A mechanical relay used at microwave frequencies

A digital signal processor for audio

E2 · Published 28 September 2026 · By The Ham Desk

Semiconductor Devices: Which One, and Why That One

Extra class device questions are about selection. Given a job - switch this RF path, tune this oscillator, amplify this weak signal - a small set of semiconductor types each solve it in a particular way, and knowing the reason is what the exam asks for.

Band 70 cm
Study time 18
Level Core
questions 10

Waveform and antenna diagram for Advanced Circuits and Devices — Advanced Circuits and Devices — waveform overlay and radiation pattern

Pre-flight briefing

Two families of transistor

The bipolar junction transistor is a current-controlled device: a small base current allows a much larger collector current to flow, and its transconductance is high, which makes it the classic choice for low-noise RF amplifiers and for power stages up to a few hundred megahertz. Its input impedance is low and its performance falls as junction capacitance comes to dominate at higher frequencies.

The field-effect transistor is voltage-controlled: a voltage on the gate controls the current between source and drain, and because the gate is a reverse-biased junction or an insulated layer, the input impedance is very high. The MOSFET is the workhorse of HF and VHF power amplifiers because it is easy to bias, tolerant of mismatch and available in devices that will dissipate hundreds of watts. Its drawback is gate capacitance, which limits gain at the highest frequencies, and a susceptibility to damage from static and from excessive gate voltage.

Gallium arsenide and gallium nitride devices extend this further. GaAs FETs and MMICs - monolithic microwave integrated circuits, complete amplifiers on a single chip - are standard above 1 GHz, and GaN devices handle far more power per unit of die area than silicon, which is why they have taken over high-power microwave and pulsed applications.

Diodes that are not rectifiers

Several diode types do jobs that have nothing to do with converting AC to DC. The varactor is a diode operated in reverse bias, where the depletion region acts as a voltage-controlled capacitor; that is how a synthesised transceiver's VCO is tuned and how a phase-locked loop locks to a reference. The PIN diode has an intrinsic layer between the p and n regions, and at RF it behaves as a current-controlled resistor: with no bias it is nearly an open circuit, with forward current it is a few ohms, which makes it ideal for RF switching, attenuators and T/R switching in a transceiver.

The Zener diode is designed to break down at a precise reverse voltage and is used as a voltage reference and as a simple regulator; the avalanche breakdown is non-destructive as long as the current is limited. The tunnel diode exploits negative resistance to amplify and oscillate at microwave frequencies, and the step recovery diode generates the sharp pulses used in frequency multipliers and comb generators.

Analog and digital building blocks

An operational amplifier is a high-gain differential amplifier with a very high input impedance, and its behaviour is set almost entirely by the feedback network around it - gain, bandwidth, input and output impedance all follow from the resistors and capacitors you connect. That is why a single op-amp type can serve as an audio preamplifier, an active filter or an RF amplifier at low frequencies.

Digital logic comes in families with different trade-offs. TTL is fast and consumes a fairly constant current per gate; CMOS consumes almost nothing while static and draws current only during transitions, which makes it the choice for battery equipment but also makes it vulnerable to static discharge and requires unused inputs to be tied to a defined level. Above a few hundred megahertz, the same functions are implemented in gallium arsenide or in a programmable logic device.

Every one of these devices has a maximum junction temperature, and that - together with the thermal resistance from junction to case and from case to heat sink - is what limits how much power a given device can handle. A device with a good heat sink dissipates far more than the same device standing in free air, which is why power amplifier design is as much about thermal engineering as about the circuit.

Terms you must be able to define
Term	What the examiner wants to hear
Varactor	A reverse-biased diode whose capacitance varies with applied voltage, used for tuning.
PIN diode	A diode with an intrinsic layer that acts as a current-controlled RF resistor for switching and attenuation.
MOSFET	A voltage-controlled field-effect transistor, common in HF and VHF power amplifiers.
MMIC	Monolithic microwave integrated circuit: a complete amplifier fabricated on one chip.
CMOS	Complementary metal-oxide-semiconductor logic, drawing current only during switching.

Question deck 10 questions

E1B01 Recall

What is the essential difference between a bipolar transistor and a field-effect transistor?

E1B02 Core

What is an advantage of a MOSFET over a bipolar transistor in an HF power amplifier?

E1B03 Core

What is a varactor diode used for?

E1B04 Core

Why is a PIN diode useful as an RF switch?

E1B05 Core

What is a Zener diode normally used for?

E1B06 Core

What determines the gain and bandwidth of a circuit built around an operational amplifier?

E1B07 Stretch

Why does CMOS logic consume very little power when it is not switching?

E1B08 Core

What is an MMIC?

E1B09 Stretch

What sets the limit on how much power a transistor can dissipate?

E1B10 Stretch

What is a tunnel diode used for?

Reference sheet

The explanations for all ten questions sit on this sheet. Pick an answer on the left and the matching card lights up.

E1B01 Explanation 1

Key D — The bipolar is controlled by current at its base, the FET by voltage at its gate

Current control versus voltage control is the defining difference, and it produces the practical differences that follow: the FET's very high input impedance, the bipolar's higher transconductance, and the different biasing and protection requirements of each.

Rule Fundamentals - transistors

E1B02 Explanation 2

Key C — Its high input impedance makes biasing simpler and it tolerates mismatch better

Because the gate is essentially a capacitor, the device draws almost no drive current, which simplifies the bias network and makes the amplifier less sensitive to drive-level changes. MOSFETs still generate heat and still need heat sinking - the thermal limit has not gone away.

Rule Fundamentals - RF power devices

E1B03 Explanation 3

Key B — Providing a capacitance that varies with applied voltage, for tuning oscillators and filters

Operated in reverse bias, the varactor's depletion region widens as voltage rises and its capacitance falls. Applying a control voltage through a resistor to the varactor tunes the resonant circuit, which is the basis of every voltage-tuned oscillator and phase-locked loop.

Rule Fundamentals - varactor tuning

E1B04 Explanation 4

Key A — With no bias it looks like an open circuit, and with forward current it looks like a few ohms

The intrinsic layer makes the diode's RF resistance a direct function of the forward bias current and nearly independent of the RF signal itself, so it can switch or attenuate with very low distortion. That is why PIN diodes appear in antenna switching, step attenuators and transmit/receive changeover circuits.

Rule Fundamentals - PIN diodes

E1B05 Explanation 5

Key D — Providing a stable reference voltage or simple regulation

A Zener is designed to break down at a precise reverse voltage and to survive doing so, provided the current through it is limited by a series resistor. That makes it useful as a reference, a clamp, and as protection for circuits that must not see more than a set voltage.

Rule Fundamentals - Zener diodes

E1B06 Explanation 6

Key C — The feedback network connected around the amplifier

An op-amp has enormous open-loop gain, so once feedback is applied the closed-loop behaviour is set almost entirely by the external components. That is what makes one device type usable as an amplifier, a filter, an integrator or a comparator depending only on what you connect around it.

Rule Fundamentals - operational amplifiers

E1B07 Explanation 7

Key B — Its complementary transistor pair means one device is always off, so almost no current flows in either state

In a CMOS gate the pull-up and pull-down devices are never on at the same time, so the only current in the static state is leakage. Current is drawn only while the gate is changing state, which makes CMOS attractive for battery-powered equipment - and also makes the inputs sensitive to static and to floating levels.

Rule Fundamentals - logic families

E1B08 Explanation 8

Key A — A monolithic microwave integrated circuit: a complete circuit fabricated on a single semiconductor chip

MMICs put an entire amplifier, mixer or switch on one gallium arsenide or silicon germanium die, which removes the parasitic inductance of bond wires and discrete components. They are why modern microwave equipment is small, repeatable and cheap compared with a hand-built equivalent.

Rule Fundamentals - microwave integrated circuits

E1B09 Explanation 9

Key D — Maximum junction temperature combined with the thermal resistance to the heat sink

The junction must stay below its rated maximum, and the temperature rise equals the dissipated power multiplied by the thermal resistance from junction to ambient. Improving the heat sink lowers that resistance, which is why a device can be run harder bolted to a large sink than it can in free air.

Rule Fundamentals - thermal design

E1B10 Explanation 10

Key D — Amplification and oscillation at microwave frequencies exploiting its negative resistance region

The tunnel diode's current-voltage curve has a region where increasing voltage reduces current - negative resistance - which can be used to cancel circuit loss or to sustain oscillation. It is fast and quiet but low in power, which confines it to specialised microwave and detector work.

Rule Fundamentals - negative resistance devices

How to use this unit

Work the ten questions in one sitting with the reference sheet covered, then uncover it and read every explanation - including the ones you got right. The wording of the real exam is different, so learn the reasoning rather than the answer letters.