Discrete semiconductor devices

A gate that takes no current

A MOSFET has three terminals. The load current flows between the drain and the source, and the gate decides whether it flows at all. The gate sits on a thin insulating layer of oxide, out of contact with the silicon beneath, so it behaves as a very small capacitor rather than as a way in. Put a voltage on it and its field reaches through the oxide and draws carriers into a conducting channel underneath.

Nothing at all happens until the gate-source voltage passes the threshold voltage V_T$V_{T}$V_T. Below it there is no channel, and the drain current is zero however large the drain voltage. Past it the channel opens and widens, and the current climbs steeply, roughly with the square of the excess above threshold.

Switching a load

That shape of characteristic makes a superb switch. Wire the load in series with the drain and drive the gate from a logic output. At 0 V the channel is shut, the drain-source resistance is megohms, and the load sees microvolts. At 5 V the channel is fully open, its resistance falls to a fraction of an ohm, and almost the whole supply lands on the load.

Work the on state as a potential divider. Lamp and channel carry 12/24.05 = 0.499 A between them, the lamp keeps 11.98 V, and the MOSFET dissipates I²R$I^{2}R$I^2R = 0.012 W against nearly 6.0 W in the lamp. A component wasting a hundredth of a watt to control six of them needs no heat sink.

The opening claim dies here. The gate is insulated, so no steady current enters it. A logic output has only to charge a gate capacitance of a nanofarad or two, which it does in microseconds, and in between it supplies nothing at all.

Holding a rail steady

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. 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 refuses to change is what a supply rail wants. 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 the whole of the design:

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 swallows 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 the device is a linear light meter needing no calibration curve. And it 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.