# Transistors

- Total cards: 16

Diodes, BJTs, MOSFETs, Dennard scaling, and why silicon won.

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## Diode and BJT mechanism

### Q1. What is a diode?

A two-terminal semiconductor that allows current to flow one way but not the other.

### Q2. Does a diode act as a conductor or insulator when conventional current flows from $\text{battery} \to \text{P} \to \text{N} \to \text{battery}$? Why?

Insulator.

Electrons saturate the holes in P, leaving no differential for the electrons in N to jump towards. Meanwhile, the extra electrons in N flow back to the battery.

### Q3. Does a diode act as a conductor or insulator when conventional current flows from $\text{battery} \to \text{N} \to \text{P} \to \text{battery}$? Why?

Conductor.

Electrons saturate N, push into the depletion region, then jump across to the holes in P, which then return to the battery.

### Q4. In a BJT transistor, if a small positive current is allowed to flow from base to emitter, what is allowed to happen?

A large positive current is allowed to flow from collector to emitter.

![BJT control flow](/images/transistors/bjt-current-flow.png)

*Image from Ben Eater's transistor video.*

### Q5. If electrons are flowing from emitter to base (i.e. you turn the transistor on), why does that enable those electrons to now reach the collector?

You're allowing electrons from the emitter to get all the way up into the base, so close that they can jump across the depletion region between base and collector.

![NPN BJT electron flow: emitter → base → collector](/images/transistors/bjt-electron-flow.png)

*Image from Ben Eater's transistor video.*

## MOSFET vs BJT

### Q1. Sketch in your head what a MOSFET looks like.

Top to bottom: gate (metal) on top of an SiO₂ insulator, sitting on a P-type substrate. Two N-doped wells flank the gate on the left and right — the source and the drain. When the gate is positive, a channel of electrons forms in the substrate just under the insulator, connecting source to drain.

![NMOS cross-section](/images/transistors/mosfet-cross-section.png)

### Q2. Why does a positive charge at the gate open up the channel underneath the insulator? (Consider an NMOS.)

It pulls up the electrons in the P-type substrate, filling up the holes at the top — the channel.

### Q3. Why is a MOSFET more power efficient than a bipolar junction transistor?

In a MOSFET, the input is a voltage you set. In a BJT, it's a current you must keep feeding.

### Q4. Conceptually, why doesn't it require a continuous current to keep the MOSFET channel between source and drain open?

Thanks to the insulator between the gate and the channel, there's no power dissipated. The channel is powered just by voltage (basically potential energy).

### Q5. What is the only time you have to pay an energy cost in a MOSFET?

When you turn the transistor on or off (i.e. when the gate voltage changes).

### Q6. In a MOSFET, you can think of the positive gate / SiO₂ insulator / negative channel triplet as what kind of component?

A capacitor.

## Dennard scaling

### Q1. What is Dennard scaling?

As transistors shrink, their power density stays constant, so total power usage stays in proportion with chip area.

### Q2. How did MOSFETs enable Dennard scaling?

If you shrink both the voltage and the oxide thickness, the transistor keeps working the same way.

## Since 1947

### Q1. The original 1947 transistor was made of germanium, which also has 4 valence electrons like silicon. Why can't germanium be used to build MOSFETs?

Its native oxide (GeO₂) is water-soluble and electrically defective. MOSFETs need a thin, stable, near-perfect insulator on the channel — only silicon grows one (SiO₂).

### Q2. At a high level, what is the difference between planar, FinFET, and gate-all-around?

- Planar: gate sits flat atop the source-to-drain channel.
- FinFET: gate drapes the channel from three sides.
- Gate-all-around: gate fully wraps the channel from all sides.

### Q3. MOSFET technology has progressed from planar (1965–2011) to FinFET (2011–2022) to gate-all-around (2022 onwards). Why?

Better gate control means you can shrink the source-to-drain distance (the "gate length") without source/drain fields leaking current through.