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Home Transistor High Frequency Applications The Transistor at High Frequencies PointContact and Junction Transistors  
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Frequency Comparison of PointContact and Junction TransistorsAuthor: Leonard Krugman At this point, a brief explanation of why the pointcontact transistor is capable of a higher operating frequency than the junction type is in order. The high frequency effects on the equivalent circuit parameters are essentially the same for both types. Actually, the major difference is in the mechanics of conduction.
ρ = 12 ohmcm, and I_{c}  3 ma, for which T = 1,570 μμsecs. Ignoring all other factors, this limits the upper frequency response of the pointcontact transistor to about 600 megacycles. In the junction transistor, movement of the current carriers is primarily by diffusion, and is not appreciably affected by the electrode potential fields. In equation form, where T is the diffusion time through the base layer, W is thickness of the base layer in centimeters, and D is the diffusion constant in cm^{2}/sec. Typical values for a PNP transistor are W = 2 × 10^{3} cm and D = 33 cm^{2}/sec (for an NPN type, D is about 69 cm^{2}/sec), for which T = 0.121 μsecs. Ignoring all factors but the diffusion time, the upper frequency for this typical PNP type is approximately 8 me, and for the NPN type about 16 mc.


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