Transistor Tetrode

Author: Leonard Krugman

The frequency response of the conventional junction transistor is limited by several factors. First, the frequency cutoff (the frequency at which the current gain drops sharply) is inversely proportional to both the base resistance and to the square of the thickness of the junction layer. In addition, the frequency cutoff is also inversely proportional to the collector junction capacitance, considered only at high frequencies. Figures 2-11 (A) and 2-11 (B) illustrate the structural and symbolic representations of the junction transistor tetrode.

Fig. 2-11. Tetrode junction transistor: (A) structural representation, (B) symbolic representation.

In this transistor, a fourth electrode, designated as b₂, is included. The fourth electrode is connected to the P junction layer in the same manner as the conventional base electrode, but the connection is made on the opposite side of the layer. The base resistance is reduced substantially when a negative bias is applied to the second base electrode. The bias prevents that part of the emitter junction which is near b₂ from emitting electrons into the P layer. Thus all of the transistor action takes place near the base. This effectively reduces the base resistance; as a result, the frequency response increases.

For proper operation, the second base electrode is biased to about -6 volts with respect to the base. The resulting bias current is approximately one milliampere. In a typical case this bias reduces the base resistance from 1,000 to 40 ohms; the change in emitter resistance is negligible. The current gain is reduced from .95 to .75, and the collector resistance is reduced from 3.0 to 1.5 megohms. The frequency response cutoff is increased from 0.5 to 5 megacycles. Thus, an increased bandwidth is obtained at the expense of lower available gain.

The effect of the junction area thickness is decreased by using very thin P layers (roughly .0005 inch). The collector junction capacitance is reduced by decreasing the collector junction area.