{"id":"source-drain","level":"transistor","name":"Source and drain regions","shortName":"Source / drain","role":"Connects the controlled channel to reservoirs that supply and collect carriers.","description":"Source and drain are engineered semiconductor regions at the ends of the channel. They must connect efficiently to both the channel and metal contacts, making their resistance a substantial part of device design. Doping and material choice affect carrier availability, while epitaxial regions can also introduce useful strain. Advanced p-type device research includes silicon-germanium and germanium-rich approaches, but one research result should not be treated as the recipe for every commercial transistor. The atlas keeps source/drain material distinct from its metal contact. That separation explains why improving the central channel alone may deliver less benefit than expected: current still has to enter and leave through these surrounding structures.","specs":[{"label":"Electrical role","value":"Carrier reservoirs","context":"Serve the channel through access regions."},{"label":"Engineering variables","value":"Doping, material, strain","context":"Process- and device-dependent."},{"label":"Resistance budget","value":"Includes access resistance","context":"Separate from intrinsic channel behavior."}],"interfaces":[{"name":"Metal contact","direction":"in","signal":"Charge supplied through terminal interface"},{"name":"Channel","direction":"out","signal":"Carrier injection and collection"}],"tradeoff":"Low resistance improves drive current. Material and activation choices must remain compatible with the full device.","whyAI":"Every switching event depends on carrier access. Source/drain engineering prevents that access from limiting useful compute.","related":["doping","channel","contact","spacer"],"sources":[{"title":"imec — Source/Drain Contact Research, IEDM 2017","url":"https://www.imec-int.com/en/articles/imec-achieves-record-low-contact-resistivity-on-ga-doped-ge-source-drain-contacts-for-pmos-transistors"}],"scale":"Nanometer-scale terminal regions; illustrative device section","kind":"source-drain","mechanism":["Provide terminal semiconductor regions at the two ends of the controlled channel.","Engineer carrier supply and access through material, doping, and interface choices.","Connect those regions to metal contacts and the surrounding circuit."],"misconception":"Source/drain regions and metal contacts are adjacent parts with different roles, not two names for the same object.","check":{"question":"Can source/drain access limit a transistor with an excellent channel?","options":["Yes; access and interface resistance remain in the current path","No; channel resistance is the only physical resistance","Only if the chip has no clock"],"answer":0,"explanation":"Useful current must enter and leave the channel through the rest of the device. Improving one portion does not erase the others."},"branch":"Device physics","subbranch":"Carrier access","science":[{"title":"Series resistance budget","equation":"R_on ≈ R_channel + R_source + R_drain + R_contacts","explanation":"A channel improvement helps only the fraction of total resistance that belongs to the channel. Access regions and contacts can therefore limit the benefit of a better central device.","assumptions":"Small-signal or resistive on-state bookkeeping over a specified bias regime; actual transistor I–V behavior is nonlinear and the terms can be bias-dependent."}],"history":[],"learningObjective":"Partition on-resistance into channel, access and contact terms.","engineeringExample":"Reducing only the channel term cannot guarantee lower total R_on if contacts and access regions dominate.","evidenceNotes":"The series decomposition is useful for diagnosis; extraction depends on test structures and bias.","visualFamily":"device-crosssection","prerequisites":["mosfet","doping"],"labId":"library","scope":"Use the switching investigation to connect physical charge, circuit load and arithmetic; the numerical library has no standalone model of this lesson’s full mechanism.","investigationId":"switching","labScope":"Use the switching investigation to connect physical charge, circuit load and arithmetic; the numerical library has no standalone model of this lesson’s full mechanism."}