conductive path.

But why can we prevent the formation of a metal/semiconductor diode by
adding extra-heavy doping to the semiconductor? It's because of
quantum mechanics: the heavier the doping, the thinner the
insulating depletion layer is formed at any particular reverse
voltage. To form a diode, light doping is required. With heavy
enough doping, the depletion layer becomes so thin that electrons can
"tunnel" quantum-mechanically through this insulating region. The
metal contact becomes like a tunnel diode, but a tunnel diode that
turns fully on at all values of applied voltage.

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty http://staff.washington.edu/wbeaty/
beaty chem.washington.edu Research Engineer
billb eskimo.com UW Chem Dept, Bagley Hall RM74
206-543-6195 Box 351700, Seattle, WA 98195-1700

4. neon

1,325
0
Oct 21, 2006
don't forget if electrons flow this way holes flow opposite.metals are conductors only pn is a semiconductor with controlled inpurity injected to minority holes or electrons

Last edited: Jun 11, 2007