Semiconductor Fabrication

Contact

Q: What is the difference between an ohmic contact and a Schottky contact?

An ohmic contact conducts current symmetrically in both directions with a low, linear (constant) resistance and negligible voltage drop, so it is used for source, drain, and emitter terminals where the contact must not distort the signal. A Schottky contact is rectifying: it forms a metal-semiconductor barrier that conducts strongly under forward bias and blocks under reverse bias, which is exactly the behavior wanted for a Schottky diode or a MESFET/HEMT gate. The same metal-semiconductor system can be made ohmic or Schottky depending on doping and processing. Heavy surface doping (above 1e19 per cm3) thins the barrier so electrons tunnel through it, producing ohmic behavior, while a lightly doped surface preserves the rectifying Schottky barrier. Alloying or annealing an ohmic stack (for example AuGeNi on GaAs at 400 to 450 C) drives in dopant and forms a low-resistivity interfacial layer.

Q: How is specific contact resistivity measured with the TLM method?

The transfer length method (TLM) uses a row of identical contact pads spaced at increasing gaps L1, L2, L3 on a uniform semiconductor layer. The total resistance between adjacent pads is measured and plotted against gap spacing. The plot is a straight line: its slope gives the sheet resistance Rsh, and its y-intercept at zero spacing equals 2 times Rc, where Rc is the contact resistance of one pad. The x-intercept gives twice the transfer length LT, and the specific contact resistivity is rho_c = Rsh times LT squared. For RF ohmic contacts on GaAs or GaN, good values are in the 1e-6 to 1e-7 ohm-cm2 range. Lower specific contact resistivity reduces parasitic source resistance, which directly improves transconductance, gain, and noise figure in an LNA.

Q: Why does contact resistance matter for RF transistor noise figure and gain?

Source and drain contact resistance adds directly in series with the intrinsic transistor channel. The extra source resistance Rs degenerates the effective transconductance to gm_eff = gm / (1 + gm times Rs), reducing gain and cutoff frequency fT. In a low noise amplifier the source resistance is also a thermal-noise generator, so every fraction of an ohm of parasitic contact resistance raises the minimum noise figure Fmin. For a millimeter-wave GaN HEMT a source contact of even 0.2 to 0.3 ohm-mm can add several tenths of a dB to noise figure and shave gigahertz off fT. This is why fabrication targets specific contact resistivity below 1e-6 ohm-cm2 and uses refractory or alloyed metal stacks that stay stable at high channel temperatures.

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At the device level, the metal-to-semiconductor junction that carries current into or out of a terminal such as a source, drain, gate, or diode anode. Depending on the surface doping and the metal stack, this junction behaves either as an ohmic interface, with low and linear resistance, or as a rectifying Schottky barrier. Ohmic contacts on GaAs and GaN are typically alloyed metal stacks (AuGeNi or refractory Ti-based systems) annealed near 400 to 870 °C to reach a specific contact resistance below 10^-6 Ω·cm². Because source and drain contacts sit in series with the channel, minimizing their resistance through good metallization is essential for high transconductance, gain, and a low noise figure in millimeter-wave transistors.

Category: Semiconductor Fabrication

Specific Contact Resistivity: 10^-6 to 10^-7 Ω·cm²

Ohmic Anneal: 400 to 870 °C

How Metal Meets the Semiconductor

Every active semiconductor device must connect its internal regions to the outside world through metal pads, and the quality of those connections sets a hard floor on performance. When a metal touches a semiconductor, the difference between the metal work function and the semiconductor electron affinity creates an energy barrier at the interface, called the Schottky barrier height, φ_B. The way carriers cross that barrier decides whether the junction is ohmic or rectifying. If the barrier is high and the semiconductor is lightly doped, carriers must climb over it by thermionic emission, giving a diode-like rectifying characteristic. If the surface is doped very heavily, the depletion region becomes so thin that electrons tunnel straight through, collapsing the rectification into a near-linear, low-resistance ohmic response.

For RF and millimeter-wave transistors the source and drain require ohmic contacts, while the gate of a MESFET or HEMT deliberately uses a Schottky contact to modulate the channel. Achieving a good ohmic contact on a wide-bandgap material such as GaN is harder than on silicon because the barrier height is large; designers thin the barrier with a heavily doped cap layer or by alloying, where a high-temperature anneal drives metal and dopant into the surface to form a low-resistivity interfacial layer. A Ti/Al/Ni/Au stack annealed at 800 to 870 °C is a common GaN HEMT recipe, while AuGeNi alloyed near 420 °C is the classic GaAs ohmic.

The figure of merit is specific contact resistivity ρ_c, the resistance normalized to contact area in Ω·cm². Because source contact resistance adds in series with the channel, even tenths of an ohm-millimeter degrade the effective transconductance and raise the minimum sheet resistance seen between terminals, directly limiting f_T and noise figure.

Contact Resistance and Transfer Length

Total measured contact resistance (TLM pad):
R_c = (R_sh × L_T) / W coth(d / L_T) ≈ (R_sh × L_T) / W (d > 1.5·L_T)

Transfer length:
L_T = √(ρ_c / R_sh)

Specific contact resistivity from TLM:
ρ_c = R_sh × L_T² (Ω·cm²)

Effective transconductance with source resistance:
g_m,eff = g_m / (1 + g_m × R_s)

Where R_sh = sheet resistance (Ω/sq), L_T = transfer length, W = contact width, d = contact length, ρ_c = specific contact resistivity, R_s = series source resistance. Example: R_sh = 300 Ω/sq and ρ_c = 3×10^-6 Ω·cm² give L_T = √(3×10^-6/300) = 1 μm.

Ohmic vs. Schottky Contact Systems

Contact Type	Semiconductor	Typical Metal Stack	Anneal	Specific Resistivity / Barrier	Used For
Ohmic	n-GaAs	AuGeNi / Au	~420 °C	1 to 5 × 10^-6 Ω·cm²	FET source / drain
Ohmic	n-GaN (HEMT)	Ti/Al/Ni/Au	800 to 870 °C	1 to 5 × 10^-6 Ω·cm²	HEMT source / drain
Ohmic	n+ Si	TiSi₂ / W	600 to 700 °C	10^-7 to 10^-6 Ω·cm²	CMOS S/D
Schottky	n-GaAs	Ti/Pt/Au	None	φ_B ≈ 0.8 eV	MESFET / diode gate
Schottky	n-GaN	Ni/Au	None	φ_B ≈ 1.0 eV	HEMT gate, mixer diode

Common Questions

Frequently Asked Questions

What is the difference between an ohmic contact and a Schottky contact?

An ohmic contact conducts symmetrically with low, linear resistance and is used for source, drain, and emitter terminals. A Schottky contact is rectifying, forming a metal-semiconductor barrier used for diode and MESFET/HEMT gates. The same metal system can be either: heavy surface doping (above 10¹⁹ cm^-3) thins the barrier so electrons tunnel through, giving ohmic behavior, while light doping preserves the rectifying Schottky barrier. Alloying an ohmic stack, such as AuGeNi on GaAs near 420 °C, forms a low-resistivity interfacial layer.

How is specific contact resistivity measured with the TLM method?

The transfer length method places identical pads at increasing gaps and plots total resistance versus spacing. The slope gives sheet resistance R_sh, the y-intercept equals 2R_c, and the x-intercept equals twice the transfer length L_T. Then ρ_c = R_sh × L_T². Good RF ohmic contacts on GaAs or GaN reach 10^-6 to 10^-7 Ω·cm². Lower ρ_c reduces parasitic source resistance, improving transconductance, gain, and noise figure.

Why does contact resistance matter for RF transistor noise figure and gain?

Source and drain contact resistance adds in series with the channel. The extra source resistance R_s degenerates transconductance to g_m,eff = g_m / (1 + g_mR_s), cutting gain and f_T; in an LNA it is also a thermal-noise source that raises F_min. For a millimeter-wave GaN HEMT, even 0.2 to 0.3 Ω·mm of source contact can add tenths of a dB to noise figure. Targeting ρ_c below 10^-6 Ω·cm² with refractory or alloyed stacks keeps the contact stable at high channel temperature.

Related Terms

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