B1 · Diodes

Diode I-V: A Valve That Flows One Way

How does the flow change when you put an external voltage across a pn junction? Sweep the voltage from reverse to forward yourself and watch the asymmetry that surges only one way.

Sweep the voltage and trace the curve

Use the slider to change the voltage across the diode. The operating point moves along the curve: nearly flat in reverse, then rising steeply past a certain point in forward.

Applied voltage VV = -0.30 V

Operating-point current I

I ≈ -Is  (≈ 0)

The barrier is even higher. Aside 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 we add an external voltage on top. Put positive on the p-side and negative on the n-side (forward) and the external voltage shaves the internal barrier down; reverse it and the barrier grows taller. What sets the flow is, in the end, the height of the barrier the carriers must climb.

Forward: current rises exponentially

The number of carriers that clear the barrier hangs exponentially on the barrier height (the same form as the Boltzmann factor of A1). So even a small drop in the barrier from a forward voltage makes the current grow explosively. In a silicon diode the curve bends like a knee around 0.7 V and shoots up. 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 ≈ ?

In deep reverse the exponential vanishes, leaving -Is.

On your ownV >> VT → I ≈ ?

In forward the exponential dominates and surges.

Reverse and rectification

In reverse the barrier grows taller, so majority carriers can barely cross and only a very small saturation current -Is flows. The diode is thus a one-way valve: large current in forward, near zero in reverse. Apply an alternating voltage and it passes only the positive half-cycles, turning AC into pulsating DC. This is rectification, the first stage of every power adapter.

Back to the first screen

When you left the slider in reverse the curve was flat, and as you pushed it forward it shot up steeply past a knee. Same device, yet one direction of voltage completely changes the flow. The root of that asymmetry is the barrier of A4. The external voltage only shaves or raises the barrier, and because the number of carriers clearing it responds exponentially, the curve surges only one way. This one-way valve is the core of rectification, which turns AC into DC.

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

What comes next

Now you hold a one-way valve. The next unit shapes signals with it: clipping, which cuts off the part beyond a set voltage, and clamping, which shifts the whole waveform baseline. The on and off of the same single diode becomes a tool for processing signals.