# Semiconductors and Transistors

Computer Science I · Physics · https://tryals.app/learn/computer-science-i/semiconductors-and-transistors

## From Physics to Logic

Every digital logic gate is built from **transistors**, microscopic electronic switches made from **semiconductors**. Materials fall into three electrical classes:

| Class | Behavior | Example |
| --- | --- | --- |
| **Conductors** | Charge flows freely | Metals |
| **Insulators** | Charge barely moves | Glass |
| **Semiconductors** | Conductivity is controllable | **Silicon** |

Pure silicon conducts poorly, but its conductivity is tuned via **doping** (adding impurity atoms):

- **n-type**: dopants adding extra mobile **electrons** (negative carriers).
- **p-type**: dopants creating **holes**, missing electrons that act as mobile **positive** carriers.

Joining p-type and n-type silicon creates a **p-n junction (diode)**, which conducts electricity in only **one** direction.

## The Power of the Switch

A **transistor** combines these junctions. Its central trick: a small voltage on a **control terminal (gate)** switches a much larger current between the other two terminals **on or off**. 

This voltage-controlled switching maps directly to binary logic: 1 is **on**, 0 is **off**. Billions of these tiny silicon switches form the gates, adders, and processors of modern computers, scaling via **Moore's law**.

> **Common pitfall:** Thinking a transistor is a passive resistor or wire. Its defining feature is that a **small** signal on its **gate** controls a **large** current, acting as an active **switch/amplifier**. A plain resistor cannot do this.

## Practice questions

5 of this lesson's 13 practice questions, with answers. The full set is in the app.

### 1. Continuing the same doubling every two years from 1 million transistors at year 0, how many million transistors would a chip hold at year 10?

**Answer:** 32

**Why:** Five doublings multiply the count by 32, giving 32 million. Miscounting the doublings is the usual error: it is the number of two-year periods that matters, not the number of years.

Page: https://tryals.app/practice/computer-science-i/semiconductors-and-transistors/continuing-the-same-doubling-every-two-years-from-1-million

### 2. Transistors are approaching a few nanometres, where electrons tunnel through barriers meant to block them. Why is that a fundamental obstacle rather than a manufacturing one?

A. Because silicon crystals become unstable below a few nanometres
B. Because heat dissipation becomes impossible at that density
C. Because tunnelling is a property of matter, not a defect
D. Because lithography cannot resolve features that small

**Answer:** C. Because tunnelling is a property of matter, not a defect

**Why:** Tunnelling is what electrons do at those scales, so a flawless barrier still leaks and the current is wasted as heat. It is the physics setting the limit rather than the process, which is why the industry moved to FinFET and gate-all-around geometries, changing the shape rather than continuing to shrink.

Page: https://tryals.app/practice/computer-science-i/semiconductors-and-transistors/transistors-are-approaching-a-few-nanometres-where-electrons-tunnel

### 3. Complete the description of what makes a transistor useful for computing.

**Answer:** A transistor computes because **a small voltage on its gate switches a much larger current on or off**.

**Why:** Control is the whole point: a small gate signal decides whether a large current flows, which is precisely a switch, and a switch gives you 1 and 0. Rectifying alternating current is what a diode does, and it cannot compute.

Page: https://tryals.app/practice/computer-science-i/semiconductors-and-transistors/complete-the-description-of-what-makes-a-transistor-useful-for

### 4. Select every statement that is TRUE of semiconductors and transistors.

A. p-type doping supplies holes that act as positive carriers
B. A p-n junction conducts equally well in both directions
C. A transistor is a voltage-controlled switch
D. n-type doping supplies extra mobile electrons
E. Silicon is a semiconductor whose conductivity can be tuned by doping

**Answer:** A. p-type doping supplies holes that act as positive carriers; C. A transistor is a voltage-controlled switch; D. n-type doping supplies extra mobile electrons; E. Silicon is a semiconductor whose conductivity can be tuned by doping

**Why:** Doping, the two carrier types and the controlled switch are all exactly as taught. A p-n junction conducts in one direction only, and that asymmetry is the entire reason diodes exist as rectifiers and LEDs.

Page: https://tryals.app/practice/computer-science-i/semiconductors-and-transistors/select-every-statement-that-is-true-of-semiconductors-and-transistors

### 5. Doping adds impurity atoms to silicon to supply mobile charge carriers. Sort each description.

**Answer:**

- n-type: Extra mobile electrons are supplied, The majority carriers are negative
- p-type: Holes act as mobile positive carriers, The majority carriers are absences of electrons

**Why:** n-type supplies negative carriers, p-type supplies positive holes, and joining the two makes the p-n junction that conducts in only one direction. The names encode the carrier's sign, which is the fastest way to keep them straight.

Page: https://tryals.app/practice/computer-science-i/semiconductors-and-transistors/doping-adds-impurity-atoms-to-silicon-to-supply-mobile-charge