B1 · Diodes

Diode I-V: A Valve That Flows One Way

How does the flow change when you apply an external voltage to a pn junction? In this unit you sweep the voltage from reverse to forward yourself and see the asymmetry, with current exploding in one direction only.

Sweep the voltage and follow the curve

Change the voltage across the diode with the slider and the operating point moves along the curve, with the vertical guide line marking the voltage at that moment. In the reverse region on the left the curve is almost flat, and in the forward region on the right it shoots up steeply past a certain point. Find for yourself where that bending knee is.

Applied voltage VV = -0.30 V

Operating-point current I

I ≈ -Is  (≈ 0)

The barrier has risen further. Apart from a tiny saturation current -Is, almost nothing flows.

Reverse · blocked

An external voltage pushes the barrier

In A4 the junction built its own internal barrier. Now an external voltage is added: positive on the p side and negative on the n side (forward bias) and the external voltage shaves the internal barrier down; connect it the other way (reverse bias) and the barrier rises further. In the end, what decides the flow is the height of the barrier the carriers must climb.

Forward: current rises exponentially

The number of carriers that get over the barrier depends exponentially on its height (the same form as the Boltzmann factor in A1). So even a small forward voltage that lowers the barrier slightly makes the current grow explosively, and in a silicon diode the curve bends like a knee around 0.7 V and shoots up steeply. This is the exp(V/VT) term of the Shockley equation.

ObserveI = Is (exp(V/VT) - 1)

The Shockley equation holds the whole diode current.

ChooseVT = ? (≈ 26 mV)

The thermal voltage is kT/q, about 26 mV at room temperature.

Fill inV << 0 → I ≈ ?

Deep in reverse, the exponential vanishes and only -Is remains.

On your ownV >> VT → I ≈ ?

In forward the exponential dominates and surges.

Reverse and rectification

In reverse bias the barrier rises further, so majority carriers can barely cross and only a tiny saturation current -Is flows. So a diode is a one-way valve that passes a large current in the forward direction and nearly 0 in reverse. Apply AC and it passes only the positive half-cycles, turning AC into pulsating DC; this is rectification, the first step in which a typical power adapter turns AC into DC. Apply too large a reverse voltage and at some point current suddenly flows, a breakdown covered in the Zener diode unit.

Back to the first screen

With the slider in reverse the curve was flat, and as you pushed it forward it passed the knee and shot up steeply. The same device, yet one change in the direction of the voltage changed the flow completely. The root of that asymmetry is the barrier of A4. The external voltage only shaves the barrier down or raises it, but because the number of carriers that cross responds exponentially, the curve explodes in one direction only. This one-way valve is the heart of rectification, which turns AC into DC.

A diode is a pn junction with an external voltage applied, acting as a one-way valve. Forward bias lowers the internal barrier and the current explodes exponentially; reverse bias raises the barrier and only a tiny saturation current -Is flows. This one-way asymmetry is rectification, which turns AC into pulsating DC. The Shockley equation is I = Is (exp(V/VT) - 1), with thermal voltage VT = kT/q.

What comes next

Now you have a one-way valve in hand. In the next unit you use it to shape signals: clipping, which cuts off the part above a set voltage, and clamping, which shifts the baseline of the whole waveform. Just one diode switching on and off becomes a tool for shaping signals.