10.1 Diode characteristics and models

Diode Biasing

P-N Junction and Biasing

A P-N junction forms when a P-type semiconductor region sits in direct contact with an N-type region. At the boundary, a depletion region develops where mobile carriers have diffused away, creating a built-in potential barrier that opposes further current flow.

How you apply voltage across this junction determines whether the diode conducts:

• Forward bias means connecting the positive terminal to the P-type side and the negative terminal to the N-type side. This shrinks the depletion region and lowers the potential barrier, letting majority carriers cross the junction: holes flow from P to N, electrons from N to P.
• Reverse bias is the opposite: negative terminal to P-type, positive to N-type. This widens the depletion region and raises the barrier, blocking majority carrier flow.

Breakdown voltage is the reverse voltage at which the diode suddenly starts conducting heavily in reverse. Two mechanisms cause this: avalanche breakdown (carriers gain enough energy to knock loose more carriers in a chain reaction) and Zener breakdown (the electric field across the junction becomes strong enough to pull electrons directly out of their bonds). If current isn't limited during breakdown, the diode can be permanently damaged.

Diode Current Flow

In forward bias, almost no current flows until the applied voltage exceeds the threshold voltage (around 0.6–0.7 V for silicon diodes). Once past that threshold, current increases exponentially with voltage. A small increase in voltage produces a large increase in current.

In reverse bias, only a tiny reverse saturation current flows. This stays roughly constant across a wide range of reverse voltages. Once the breakdown voltage is reached, reverse current shoots up dramatically.

Diode Models

Real diodes are nonlinear devices, so engineers use simplified models to make circuit analysis practical. The two models you need to know trade off simplicity against accuracy.

Ideal Diode Model

This model treats the diode as a perfect switch:

• Forward bias: The diode is a short circuit (zero voltage drop, zero resistance).
• Reverse bias: The diode is an open circuit (infinite resistance, no current).

It's the simplest model and works well for understanding the basic function of rectifiers, clippers, and clampers. The tradeoff is that it ignores the real voltage drop across a conducting diode, so your calculated voltages will be off by roughly 0.7 V.

Constant Voltage Drop Model

This model adds one layer of realism: when the diode is forward biased and conducting, it maintains a fixed voltage drop of about 0.7 V (for silicon). You can think of it as an ideal diode in series with a 0.7 V battery.

• Forward bias: The diode conducts, but 0.7 V always appears across it regardless of how much current flows.
• Reverse bias: The diode is still an open circuit, same as the ideal model.

This is the model you'll use most often in intro-level circuit analysis. It's accurate enough for most hand calculations and accounts for the voltage "lost" across each diode in a circuit.

Diode Characteristics

I-V Characteristic Curve

The I-V curve is a graph of diode current (vertical axis) versus voltage across the diode (horizontal axis). It captures the diode's full behavior in one picture.

• Forward bias region: Current stays near zero until the voltage approaches the knee voltage (about 0.6–0.7 V for silicon). Past the knee, current rises exponentially. The Shockley diode equation describes this relationship: $I = I_S \left( e^{V / nV_T} - 1 \right)$, where $I_S$ is the reverse saturation current, $n$ is the ideality factor (typically 1–2), and $V_T$ is the thermal voltage (about 26 mV at room temperature).
• Reverse bias region: A small, nearly constant reverse saturation current flows. The curve is almost flat here.
• Breakdown region: At the breakdown voltage, the curve drops sharply downward as reverse current increases rapidly.

Reverse Saturation Current

The reverse saturation current ($I_S$) is the small leakage current that flows when the diode is reverse biased. For silicon diodes, it's typically in the nanoampere (nA) range.

This current exists because minority carriers (holes in the N-type region, electrons in the P-type region) still diffuse across the junction even when the diode is reverse biased.

Temperature has a strong effect on $I_S$: it roughly doubles for every 10°C rise in temperature, because higher temperatures generate more minority carriers. This is why diode circuits can behave differently in hot environments, and it's something to watch for in high-temperature applications.