Tetrode Transistor
A tetrode transistor is a bipolar junction transistor with four terminals instead of three: the usual emitter, collector and base connections, plus a second base contact attached to the opposite side of the thin base layer. A small bias current driven between the two base contacts creates a voltage drop across the base, and that drop squeezes the emitter's injection into the narrow strip nearest the first base contact. The device belongs to the early 1950s, when Shockley's 1948 theory of the junction transistor was well understood and the practical complaint was speed: the base of a real transistor was too big, too resistive and too capacitive for radio-frequency work, and the tetrode was an electrical rather than geometrical way of making the active region smaller.
The problem the second base contact attacked was base resistance and collector capacitance, the two quantities that together set a transistor's high-frequency ceiling. In an ordinary junction transistor the base current has to travel sideways through a thin, lightly doped layer to reach its contact, so the layer behaves as a resistor in series with the input, and the voltage drop along it makes the emitter inject most of its current near the edge closest to the contact, a crowding effect. Meanwhile the collector junction, which is much larger than the part actually doing useful work, contributes capacitance without contributing gain. A common figure of merit, the maximum oscillation frequency, is roughly the square root of fT divided by 8π × rb' × Ccb, where fT is the transition frequency, rb' the base spreading resistance and Ccb the collector-base capacitance, so cutting either rb' or Ccb pays off directly.
Biasing the second base contact steers the active region electrically, which is what distinguishes the tetrode from a simply smaller transistor. With b2 held a few volts away from b1, current flows along the base layer and the potential drops steadily across it, so the emitter-base junction far from b1 is pushed toward reverse bias while the part near b1 stays forward biased. Injection therefore happens only in a narrow region, the base current path becomes short, and the portion of the collector junction that actually collects carriers is small. Because changing the b2 bias changes the size of that region, it also changes the gain, which made the extra terminal useful for automatic gain control in radio receivers and for mixer circuits in which one signal is applied to the emitter and another to a base contact.
The name comes from the vacuum tube, and the analogy is real but only partial. The vacuum tube tetrode added a screen grid between control grid and plate, an idea associated with Walter Schottky around 1919 and made practical by Albert Hull at General Electric in the 1920s, to cut the grid-to-plate capacitance that otherwise caused radio-frequency amplifiers to oscillate. The transistor tetrode shares the engineering goal of taming an unwanted capacitance and adding a fourth electrode, but its second base contact is not a screen and does not shield anything; it works by reshaping where current flows. The most common misconception is that a tetrode transistor is two transistors or a device with two emitters, when it is a single junction transistor with one emitter, one collector and a base contacted twice.
The tetrode transistor faded because better geometry solved the problem at its source. Making it required a more elaborate structure, an extra bias supply and an extra lead, and the benefit was a modest improvement over what shrinking the device could deliver. Once diffusion allowed base layers a small fraction of a micrometre thick and the mesa and planar structures cut junction area dramatically, high-frequency transistors with three terminals outperformed tetrodes at lower cost and complexity. Germanium tetrodes were made for radio-frequency and mixer service, but they were a transitional design, not a lasting family, which is why they are better known from the history books than from the circuits of the 1960s onward.
The idea lives on in the dual-gate MOSFET, which puts two gates in series along one channel. Gate 1 carries the signal and gate 2 acts as a control and shield: the second gate lowers the capacitance between gate 1 and the drain, much as a cascode stage would, and its bias adjusts the gain smoothly. Dual-gate MOSFETs were used for decades in television tuners, FM receivers, mixers and gain-controlled amplifiers, and the physics differs entirely from the bipolar tetrode because the device is a field-effect transistor, though the motive, taming a parasitic capacitance and gaining a control terminal, is the same. A related lesson persists in modern four-terminal MOSFETs, where the body terminal is a fourth connection that engineers must deliberately bias or tie off.
Read tetrode transistor as the early-1950s attempt to speed up a junction transistor by electrically pinching its base with a fourth terminal, a clever stopgap that geometry later made unnecessary.