--- title: "Hardware diode" date: "2024-07-18" created: 2024-07-18T11:01+10:00 updated: 2024-07-18T11:01+10:00 tags: - power-electronics aliases: permalink: hardware-diode description: image: --- # Hardware diode ## Basic Structure - A [PN junction](../../Concepts/Microelectronics/PN%20Junction.md) - Symbol is the shape of an arrow which points in the direction it conducts conventional current in (positive charges) - When forward biased, current flows from anode to cathode - [^1] ### VI Characteristic - As voltage increases, we reach a threshold where the current starts to conduct and current rises - this threshold V is typically 0.7-1V for a silicon diode - [^2] - 1/Ron is steep - alot of currrent increase for a small increase in voltage - When we assume the diode is ideal - voltage drop across the diode is 0V - When a reverse voltage is applied (from cathode to anode) - no current flow in steady state ### Power Diode Physical Structure - Current tends to flow through the thickness of the wafer - different to [diode connected transistor](../../Concepts/Microelectronics/diode%20connected%20transistor.md) in microelectronics where current flows across the plane of the device rather than through it - three sections: - P+ - N- epi (epi means "on top of" - literally grown on top of the substrate) - N+ substrate - [^3] - the [PN junction](../../Concepts/Microelectronics/PN%20Junction.md) blocks/allows current flow - the depletion region occurs mainly in the N- epi region since it is less doped - there is a relationship between donor density and acceptor densities in P+ -  %%[🖋 Edit in Excalidraw](../../Public%20Extras/Doodles/Hardware%20Diode-2024-07-18-drawing.md)%% - When we apply a reverse bias voltage we attract all the free charge carriers out of that region - this area acts like a dielectric with a central insulator (no free charge carriers) - looks like a capacitance - %%[🖋 Edit in Excalidraw](../../Public%20Extras/Doodles/Hardware%20Diode-2024-07-18-drawing_0.md)%% - In [power electronics](../../Spaces/University/Thesis/notes/Power%20Electronics%201.md), we want a high breakdown voltage (increases voltage capability) - The depletion region extends into the N-epi depending on the reverse breakdown voltage - depth of depletion region increase is almost linear - for bigger reverse breakdown voltage capability, we need to increase the N- epi thickness - the current has to flow through more semiconductor from anode to cathode - this in turn increases the on state voltage drop (forward biasing voltage) ## Exceeding the Reverse Breakdown Voltage - If exceed breakdown voltage, can't usually go back to normal (unless it is a zener diode) - N- epi - the edge of the space charge region (free of charge) extends into substrate - punch through effect - allows diode current to flow from cathode to anode - called avalanche current, generates heat and generally destroys the device - typically no recovery from a reverse breakdown voltage being exceeded - semiconductors are very intolerant to exceeding reverse breakdown voltage ## Reverse Recovery - When a reverse bias is applied, a depletion region forms - this looks like a capacitor with capacitance $C=\frac{\epsilon_0\epsilon_r A}{d}$ - major challenge is moving from forward biased to reverse biased we have to establish [^1]: https://media.geeksforgeeks.org/wp-content/uploads/20230210133728/Diode-and-Zener-Diode-1.png [^2]: https://homework.study.com/cimages/multimages/16/image35800388703943066704.png [^3]: https://www.engineersgarage.com/wp-content/uploads/2021/07/TCH28-04.png