TN2124K1-G MOSFET: Sourcing Guide and Key Specs
The TN2124K1-G is one of those parts that keeps showing up on BOMs long after designers stopped putting it into new layouts. It is a 240V N-channel depletion-mode MOSFET in a TO-92 package, originally from Supertex, which Microchip absorbed back in 2014. If you are maintaining legacy industrial controls, audio equipment, or off-line power supply startup circuits, you have probably had to hunt for this transistor at least once. This guide covers what the part actually does, what to check before buying, and the pitfalls that catch buyers out in the open market.
What the TN2124K1-G Actually Does
Depletion-mode MOSFETs are normally on at zero gate voltage. That is the opposite of the enhancement-mode devices most engineers work with daily, and it is exactly why the TN2124K1-G survives in certain designs. It conducts by default, and you pull the gate negative to turn it off.
The headline specs: 240V drain-source breakdown, roughly 100mA continuous drain current, and a gate threshold around -1.5V to -3.5V depending on the lot. The TO-92 package limits power dissipation to about 740mW, so this is a small-signal and startup-duty part, not a power switch.
Where does it get used? The classic application is the high-voltage startup circuit in off-line SMPS designs. The depletion FET feeds the controller IC's Vcc capacitor directly from the rectified mains, then shuts off once the auxiliary winding takes over. You also see it in constant-current sources, solid-state relay input stages, and as a cascode device in high-voltage linear regulators. Some tube amplifier and pro-audio designs use it in similar roles.
Why Sourcing This Part Gets Complicated
Here is the honest picture. Depletion-mode MOSFETs are a niche product family, and the TO-92 through-hole package is losing ground across the industry. Wafer capacity keeps shifting toward automotive and high-volume power devices, which squeezes older through-hole lines. A recent EE News Europe analysis of Chips Act 2.0 makes a point worth noting: Europe and the broader market cannot compete at every layer of the value chain, and legacy process nodes are exactly the layer that gets deprioritized when capacity decisions get made. For buyers, that translates into longer quoted lead times and more frequent allocation on parts like the TN2124K1-G.
Franchised stock exists, but it moves slowly and sometimes sits in bonded warehouses overseas. That pushes many buyers toward independent distributors and brokers, where the risk profile changes. We have seen plenty of TO-92 high-voltage parts come through the grey market with re-marked bodies. A cheap 2N7000 or a low-voltage device relabeled as a 240V depletion FET will pass a casual visual check and fail the moment your startup circuit sees rectified mains.
Checks to Run Before You Buy TN2124K1-G
A few practical steps will filter out most of the bad stock:
- Verify the lot code format. Supertex-era and Microchip-era marking differs. Ask the seller for photos of the actual reels or bags, not stock images, and compare the date code format against known-good examples from your own history or the manufacturer's documentation.
- Request a sample for electrical test first. A curve tracer or even a basic component tester will separate real depletion devices from remarked enhancement parts in minutes. An enhancement MOSFET is off at Vgs=0; the TN2124K1-G should conduct. That single test kills most counterfeits.
- Check Vgs(th) spread. Legitimate parts from the same lot show tight threshold voltage distribution. A bag of parts with wildly varying thresholds is a red flag for mixed rework stock.
- Ask about traceability. Even in the open market, a serious seller can show where the stock came from: an OEM excess list, a contract manufacturer line-down, a franchised channel overflow. No paperwork at all means you are the final quality gate.
Substitutes and Cross-Reference Reality
When TN2124K1-G stock dries up, engineers ask about crosses. The honest answer is that depletion-mode HV MOSFETs in TO-92 have a thin field of alternatives. The LND150 from the same Supertex/Microchip family is a common discussion point, but it is a 500V part with different threshold and capacitance behavior. In a startup bleed circuit that may not matter. In a precision current source, it will.
IXYS (now Littelfuse) also makes depletion devices, mostly in SOT-89 and TO-220, which forces a board change if your layout is built around TO-92. Some teams bite the bullet and redesign the startup section with a resistor-dropper plus high-voltage regulator instead, which removes the depletion FET entirely. That is a real cost-benefit call: a small respin versus a lifetime of allocation headaches.
One scenario we see repeatedly: a contract manufacturer quotes a legacy industrial controller build, and the TN2124K1-G line item blows up the schedule because the buyer assumed TO-92 transistors are commodity stock. They are not, at least not in this voltage class. Build in four to eight weeks of buffer for this part, or lock stock ahead of production.
The Broader Picture for Legacy Parts Buyers
Capacity pressure is not letting up. The weekly roundup from Semiconductor Engineering keeps showing investment flowing toward advanced nodes, silicon photonics, and edge AI silicon. Almost none of that money touches legacy through-hole discrete lines. The practical read: if a part like the TN2124K1-G is designed into products with five or more years of remaining life, buying lifetime stock now is cheaper than redesigning later.
At XingHuan International, we handle a steady volume of legacy and hard-to-find discrete semiconductors, and the TN2124K1-G is a recurring request. Our approach is straightforward: we verify source and lot before quoting, and we can arrange electrical screening on incoming stock when the application justifies it. If your BOM has other aging discretes alongside this FET, it is usually worth auditing the whole list in one pass rather than firefighting part by part.
FAQ
Q: Is the TN2124K1-G still in production, or is it obsolete?
A: Microchip has listed Supertex depletion MOSFETs as active for years, but availability fluctuates and factory lead times can stretch past 16 weeks. Treat it as a plan-ahead part rather than something you can spot-buy from franchise channels.
Q: How can I quickly test whether a TN2124K1-G is genuine?
A: Tie gate to source and check drain-source conduction with a component tester or curve tracer. A genuine depletion device conducts at Vgs=0; a remarked enhancement MOSFET like a 2N7000 will show open. Then confirm breakdown rating with a proper HV tester if the application is safety-relevant.
Q: What is a workable substitute if I cannot find TN2124K1-G stock?
A: The Microchip LND150 is the closest functional relative, but it is 500V rated with different threshold characteristics, so verify it in your actual circuit. For startup-duty applications, many teams redesign with a resistive dropper and HV regulator to eliminate the depletion FET dependency entirely.