Zener diodes and semiconductor sensors

Holding a rail steady

Each of the three devices here converts a physical quantity into a voltage or a current that the rest of a circuit can use: a zener diode holds a supply rail near its breakdown voltage, a photodiode produces a current proportional to illumination, and a Hall sensor produces a voltage proportional to magnetic flux density.

Run an ordinary diode backwards and it blocks until the reverse voltage grows large enough to destroy it. A zener diode is built to break down gently at a chosen voltage V_Z$V_{Z}$V_Z and to survive the experience indefinitely, so long as its rated current and power are respected. Past breakdown its characteristic is nearly vertical, so the current through it can swing by tens of milliamps while its pd barely moves.

A component whose voltage stays almost constant is what a supply rail needs. So the zener goes across the load, reverse biased, with a series resistor between it and the raw supply. The load gets V_Z$V_{Z}$V_Z, everything left over is dropped across the resistor, and when the supply wanders it is the resistor's share that changes.

The resistor sets the current, and the current is what the design turns on:

Too small a current and the diode never properly reaches breakdown. Too large and it overheats. In the figure the resistor carries 10.0 mA at 9 V in and 25.4 mA at 15 V in, and the zener carries whatever the load leaves.

Light into current

A photodiode is a diode with a window, used reverse biased so that almost nothing flows in the dark. A photon absorbed in the depletion layer frees an electron-hole pair, and the strong field there sweeps the two apart before they can recombine. Every absorbed photon therefore adds to a small reverse current, the photocurrent, and doubling the illumination doubles the pairs freed each second.

Those flat lines carry two messages. Photocurrent is proportional to illumination, so a single constant, the responsivity, describes the device where a curved characteristic would have needed a whole calibration graph. That is not the same as needing no calibration at all. The constant itself still has to be found against a known source, and it depends on the wavelength of the light, differs from one device to the next and drifts with temperature, while the small dark current that flows with no light on the window has to be subtracted as well. Linearity makes the calibration short, not unnecessary.

The second message is that the current hardly depends on the applied pd, so the diode behaves as a current source. Read it by dropping its current across a resistor: 40 μA through 50 kΩ gives 2.0 V, and 60 μA gives 3.0 V.

Against a light-dependent resistor the photodiode wins on speed and on linearity. Carriers in an LDR take milliseconds to build up and longer to disperse, so it cannot follow anything flickering faster than a few hundred hertz, and its resistance is nowhere near proportional to the light. A photodiode responds in nanoseconds, which puts one at the end of every optical fibre. Where the light changes slowly, the cheaper LDR is fine.

Measuring a field, and noticing a magnet

Send a current along a thin slab of semiconductor and put a magnetic field through its face. Each moving carrier feels a force BQv$BQv$BQv at right angles to both, so carriers pile up along one edge and leave the opposite edge short of them. The separated charge builds a field that opposes further pile-up, and matters settle once the two forces balance. What remains is a steady Hall voltage across the faces.

Hold the current constant and that voltage is proportional to the flux density, so the slab is a direct-reading field meter. A Hall probe is exactly that, calibrated. Its reading is largest when the field runs perpendicular to the slab's face, so a probe is rotated for a maximum before the number is taken. The sensor above gives 2.4 mV in 0.20 T and 4.8 mV in 0.40 T, a sensitivity of 12 mV per tesla.