Diode Intro

A diode is like a one way valve:

• No flow until you push hard enough, then it flows freely
• Blocks reverse flow…until it breaks
• Like a check valve

How do they work? Diodes are made through Semiconductor Doping, which intentionally introduces impurities into a semiconductor.

Exponential Current Relationship

The current through a diode, $I_D$, can be written as:

$$I_D=I_S\left(e^{\frac{V_D}{V_T}}-1\right)\approx I_S{e}^{\frac{V_D}{V_T}}$$

$I_D$ is a constant that depends on the the semiconductor doping, and tends to be a very small value.

The key part is $e^{\frac{V_D}{V_T}}$, where $V_D$ is the diode voltage and $V_T$ is the thermal voltage (average voltage of particles given a certain temperature, around $25\text{ mV}$ at room temperature). The takeaway here is that diodes are very temperature sensitive.

Effects of $V_D$ on $I_D$:

• What happens to $I_D$ when $V_D$ changes to some $V_D+\Delta V_D$? Big changes!
• What is required of $V_D$ to cause a “normal sized” $\Delta I_D$ ? Need a very small $\Delta V_D$

Due to the above sensitivity, we will assume $V_D$ is constant near an operating point.

Note on terminology:

• $V_D$ is the voltage across the diode
• $V_F$ is the forward voltage required to turn on the diode (see Diode Models)