15 Semiconductors: Doping and Diode Behaviour

15.1 Start With the Decision

A diode works because added atoms shape which charge carriers can move. That material change creates the junction and its one-way behaviour.

15.2 Route Overview

This is part 1 of 2. Continue with Semiconductors: Fabrication and Switching Devices.

15.3 Part Objectives

• Explain n-type and p-type doping with charge carriers.
• Relate junction voltage to diode current and use.

15.4 Start Simple

Choose the Switch From the Real Load

A microcontroller is a small computer built for direct control. General Purpose Input Output (GPIO) is a pin it can read or drive. Picture that pin turning a motor on through an electronic switch. The pin can signal the change, but it cannot safely supply the motor current.

Start with the load voltage, running current, start-up current, direction, and heat limit. Then choose a diode or switch that is fully on at the actual control voltage. Add a safe path for stored energy when a coil turns off.

Measure the control pin, voltage across the switch, load current, and part temperature. Test start, stop, a stalled load, reversed supply, and a reset. A part name or headline rating is not proof that it will be safe in this circuit.

The simple one-way picture has limits. Real parts leak, warm, switch over time, and have exact rated conditions. The deeper sections explain material, junctions, and device curves so the practical choice stays tied to measured limits.

Imagine current allowed through a part in one direction but blocked in the other. Doping creates the P-N junction that makes that behavior useful for rectifiers, clamps, LEDs, and protection paths. Start with the direction of current, the voltage needed to turn the junction on, and the reverse condition the circuit must survive.

15.5 Learning Objectives

• Distinguish N-type (excess electrons) from P-type (holes) semiconductors and explain how doping concentration determines conductivity
• Explain PN junction behavior in forward bias (depletion narrows, current flows) and reverse bias (depletion widens, current blocked) using carrier physics
• Contrast BJT (current-controlled, continuous base drive) against MOSFET (voltage-controlled, near-zero gate current) switching behavior for IoT load control
• Select logic-level MOSFETs with Vgs(th) below your MCU GPIO voltage (3.3V or 5V) and verify Rds(on) at that gate voltage
• Calculate MOSFET power dissipation using P = I²×Rds(on) at actual gate voltage and determine if heatsinking is required

15.6 Why This Chapter Matters

Follow these connections in order: This is the chapter that explains why modern electronics work at all. Without it, diodes and transistors can feel like magic parts with arbitrary rules. In real projects, this knowledge shows up whenever you choose a protection diode, interpret an LED forward voltage, or decide whether a MOSFET will really turn on from a 3.3 V GPIO pin. You do not need to become a semiconductor physicist, but you do need enough mental model to stop copying circuits blindly.

15.7 How To Use This Chapter

• Read it in two passes if needed. First understand the big ideas: doping creates controllable materials, PN junctions create one-way behavior, and that one-way behavior becomes useful devices.
• Use the deeper formulas and carrier-physics details as support, not as the first thing to memorize.
• As you read, keep asking: how does this explain a real IoT part I already know, such as an LED, a reverse-polarity diode, or a transistor switch.

15.8 Semiconductor Foundations

Stop one on the roadmap: the raw material. Before anything can conduct on command, we need a material that can be persuaded to conduct — and pure silicon, on its own, is surprisingly bad at the job.

15.8.1 Pure Semiconductors

Common Materials:

• Silicon (Si) - 95% of all semiconductors
• Germanium (Ge) - Historical use, now rare
• Gallium Arsenide (GaAs) - High-frequency applications

Pure semiconductors aren’t very useful because they don’t conduct well at room temperature.

Atomic Structure

The next selection should be grounded in Figure 15.1. Look for the labels that establish simplified silicon shell model.

In the visual Figure 15.1, Simplified Silicon Atom establishes one fact and 2nd shell another; Z = 14 connects them to the operating case. The diagram is showing simplified silicon shell model. That connection advances the section’s running technical explanation.

A simplified silicon shell model helps explain why four outer-shell electrons matter for bonding and why donor or acceptor dopants change the available charge carriers.

Deep dive: Putting Numbers to It

Doping concentration determines semiconductor conductivity. Pure silicon has ~$5 \times 10^{22}$ atoms/cm³ but only ~$1.5 \times 10^{10}$ free carriers/cm³ at room temperature (300K).

N-type doping with phosphorus (1 in 10⁶ atoms):

$n = \frac{5 \times 10^{22}}{10^6} = 5 \times 10^{16} \text{ free electrons/cm}^3$

This increases free carrier concentration by a factor of ~$\frac{5 \times 10^{16}}{1.5 \times 10^{10}} \approx 3 \times 10^{6}$, roughly 3 million times more than intrinsic silicon!

Carrier mobility determines switching speed. In silicon at 300K:

• Electron mobility: $\mu_n = 1400 \text{ cm}^2/(V \cdot s)$
• Hole mobility: $\mu_p = 450 \text{ cm}^2/(V \cdot s)$

N-type semiconductors switch 3× faster than P-type because electrons have higher mobility — this is why NMOS transistors are preferred for high-speed digital logic.

Depletion width in a PN junction with 1V reverse bias (the potential that matters is the total across the junction, $V = V_{bi} + V_R \approx 0.7 + 1 = 1.7$ V):

$W = \sqrt{\frac{2\epsilon V}{q} \left(\frac{1}{N_A} + \frac{1}{N_D}\right)} \approx 0.1\mu m \text{ to } 1\mu m$

This microscopic insulating region prevents current flow in reverse bias, enabling diode rectification.

Doping Calculator

Adjust the doping ratio to see how many free carriers are created and the conductivity improvement factor over intrinsic silicon.

15.8.2 Doping: Creating Useful Semiconductors

We just saw that pure silicon barely conducts. The fix is deliberate contamination — and it is the single most important trick in all of electronics.

Doping = Adding impurities (dopants) to semiconductors to control conductivity

Inspect Figure 15.2 with Each silicon atom shares 4 electrons and Conducts poorly until doped in mind. Look at how it presents semiconductor Doping Process: Creating N-Type and P-Type Materials before applying doping: creating useful semiconductors.

Follow the diagram in Figure 15.2 from Each silicon atom shares 4 electrons under Semiconductor Doping Process. That stable lattice explains the neighbouring statement Conducts poorly until doped; donor and acceptor additions then create majority carriers without turning silicon into an ordinary metal. The progression supplies the material basis for the diode and transistor behaviour that follows, including why polarity and junction bias matter.

Checkpoint: Doping

You now know:

• Silicon dominates real devices (95% of all semiconductors), but pure semiconductors conduct poorly at room temperature.
• Doping means intentionally adding impurity atoms to control conductivity.
• Even a tiny dose works: about 1 dopant per million silicon atoms raises the free-carrier count roughly 3 million times over pure silicon.

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15.9 N-type and P-type Semiconductors

Doping creates extra charge carriers — but which kind of carrier you get depends entirely on which element you add. That one choice splits the semiconductor world into two families.

15.9.2 P-type (Positive)

Doping: Add elements with fewer electrons (e.g., Boron)

Result: Deficit of electrons creates “holes” (positive charge carriers)

Atom Type	Valence Electrons	Result
Silicon atom	4	Normal semiconductor lattice
Boron dopant	3	One missing electron creates a mobile hole

Properties:

• Majority carriers: Holes (+)
• Minority carriers: Electrons (-)
• Holes act as positive charge carriers

The running circuit argument reaches Figure 15.3, where n-type vs P-type semiconductor energy band diagram showing electron and hole distributions can be inspected directly before p-type (positive) proceeds.

First identify N-Type vs P-Type Semiconductors in Figure 15.3, then contrast it with N-Type Semiconductor and test the claim beside Band Gap. These visible labels explain n-type vs P-type semiconductor energy band diagram showing electron and hole distributions. They keep p-type (positive) tied to evidence that can be inspected on the schematic, part, or trace.

15.9.3 How It Works: PN Junction and Diode Bias

On their own, N-type and P-type are just two flavors of doped silicon. The interesting behavior appears the moment you join them into a single crystal.

Diode Forward and Reverse Bias

The big picture: A PN junction diode conducts current easily in one direction (forward bias) but blocks it in the reverse direction, acting as an electronic one-way valve controlled by voltage polarity.

Step-by-step breakdown:

1. Forward Bias (Conducting): Connect P-side to positive voltage, N-side to ground, reducing depletion zone width from 500nm to ~10nm, allowing electron flow across junction with only 0.7V voltage drop - Real example: 1N4007 diode passing 1A forward current drops 0.9V, dissipating 0.9W as heat

2. Reverse Bias (Blocking): Connect N-side to positive, P-side to ground, expanding depletion zone to 5-10um, creating insulating barrier that blocks current with only <1uA leakage - Real example: Same 1N4007 diode blocking 12V reverse voltage passes only 0.5uA leakage current, negligible 0.006mW power loss

3. Breakdown (Destructive): Exceed reverse voltage rating (1000V for 1N4007), causing avalanche effect where electric field tears electrons free, creating runaway current that generates heat until junction melts - Real example: Applying 1200V reverse voltage to 1N4007 rated for 1000V causes catastrophic failure within milliseconds

Why this matters: Forward voltage drop (0.7V silicon, 0.3V Schottky) wastes power in rectifiers and protection circuits. Using Schottky diodes in 5V supplies recovers 8% efficiency compared to silicon diodes.

Phoebe’s Field Notes: Where the Diode Equation Comes From

The mathematical gist. A silicon junction has thermal voltage (V_T=kT/q), and its forward-current ratio changes as (e^{\Delta V/V_T}). At 300 K, (V_T=25.85) mV, so one decade of current needs (V_T\ln 10=59.5) mV and a 120 mV rise multiplies ideal current by about 104.

Math Bridge · guided foundationsWhy does a silicon diode look as if it has a knee?Let Eddie connect temperature, thermal voltage, and current decades. Learn the maths

15.10 Continue to the Next Part

Carry this evidence into Semiconductors: Fabrication and Switching Devices, which begins with How ICs Are Actually Made.