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“Saturation” is a generic term for regions that are useless for an amplifier (i. e., are strongly non-linear on \$I_{\mathrm B}\$ or have β less than 1), but are far from cut-off. Most authors don’t apply due efforts to formulate a usable definition. There exists at least one English book not negligent on the matter:

E. Ramshaw (Google Books info) or R. S. Ramshaw (as written on the cover)
Power Electronics Semiconductor Switches
Springer Science & Business Media, 2013

The book specifies a distinct quasi-saturation region, that is non-linear (meets “4.”), but still fits to en.Wikipedia’s “forward-active” and the collector current flows in a healthy direction. After \$V_{\mathrm{BC}}\$ passes through zero (≈ “2.”), a hard saturation mode develops, that meets the definition “1.”. Ī̲’d call it “on–on region”.

Also, Ī̲’d characterize the region immediately before switching B–C on as “(forward) low-voltage”, having low (but positive) β. For details, see comments and my answer in Why does the collector current direction remain the same in saturation and active region? and the Common base circuit with zero supply voltage thread (with some simulation in CircuitLab).

What about the en.Wikipedia definition (as of today) of regions? It can be traced to various sources where authors were unwilling to get into details of these marginal modes. Quasi-saturation is close to the hard saturation region on the voltage–voltage plane (albeit operates in a different way), and many authors were satisfied with the simplistic picture presented in @jonk’s answer. What about the stuff from es.Wikipedia under “3.”? It’s just incompetent rubbish.

“Saturation” is a generic term for regions that are useless for an amplifier (i. e., are strongly non-linear on \$I_{\mathrm B}\$ or have β less than 1), but are far from cut-off. Most authors don’t apply due efforts to formulate a usable definition. There exists at least one English book not negligent on the matter:

E. Ramshaw (Google Books info) or R. S. Ramshaw (as written on the cover)
Power Electronics Semiconductor Switches
Springer Science & Business Media, 2013

The book specifies a distinct quasi-saturation region, that is non-linear (meets “4.”), but still fits to en.Wikipedia’s “forward-active” and the collector current flows in a healthy direction. After \$V_{\mathrm{BC}}\$ passes through zero (≈ “2.”), a hard saturation mode develops, that meets the definition “1.”. Ī̲’d call it “on–on region”.

Also, Ī̲’d characterize the region immediately before switching B–C on as “(forward) low-voltage”, having low (but positive) β. For details, see comments and my answer in Why does the collector current direction remain the same in saturation and active region? and the Common base circuit with zero supply voltage thread (with some simulation in CircuitLab).

What about the en.Wikipedia definition (as of today) of regions? It can be traced to various sources where authors were unwilling to get into details of these marginal modes. Quasi-saturation is close to the hard saturation region on the voltage–voltage plane (albeit operates in a different way), and many authors were satisfied with the simplistic picture presented in @jonk’s answer. What about the stuff from es.Wikipedia under “3.”? It’s just incompetent rubbish.

“Saturation” is a generic term for regions that are useless for an amplifier (i. e., are strongly non-linear on \$I_{\mathrm B}\$ or have β less than 1), but are far from cut-off. Most authors don’t apply due efforts to formulate a usable definition. There exists at least one English book not negligent on the matter:

E. Ramshaw (Google Books info) or R. S. Ramshaw (as written on the cover)
Power Electronics Semiconductor Switches
Springer Science & Business Media, 2013

The book specifies a distinct quasi-saturation region, that is non-linear (meets “4.”), but still fits to en.Wikipedia’s “forward-active” and the collector current flows in a healthy direction. After \$V_{\mathrm{BC}}\$ passes through zero (≈ “2.”), a hard saturation mode develops, that meets the definition “1.”. Ī̲’d call it “on–on region”.

Also, Ī̲’d characterize the region immediately before switching B–C on as “(forward) low-voltage”, having low (but positive) β, see comments and my answer in Why does the collector current direction remain the same in saturation and active region? for details.

What about the en.Wikipedia definition (as of today) of regions? It can be traced to various sources where authors were unwilling to get into details of these marginal modes. Quasi-saturation is close to the hard saturation region on the voltage–voltage plane (albeit operates in a different way), and many authors were satisfied with the simplistic picture presented in @jonk’s answer. What about the stuff from es.Wikipedia under “3.”? It’s just incompetent rubbish.

“Saturation” is a generic term for regions that are useless for an amplifier (i. e., are strongly non-linear on \$I_{\mathrm B}\$ or have β less than 1), but are far from cut-off. Most authors don’t apply due efforts to formulate a usable definition. There exists at least one English book not negligent on the matter:

E. Ramshaw (Google Books info) or R. S. Ramshaw (as written on the cover)
Power Electronics Semiconductor Switches
Springer Science & Business Media, 2013

The book specifies a distinct quasi-saturation region, that is non-linear (meets “4.”), but still fits to en.Wikipedia’s “forward-active” and the collector current flows in a healthy direction. After \$V_{\mathrm{BC}}\$ passes through zero (≈ “2.”), a hard saturation mode develops, that meets the definition “1.”. Ī̲’d call it “on–on region”.

Also, Ī̲’d characterize the region immediately before switching B–C on as “(forward) low-voltage”, having low (but positive) β. For details, see comments and my answer in Why does the collector current direction remain the same in saturation and active region? and the Common base circuit with zero supply voltage thread (with some simulation in CircuitLab).

What about the en.Wikipedia definition (as of today) of regions? It can be traced to various sources where authors were unwilling to get into details of these marginal modes. Quasi-saturation is close to the hard saturation region on the voltage–voltage plane (albeit operates in a different way), and many authors were satisfied with the simplistic picture presented in @jonk’s answer. What about the stuff from es.Wikipedia under “3.”? It’s just incompetent rubbish.

“Saturation” is a generic term for regions that are useless for an amplifier (i. e., are strongly non-linear on \$I_{\mathrm B}\$ or have β less than 1), but are far from cut-off. Most authors don’t apply due efforts to formulate a usable definition. There exists at least one English book not negligent on the matter:

E. Ramshaw (Google Books info) or R. S. Ramshaw (as written on the cover)
Power Electronics Semiconductor Switches
Springer Science & Business Media, 2013

The book specifies a distinct quasi-saturation region, that is non-linear (meets “4.”), but still fits to en.Wikipedia’s “forward-active” and the collector current flows in a healthy direction. After \$V_{\mathrm BC}\$ passes through zero (≈ “2.”), a hard saturation mode develops, that meets the definition “1.”. Ī̲’d call it “on–on region”.

Also, Ī̲’d characterize the region immediately before switching B–C on as “(forward) low-voltage”, having low (but positive) β, see comments and my answer in Why does the collector current direction remain the same in saturation and active region? for details.

What about the en.Wikipedia definition (as of today) of regions? It can be traced to various sources where authors were unwilling to get into details of these marginal modes. Quasi-saturation is close to the hard saturation region (albeit operates in a different way), and many authors were satisfied with the simplistic picture presented in @jonk’s answer. What about the stuff from es.Wikipedia under “3.”? It’s just incompetent rubbish.

“Saturation” is a generic term for regions that are useless for an amplifier (i. e., are strongly non-linear on \$I_{\mathrm B}\$ or have β less than 1), but are far from cut-off. Most authors don’t apply due efforts to formulate a usable definition. There exists at least one English book not negligent on the matter:

E. Ramshaw (Google Books info) or R. S. Ramshaw (as written on the cover)
Power Electronics Semiconductor Switches
Springer Science & Business Media, 2013

The book specifies a distinct quasi-saturation region, that is non-linear (meets “4.”), but still fits to en.Wikipedia’s “forward-active” and the collector current flows in a healthy direction. After \$V_{\mathrm{BC}}\$ passes through zero (≈ “2.”), a hard saturation mode develops, that meets the definition “1.”. Ī̲’d call it “on–on region”.

Also, Ī̲’d characterize the region immediately before switching B–C on as “(forward) low-voltage”, having low (but positive) β, see comments and my answer in Why does the collector current direction remain the same in saturation and active region? for details.

What about the en.Wikipedia definition (as of today) of regions? It can be traced to various sources where authors were unwilling to get into details of these marginal modes. Quasi-saturation is close to the hard saturation region on the voltage–voltage plane (albeit operates in a different way), and many authors were satisfied with the simplistic picture presented in @jonk’s answer. What about the stuff from es.Wikipedia under “3.”? It’s just incompetent rubbish.