Profile

Elektro Lab

Analog soul, digital mind

MOSFETs: From First Principles to Efficient Power Switching

By Dhruvjit February 8, 2026 Posted in Electronic Components
Tags: mosfet power-electronics switching gate-drive thermal-design

Core Behavior and First-Principles View

MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors) are the dominant switching devices in modern low- and mid-power electronics. They are used in:

To design MOSFET-based systems reliably, we must understand:

A MOSFET is a voltage-controlled current device.
Unlike BJTs, it does not require continuous input current — only charge to the gate.

Internal Structure and Channel Formation

Internal Structure

An N-channel enhancement MOSFET consists of:

When:

VGS>VTHV_{GS} > V_{TH}

An inversion layer forms under the oxide, creating a conductive channel between source and drain.

The gate voltage controls the density of charge carriers in this channel.

Operating Regions and Mathematical Model

Cutoff Region (OFF State)

If:

VGS<VTHV_{GS} < V_{TH}

Then:

ID≈0I_D \approx 0

The MOSFET is OFF.

Linear (Triode) Region

When:

Drain current:

ID=kn[(VGS−VTH)VDS−VDS22]I_D = k_n \left[(V_{GS}-V_{TH})V_{DS} - \frac{V_{DS}^2}{2}\right]

Where:

kn=μnCoxWLk_n = \mu_n C_{ox} \frac{W}{L}

In power switching, this region behaves like a controlled resistor.

Saturation Region

When:

VDS≥VGS−VTHV_{DS} \ge V_{GS} - V_{TH}

Drain current:

ID=12kn(VGS−VTH)2I_D = \frac{1}{2}k_n(V_{GS}-V_{TH})^2

In switching applications, we avoid operating in saturation.
We drive the MOSFET fully into the linear region to minimize resistance.

MOSFET as a Practical Switch

In switching applications:

On-state voltage:

VDS(on)=ID⋅RDS(on)V_{DS(on)} = I_D \cdot R_{DS(on)}

Datasheets therefore emphasize:

Conduction Loss

When the MOSFET is fully ON:

Pcond=ID2RDS(on)P_{cond} = I_D^2 R_{DS(on)}

Example:

If:

Then:

Pcond=102×0.02=2WP_{cond} = 10^2 \times 0.02 = 2W

Important:

Switching Behavior and Gate Charge

The MOSFET gate behaves like a capacitor.

Total gate charge:

Qg=Ciss×VGSQ_g = C_{iss} \times V_{GS}

Switching occurs in phases:

  1. Gate voltage rises
  2. Channel forms
  3. Miller plateau begins
  4. Drain voltage falls
  5. Device fully enhances

The Miller plateau is where drain voltage transitions.

Switching Loss

During transitions, voltage and current overlap:

Pswitch=12VDSID(tr+tf)fsP_{switch} = \frac{1}{2} V_{DS} I_D (t_r + t_f) f_s

Where:

At higher frequencies, switching loss dominates conduction loss.

Gate Driver Design

Gate driver must supply current:

Igate=QgtswitchI_{gate} = \frac{Q_g}{t_{switch}}

Example:

If:

Then:

Igate=2AI_{gate} = 2A

GPIO pins cannot supply this current.
Dedicated gate drivers are required for fast switching.

Parasitics and Ringing

Real PCBs introduce parasitic inductance:

V=LdidtV = L \frac{di}{dt}

High ( di/dt ) causes:

Minimize:

Keep high-current loops short and tight.

Thermal Design and Junction Temperature

Total loss:

Ptotal=Pcond+PswitchP_{total} = P_{cond} + P_{switch}

Junction temperature:

TJ=TA+Ptotal×RθJAT_J = T_A + P_{total} \times R_{\theta JA}

Where:

Always validate under:

Never design to absolute maximum ratings.

Device Selection Trade-Off

Lower ( R_{DS(on)} ) → Larger die → Higher ( Q_g )
Higher ( Q_g ) → Slower switching → Higher switching loss

You must balance:

There is no perfect MOSFET.
Only optimal compromise.

Common Failure Patterns

Practical Design Checklist

✔ Oscilloscope waveforms verified
✔ Switching edges clean
✔ Ringing within safe margin
✔ Junction temperature calculated
✔ Worst-case testing performed
✔ 20% thermal safety margin

Final Insight

Reliable MOSFET design happens when:

Power electronics is the discipline of controlling energy transitions safely, efficiently, and predictably.

You Might Also Like